# Photocatalyzed elaboration of antibody-based bioconjugates

Marine Le Stum, Eugénie Romero, Gary Molander

► To cite this version:

Marine Le Stum, Eugénie Romero, Gary Molander. Photocatalyzed elaboration of antibody-based bioconjugates. Beilstein Journal of Organic Chemistry, 2025, 21, pp.616-629. 10.3762/bjoc.21.49 .
hal-05133268

HAL Id: hal-05133268

https://hal.science/hal-05133268v1

Submitted on 24 Jul 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Photocatalyzed elaboration of antibody-based bioconjugates

Marine Le Stum<sup>‡</sup>, Eugénie Romero<sup>*,‡</sup> and Gary A. Molander<sup>*,‡</sup>

## Perspective

Open Access

Address:
Université Paris-Saclay, CEA, INRAE, Département Médicaments et Technologies pour la Santé (DMTS), SCBM, 91191 Gif-Sur-Yvette, France

Email:
Eugénie Romero* - eugenie.romero@cea.fr; Gary A. Molander* - gmolandr@sas.upenn.edu

* Corresponding author ‡ Equal contributors

Keywords:
antibodies; bioconjugation; bioorthogonality; chemoselectivity; late-stage functionalization; photochemistry

Beilstein J. Org. Chem. **2025**, *21*, 616–629.

https://doi.org/10.3762/bjoc.21.49

Received: 21 December 2024

Accepted: 05 March 2025

Published: 18 March 2025

This article is part of the thematic issue "Photocatalysis and photochemistry in organic synthesis".

Guest Editor: T. Noël

<figure><img src="image_3.png" /></figure>

© 2025 Le Stum et al.; licensee Beilstein-Institut.

License and terms: see end of document.

## Abstract

Antibody-drug conjugates (ADCs) represent a promising class of targeted therapeutics, combining the specificity of antibodies with the potency of cytotoxic drugs to enhance therapeutic efficacy while minimizing off-target effects. The development of new chemical methods for bioconjugation is essential to generate ADCs and to optimize their stability, efficacy, and safety. Traditional conjugation methods often face challenges related to site-selectivity and heterogeneous product mixtures, highlighting the need to develop new, innovative chemical strategies. Photoredox chemistry emerges as a powerful tool in this context, enabling precise, mild, and selective modifications of peptides and proteins. By harnessing light to drive chemical transformations, photoredox techniques can facilitate the synthesis of antibody bioconjugates. This perspective will discuss the drive to develop and empower photoredox methods applied to antibody functionalization.

## Introduction

Antibodies represent increasingly important tools in several groundbreaking approaches to medical innovation, including basic biomedical research and therapy. One of the most critical requirements for the application of antibodies in disease detection and treatment is that they are adorned with functional units (e.g., fluorophores, radionuclides, toxic payloads) that must be efficiently and selectively installed through compatible synthetic methods.

Dozens of ADCs have been approved for clinical uses, and to date all are designed in the context of cancer therapy [1], which

combines the precision targeting of monoclonal antibodies (mAbs) with the therapeutic effects of cytotoxic drugs [2]. The ADCs are thus designed to deliver potent cytotoxic agents selectively and directly to cancer cells while minimizing damage to healthy tissues. Notably, ADCs have started to enter clinical trials for non-oncology applications as well [3].

The importance and value of ADCs are several fold:

* Precise targeting: ADCs specifically recognize their target cells because of their antibody component. This
minimizes collateral damage to healthy tissues, reducing
side effects compared to traditional chemotherapy.

* Enhanced potency: By delivering cytotoxic payloads directly to tumor or other target cells, ADCs achieve higher drug concentrations at the site of action. This potency enhances therapeutic efficacy.

* Treatment of refractory diseases: ADCs have shown remarkable success in treatment of highly refractory diseases. Their ability to overcome resistance makes them valuable options for patients who previously had limited treatment choices.

ADCs consist of three main components (Figure 1): (1) Monoclonal antibody (mAb): The antibody specifically recognizes and binds to surface antigens present on tumor or other targeted cells. (2) Linker: the linker connects the antibody to the payload. The nature of the moiety linking the drug/radiolabel/ imaging agent to the antibody plays a crucial role in the pharmacokinetic properties [4,5], therapeutic index, selectivity, and overall success of the ADC. The linker will ideally be stable in plasma for an extended period before the intended target can be reached, and after internalization, linkers play a key role in the drug-releasing event. Importantly, some exceptions exist. For example, in trodelvy, used in a treatment for patients with triple-negative breast cancer, the linker may release the drug prior to internalization. (3) Payload: The payload may be a subunit used in cellular tracking, imaging, or most commonly toxic drug therapeutics. The overall goal of ADCs is to deliver the payload directly to target cells via the antibody without affecting normal, healthy tissue.

Importantly, the use of antibodies in modern medicine is not
restricted solely to ADCs and cancer therapy. For example,
mABs now find routine use in the context of radionuclide (PET)
imaging agents, informing therapeutic decision-making [6,7].

Antibody-oligonucleotide conjugates, antibody-enzyme conju-
gates, antibody-polymer conjugates, antibody-nanomaterial
conjugates, antibody-catalyst conjugates, and antibodies
involved in protein degradation also play critical roles in bio-
medical research and therapies [2].

In whatever capacity, the use of antibodies applied to medicine
is critically dependent on the ability to anchor them to opera-
tive payloads in a highly precise manner [8-10]. Many
researchers are tackling the challenge of new strategies for
chemoselective synthetic modifications of biomolecules, includ-
ing antibodies [11]. In the case of ADCs, conjugation of the
linker/payload to the antibody must not drastically alter the
pharmacokinetics or physicochemical properties of the anti-
body [12,13]. Typically, zero to eight payloads are attached to
the antibody. Heterogeneous ADCs may thus be a mix of both
unconjugated and “overloaded” antibodies. Unconjugated anti-
bodies compete with antibodies containing payloads for
binding, which can diminish the effectiveness of the antibody-
payload materials. On the other hand, excessive loading of the
antibody can lead to antibody aggregation, increased toxicity,
decreased stability, and/or a shorter ADC half-life [14]. Optimiz-
ation of the drug/antibody ratio (DAR) and payload distribu-
tion/location thus becomes significant for ideal ADC design.

Given the complexity of biological macromolecules, there are
inherent limitations in terms of the types of reactions that can be
used to modify them. Reactions must be carried out at or near
ambient temperatures and near-neutral pH. Aqueous media is
often required to solubilize the substrates and/or to prevent
denaturation, and reactions are normally carried out under very
dilute (micromolar) conditions. As importantly, the highly func-
tionalized nature of polypeptides/proteins demands exquisite
selectivity to target specific sites on the macromolecule of
interest. In addition to the requisite features of the chemical

<figure><img src="image_4.png" /><figcaption>**Figure 1:** Representation of an antibody-drug conjugate. The antibody shown in this figure is from https://www.gettyimages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.</figcaption></figure>
methods outlined above, there are key requirements for synthe-
tic methods used in ADCs, which include high site-selectivity
and controlled stoichiometry (DAR). Additionally, the methods
should be synthetically reliable, easy, and rapid to carry out,
and they must tolerate diverse functional groups on the
payloads/linker [15,16]. These factors conspire to set a very
high bar for those processes that are amenable to covalent bio-
conjugation.

Effective bioconjugation has been dramatically facilitated by
the development of robust bioorthogonal reactions, which has
revolutionized the field of chemical biology [17]. A bioorthogo-
nal reactive group can be introduced into proteins via direct
chemical coupling on specific native amino acids, by enzy-
matic coupling, or by bioengineering for the incorporation of
non-canonical amino acids [18,19]. In fact, incorporating non-
natural amino acids significantly broadens the scope of reac-
tions that can be used in bioconjugation. As an example, click
chemistry comprises one particularly useful and important tool
in which azides and other dipolar species engage with reactive
alkenes and alkynes on non-canonical amino acids [20]. Transi-
tion-metal-mediated processes, including metathesis reactions,
aryl cross-coupling reactions, and conjugate addition reactions
with dehydroalanine derivatives round out the most predomi-
nant reactions used on non-natural amino acids. However, even
though bioengineering allows the incorporation of non-canon-
ical amino acids with astounding effectiveness and near-com-
plete selectivity, this technology is exceedingly expensive and
time-consuming, and the expertise required to carry out these
transformations is constrained to a very limited number of labo-
ratories, particularly in the field of antibodies.

Given the challenges of bioengineering antibodies with non-
canonical amino acids, the direct and chemoselective modifica-
tion of native antibodies is most attractive, albeit not without
serious obstacles. Among native amino acids, attachment to
lysine (Lys) [21] and cysteine (Cys) are the most common.
Antibodies typically contain as many as 90 lysine units, many
of which are at highly solvent-accessible sites [22,23]. Conjuga-
tion to Lys is thus rarely controlled and leads to a wide range of
DARs that can destabilize the antibody and furthermore greatly
alter the pharmacokinetics of the ADC (vide infra). Additional-
ly, undesirable Cys and Tyr modifications may compete in
some instances with the Lys conjugation.

Cysteine amino acids are less prevalent and typically more
evenly distributed in antibodies than lysine but are often tied up
in disulfide bridges that must be reduced prior to conjugation
[24,25]. Such reductions must thus be carefully controlled to
allow conjugation while ensuring the overall structural integrity
of the antibody. The free sulfhydryls can be conjugated by

several means, including alkylation with α-halo carbonyls (in
which case Lys may compete), and Michael additions (e.g., to
maleimide, which is reversible and therefore may lead to
incomplete conversion). The Michael adducts also present
chemical instability in plasma and additionally generate an
undesirable new stereocenter [26,27].

Few methods have been developed for the functionalization of
tyrosine (Tyr) and tryptophan (Trp). With a low abundance
(≈3%) in proteins, Tyr modifications are widely recognized for
their ability to profoundly impact protein properties and func-
tion. Their chemical modification has been developed using
iminoxyl radicals [28]. Although numerous methods exist for
the functionalization of Trp in proteins, their application in the
elaboration of ADCs is limited [29].

Given the many challenges in antibody modification as outlined
above, it is perhaps of no surprise that the recent renaissance of
synthetic chemistry based on novel photochemical methods has
been applied to their functionalization [30]. In the context of
photochemistry applied to bioconjugation, photochemical
affinity labelling (PAL) is a technique that has been used to
create selective chemical reactions, often for bioconjugation, by
utilizing photoaffinity reagents. These reagents are typically
photoreactive molecules that can be activated by light to form
covalent bonds with nearby molecules [31-33]. PAL involves
using a light-activated group (often a photo-crosslinker) that can
form covalent bonds when exposed to UV or visible light.
These photoaffinity tags or crosslinkers are designed to bind to
specific biomolecules, such as proteins, nucleic acids, or lipids,
in a highly selective manner. The light irradiation triggers a
chemical reaction, such as a bond formation, which allows the
conjugation of two molecules. PAL is commonly used in
targeted bioconjugation when the timing and location of the
conjugation need to be controlled. A typical example is using
azido groups or alkyne groups in conjunction with light to
initiate a covalent bond between two different molecules, such
as a drug and a targeting moiety. This technique has been
shown to be useful in applications such as cell labeling, pro-
tein-protein interactions, and photoradiosynthesis of bioconju-
gates, but the most important challenge remains the lack of
specificity to target one amino acid, and thus to have a better
control of the selectivity and the DAR, leading to more homo-
geneous ADCs. In this specific context, photocatalysis
(Figure 2) enables site-specific bioconjugation by generating
reactive intermediates (such as radicals or electron-deficient
species) that can selectively react under the agency of low-
energy visible light with specific functional groups on biomole-
cules. This allows precise control over the conjugation process,
enabling targeted modifications without affecting non-reactive
sites.
<figure><img src="image_5.png" /><figcaption>**Figure 2:** a. Photoredox catalytic cycles; b. absorption spectrum of photosensitizers. Therapeutic window indicates the most appropriate wavelength range to apply irradiation for biological applications.</figcaption></figure>

Although previous reviews have discussed the more traditional synthetic methods applied to ADCs [34-36], the discussion below focuses on photocatalytic approaches that have been used to date to elaborate antibodies, providing insight into those methods that aim to revolutionize approaches to cancer treatment and other medical applications via the use of synthetically functionalized antibodies.

## Perspective

In this perspective, how light-driven chemistry can enhance the development of innovative methods for accessing antibody-drug conjugates (ADCs) will be outlined. A brief introduction to photoredox chemistry as it relates to bioconjugation in proteins is followed by a summary of the limited photoredox approaches reported for antibodies. Finally, the potential benefits and cautionary details these chemical strate-

gies possess for creating ADCs with well-defined DAR and enhanced selectivity are discussed.

## Photocatalytic modification of proteins

Photoredox chemistry has emerged as a transformative approach in the modification of proteins, enabling researchers to achieve selective and efficient conjugation under mild conditions [37]. By utilizing visible light and transition-metal catalysts, this technique allows the generation of reactive intermediates that can facilitate various modifications, including labeling, crosslinking, and the creation of protein-drug conjugates. The incorporation of photoredox strategies has facilitated the synthesis of complex protein architectures, enabling precise control over conjugation sites and degrees of modification. The number of publications reporting photoredox bioconjugation applied to proteins over the last 20 years is significant (Figure 3).

<figure><img src="image_6.png" /><figcaption>**Figure 3:** Graph representing the average number of publications focusing on photoredox chemistry applied to protein-based bioconjugation (source: SciFinder).</figcaption></figure>
Recent reviews have highlighted advancements in this field, emphasizing methods that enable site-specific modifications of proteins with minimal disruption to their native structures [38,39].

In 2021, Bottecchia and Noël reported the utility of photoredox catalysis for the functionalization of amino acid side chains, paving the way for tailored modifications in biotherapeutics [40]. More recently, Sato et al. have reviewed photochemical strategies enabled by a range of catalysts, including photoredox catalysts, energy-transfer catalysts, and genetically encoded photocatalysts, highlighting their distinct features, mechanisms, applications, and prospects [41]. This thorough analysis showcased the promising advancements in the chemical modification of proteins.

As this field continues to expand, ongoing research efforts are focusing on optimizing reaction conditions, understanding mechanistic pathways, and exploring new catalysts to broaden the scope of photoredox applications in protein chemistry.

The integration of photoredox chemistry with protein modification has opened new avenues for designing advanced biotherapeutics, including ADCs and targeted delivery systems.

## Photoinduced modification of antibodies

Although the bioconjugation of proteins via photocatalytic pathways is well-documented, the application of this method to the functionalization of antibodies remains largely unexplored. This is primarily because of the structural complexity of antibodies, which exhibit a three-dimensional architecture and numerous potential modification sites, making selective control of functionalization sites challenging. Nonetheless, photocatalytic approaches offer a unique potential to overcome these limitations, particularly by enabling modifications under mild and controlled conditions. Unlike classical methods, such as thiol chemistry [22] or widely used click reactions [33], photocatalysis could provide innovative solutions to produce ADCs, especially in terms of selectivity and the preservation of sensitive biological structures when appropriate redox potentials of photocatalysts are applied to the targeted amino acid. Additional advantages of photoinduced reactions include the ability to perform the reactions rapidly (typically <15 minutes). It was only in the late 2010s that the first publications on the subject emerged. An overview of reported photoredox approaches for the functionalization of antibodies is outlined below.

### Histidine

In 2021, the group of Sato developed a selective functionalization method for histidine using an ampolung approach based on

singlet oxygen [42]. Through energy transfer (EnT) from the ruthenium-based photocatalyst to triplet oxygen, singlet oxygen is produced in a targeted manner, which oxidizes histidine to an endoperoxide, significantly increasing its reactivity toward nucleophiles (Figure 4A). This strategy employs a functionalized ruthenium complex and a fragment crystallizable (Fc) ligand anchored to a magnetic bead, enabling the localized generation of singlet oxygen near antibodies (Figure 4B). The short lifetime and limited diffusion of singlet oxygen ensure exclusive reactions with proximal histidine residues. This strategy maximizes the efficiency and specificity of functionalization, representing a significant advancement in the selective modification of antibodies.

This method demonstrated an innovative mode of selectivity, even though some drawbacks remain. For example, the 1-methyl-4-arylazole used in these transformations is a strong electrophile that can react with Tyr, for instance, if such an amino acid is located near the reactive site. In addition, command of the DAR might have some limits, as the distance between the catalyst and the mAbs is not well defined, meaning that the photoredox reaction might not give satisfactory reproducibility.

### Histidine/Tyr

More recently, the same research group demonstrated the divergent functionalization of tyrosine or histidine, thanks to the use of a DNA photoswitch Ru complex, either used as a standalone system or involved in an artificial metalloenzyme (ArM). The use of the ArM significantly enhances the potential of the photocatalyst for antibody modification [43]. By inserting a [Ru(bpy)₂dppz]²⁺ complex into the apo-form of riboflavin-binding protein (RFBP), a complete reversal of selectivity was achieved: the Ru complex alone enabled tyrosine modification via a photoredox reaction, whereas when embedded within the protein, the Ru-based ArM complex promoted histidine modification through an energy-transfer mechanism (Figure 5). Based on DFT calculations, this is permitted by the modification of the energy diagram of the ruthenium complex in the presence of biological and aqueous media. When applied to trastuzumab, the artificial metalloenzyme system ([Ru] ⊂ RFBP) thus yielded striking results – significantly reducing undesired tyrosine modifications on the heavy chain while enabling selective modification of histidine.

This impressive divergent method allows the same photocatalyst and electrophile to be involved in two different but selective bioconjugations of mAbs. It remains limited to reactions involving oxidative mechanistic pathways with $^1O_2$.
<figure><img src="image_7.png" /><figcaption>B) localized production of singlet oxygen near antibody</figcaption></figure>

<figure><img src="image_8.png" /></figure>

**Figure 4:** Schematic procedure developed by Sato et al. on histidine photoinduced modification. The antibody shown in this figure is from https://www.gettyimages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.

## Cys

In 2016, Bräse et al. developed a photomediated disulfide rebridging method, exploiting the disulfide bridging sites in biomolecules to introduce specific functional groups (Figure 6) [44]. The bioconjugation reaction is based on thiol–yne coupling. Originally developed on peptides and proteins, this approach was applied to a fragment antigen-binding (Fab) antibody fragment (approximately 46 kDa) containing a single interchain disulfide bond. After 4 h of irradiation under UV wavelengths, the desired rebridged Fab fragment was obtained with around 40% conversion.

Although this method shows promise, it remains to be seen whether it is feasible for a full antibody, such as an IgG1, which typically contains four un-buried interchain disulfide bonds. Thus, rebridging allows the disulfide bonds to be reformed after conjugation, but one may question whether this could affect the long-term stability or functionality of the ADC, particularly if

the new bonds do not perfectly mimic the properties of native disulfide bridges.

More recently, Lang et al. developed an approach using dual photoredox/nickel-catalyzed antibody functionalization [45], demonstrating the selective modification of Cys residues through photocatalytic methods (Figure 7). A key advantage of this strategy lies in the use of readily available and inexpensive Ru(bpy)₃, along with a water-soluble, air- and moisture-stable Ni(dabpy)Br₂ catalyst. Moreover, they demonstrated that this system works efficiently in aqueous conditions, making it highly suitable for applications involving antibodies. These characteristics make the method highly attractive for industrial applications, where both scalability and robustness are essential.

Beyond its practical advantages, the method offers notable versatility, enabling the conjugation of a wide range of aryl linkers to Cys residues. This flexibility is crucial for the devel-
<figure><img src="image_9.png" /><figcaption>**Figure 5:** Schematic procedure of the divergent method developed by Sato et al. on histidine/tyrosine photoinduced modification. The antibody shown in this figure is from https://www.gettyimages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 6:** Schematic procedure developed by Bräse et al. on photoinduced disulfide rebridging method.</figcaption></figure>

opment of ADCs, where precise control over the linker and conjugation site is vital to optimizing therapeutic efficacy and pharmacokinetics. The Lang group's approach therefore represents a significant advancement in the field of selective and scalable antibody functionalization.

Importantly, although pioneering the field of dual nickel catalysis for the functionalization of antibodies in a very elegant and practical manner, the method has been applied to Cys, which necessitates the use of a reductant to access the free form of Cys. Another consequence was the high and non-reproducible
<figure><img src="image_11.png" /><figcaption>**Figure 7:** Schematic procedure developed by Lang et al. on a photoinduced dual nickel photoredox-catalyzed approach. The antibody shown in this figure is from https://www.gettyimages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.</figcaption></figure>

DAR. The development of such methods applied to other canonical amino acids should benefit from the advantages of this approach while overcoming the encountered problems.

Glutamic acid

The Chung group explored the development of a method based on a high affinity immunoglobulin G (IgG) Fc-binding strategy to generate ADCs (Figure 8). Although such methods are generally reported with a heterogeneous DAR, the group developed a site-specific approach leading to a homogeneous DAR 2, thanks

to a photoreactive Fc-binding peptide derivative (pFcBP) con-
taining a photoleucine (pLeu) [46]. By analyzing the X-ray
crystal structure of IgG, the researchers identified high-affinity
binding sites for the ligand within the Fc domain. This struc-
tural analysis allowed precise determination of amino acid
residue positions and orientations.

Upon photoirradiation, the pLeu within the pFcBP generated a
carbene from the diazirine moiety. This transformation facili-
tated a site-specific covalent linkage to the antibody, resulting

<figure><img src="image_12.png" /><figcaption>**Figure 8:** Schematic of the procedure developed by Chang et al. on photoinduced high affinity IgG Fc-binding strategy. The antibody shown in this figure is from https://www.gettyimages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.</figcaption></figure>
in the formation of a DAR 2 ADC precursor. For their ADC
design strategy, the group selected a polyethylene glycol (PEG)
linker, as it enhances the water solubility of the ADC, while the
payload consists of the cytotoxic agent DM1, which acts as a
microtubule destabilizer. To ensure the stability and homo-
geneity of the final product, the design of the FcBP included a
norbornene motif at the N-terminal end of the peptide sequence.
The norbornene motif selectively reacts with a tetrazine located
on the spacer-payload.

This study demonstrates the potency of photoinduced methods
to access more homogeneous ADCs, hopefully reducing patient
side effects.

In summary, photoredox modifications of antibodies have gained attention in recent years, though the field is still in its early stages because of the complexity of antibody structures. These photoinduced methods offer the potential for selective and mild functionalization, which could be particularly valuable for producing homogeneous ADCs with controlled properties. Even though very few photoredox approaches have been published in the literature when compared to proteins, high-lighting the complexity of antibody architecture, new photoinduced methods for the selective modification of natural amino acids in antibodies are of high necessity.

Discussion

Tremendous inroads have already been made in the application
of photochemical methods to the construction of ADCs. High-

lighted below are additional aspects that photochemical trans-
formations could, in our opinion, bring to ADCs that are not
easily achieved through traditional, mostly two-electron mecha-
nistic approaches (Figure 9).

Chemoselectivity

As mentioned in the introduction, there are exceedingly strin-
gent requirements for the types of reactions that can be used for
the elaboration of ADCs. Among the signature features of pho-
tochemical, single-electron transformations is their ability to be
conducted in dilute aqueous media at room temperature and at
neutral pH [47]. Additionally, the distinctive mechanistic para-
digms under which photochemical reactions operate makes
them uniquely tolerant of numerous unprotected functional
groups [48]. Consequently, the elaboration of a variety of
highly sophisticated biomolecules, including ADCs, can be
carried out without protecting groups in a highly selective
manner. This chemoselectivity should extend to any molecu-
larly complex linkers and/or payloads being conjugated to the
mAbs, allowing highly efficient entry to ADCs.

DAR control

A noteworthy characteristic of photochemical transformations is the ability to unambiguously start and stop reactions instantaneously using light on and light off reaction conditions (Figure 10). Aspirationally this would allow exquisite control of the drug/antibody ratio, taking advantage of the nature of photochemical transformations to occur rapidly and selectively at the kinetically most reactive sites [49,50]. Importantly, as a general

<figure><img src="image_13.png" /><figcaption>**Figure 9:** Potential advantages of photoredox chemistry for bioconjugation applied to antibodies. The antibody shown in this figure is from https://www.gettyimages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Figure 10:** Representation of the photoinduced control of the DAR. The antibody shown in this figure is from https://www.gettyimages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.</figcaption></figure>

comment, targeting a single class of amino acids, the most sol- vent-exposed would react preferentially to those buried within hydrophobic domains of the mAbs, and ideally the relative rates of their reactivity would translate directly to tightly controlled DARs. The time of irradiation, the wavelength of the light, the lamp wattage, and diverse photocatalysts or mechanisms (e.g., energy transfer, photoredox, or electron-donor/electron-acceptor photoinduced electron transfer) might all be brought to bear on controlling the DAR. In addition to the DAR, homogeneity for conjugation at specific sites using light control might also be improved using photochemical transformations. Importantly, optimization of bioconjugation reactions with technologies such as high-throughput experimentation has already been applied on antibodies [51].

## Bioorthogonality

Bioorthogonal chemistry has transformed our capability to study and alter biological systems at the molecular level [52,53]. Owing to the often-unique mechanisms characterized by photocatalytically promoted reactions, reactivity patterns might be developed that allow selective reactions at amino acids that are not currently used for conjugation of linkers/payloads to the mAb. Potential access to modification of the mAbs at differ- ent amino acids has obvious ramifications for installation of the linker/payload because of the relative abundance and local envi- ronments of the amino acids, in addition to the inherent reactivi- ty of these diverse components. In turn, this selectivity for reac- tion at a wider range of amino acids impacts several important features of ADCs, including site selection for the linker/ payload, the DAR, and the homogeneity of the ADCs.

## Multi-payload ADCs

Most clinically approved ADCs contain a single-drug payload. However, systemic cancer chemotherapies often involve combi- nations of drugs. Combination regimens improve treatment outcomes by producing synergistic anticancer effects and slowing the development of drug-resistant cell populations.

Researchers aim to replicate combination regimens by devel- oping techniques for attaching multiple payloads to a single antibody molecule with high homogeneity [54-56]. Generating homogeneous multi-payload ADCs is complex because of the diverse reactive functional groups on antibody surfaces. As outlined above in the discussion concerning bioorthogonality, it is conceivable that photochemical methods could be developed allowing multi-payload ADCs to be prepared effortlessly, as the single-electron mechanistic approach of such reactions is com- pletely orthogonal to currently used methods, and even among photochemical methods that might be designed for different amino acids. The scheme below (Figure 11) represents schemat- ically how bioorthogonal introduction of linkers/payloads on the ADCs might be carried out using photocatalyzed ap- proaches to ADC construction. Importantly, native amino acids could be used in these processes, and thus the onerous and expensive introduction of non-canonical amino acids into the ADCs would be completely unnecessary. Photocatalyzed methods could also be used in conjunction with more tradi- tional chemical approaches at either stage to allow biorthog- onal strategies for multi-payloads ADCs.

## Antibody-protein conjugates

Antibody-protein conjugates possess tremendous promise within the domains of biotechnology and biomedical research [57,58]. To date, such entities have most often been formulated from the expression of fusion proteins, although more recently post-translational protein-protein conjugation has been recog- nized as a means to access such structures [59]. However, methods developed to date based on traditional two-electron chemistry with canonical amino acids are extremely limited and are most effectively based on maleimide hydrolyzing methods that rely on a single amino acid (Cys) conjugation. The devel- opment of viable photochemical transformations targeted toward a variety of diverse canonical amino acids would intro- duce more versatility, efficiency, and convenience in the con- struction of these increasingly important structural units.
<figure><img src="image_15.png" /><figcaption>**Figure 11:** Representation of a photoinduced control of multi-payloads ADC strategy. The antibody shown in this figure is from https://www.gettyim-ages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.</figcaption></figure>

## Cautionary considerations

Photoredox chemistry offers a powerful approach for the targeted modification of proteins and antibodies, enabling selective covalent bond formation and functionalization under mild conditions. However, its use in biomolecule modification comes with several specific features that require consideration. Some of these are detailed below.

**Photoaggregation:** The possibility of photoinduced aggregation during photoconjugation reactions represents a major challenge for the stability and efficacy of conjugated antibodies [60]. Exposure of antibodies to light, particularly UV light, can trigger photoinduced reactions that compromise their structural stability. These phenomena include the oxidation of sensitive residues such as tryptophan (Trp), tyrosine (Tyr), cysteine (Cys), and histidine (His), as well as partial unfolding of the antibodies, exposing hydrophobic regions that promote nonspecific interactions and aggregate formation. Additionally, covalent cross-links may form, involving disulfide bridges or Cys–Tyr bonds, further increasing the risk of aggregation.

Photosensitive payloads, although essential for specific therapeutic or diagnostic applications, are particularly vulnerable to these photochemical reactions. Their light-induced modifications not only exacerbate aggregation but may also reduce therapeutic efficacy and increase the risk of immunogenicity. These effects are especially concerning when the DAR is high, as a higher DAR increases the overall photosensitivity of the conjugated antibody.

Therefore, it is crucial to account for these factors during photo-conjugation reactions to mitigate aggregation phenomena. Strategies such as optimizing the length and nature of the linker, precisely controlling reaction conditions (light intensity, exposure time), or modifying conjugation sites to distance payloads

from sensitive residues may offer effective solutions. Finally, storage and handling of conjugated antibodies require special attention to minimize exposure to ambient light, which could contribute to structural instability and promote aggregation over time.

**Reaction time:** Proteins and antibodies are delicate macromolecules, and prolonged exposure to light or reactive intermediates can lead to protein denaturation or aggregation [61]. Because photoredox reactions often generate highly reactive species (such as radicals or singlet oxygen), long exposure times could cause unwanted side reactions, including the degradation of 3D structure of the protein or antibody. A short reaction time minimizes this risk by allowing the reaction to proceed quickly and efficiently before damage occurs. To avoid damage caused by high energy wavelength, the use of irradiation in the red region could be considered [62-65].

**Aqueous media:** Developing photoredox reactions compatible with aqueous media is essential for the modification of antibodies, as these biomolecules are typically dissolved in water or aqueous buffers to maintain their stability and activity. Antibodies are sensitive to their environment, and using a non-negligible amount of organic solvents or harsh chemical conditions can lead to denaturation, aggregation, or loss of functionality. Buffers, which are commonly used to stabilize antibodies by maintaining their pH and ionic strength, must also be compatible with the photoredox process to prevent unwanted side reactions or instability.

Thus, although photoredox chemistry offers a promising method for the targeted modification of antibodies, careful consideration of factors such as photoaggregation, reaction time, and aqueous media compatibility is essential to ensure the stability, efficacy, and functionality of the conjugated biomolecules.
## Conclusion

In conclusion, the development of antibody-drug conjugates (ADCs) holds great promise for advancing targeted therapeutics, particularly in the treatment of cancer and other refractory diseases. However, the challenges related to achieving selective, efficient, and stable conjugation of antibodies to their payloads remain a significant barrier to realizing the full potential of ADCs. Traditional conjugation methods often struggle with issues such as site-selectivity and heterogeneous products. Photoredox chemistry has emerged as a transformative approach, offering precise control over the modification of antibodies with the potential to enhance ADC purity, stability, and therapeutic efficacy. By utilizing light-driven chemical transformations, photoredox techniques enable selective functionalization of antibodies at specific sites, addressing the limitations of conventional methods. As research in photochemistry continues to evolve, these strategies may pave the way for the creation of more homogeneous and optimized ADCs, enhancing their therapeutic outcomes and minimizing off-target effects. This perspective highlights the exciting future of photoredox methods in the development of next-generation ADCs, ultimately contributing to the broader landscape of targeted biomedicine.

## Acknowledgements

The antibody shown in the graphical abstract is from https://www.gettyimages.de/detail/illustration/monoclonal-antibody-igg2a-lizenfreie-illustration/585105259?searchscope=image%2Cfilm&adppopup=true; ALFRED PASIEKA/SCIENCE PHOTO LIBRARY, No. 585105259. This content is not subject to CC BY 4.0.

## Funding

GAM acknowledges support from the Université Paris-Saclay for a Jean d'Alembert Research Fellowship.

## ORCID® iDs

Marine Le Stum - https://orcid.org/0009-0001-0095-2138

Eugénie Romero - https://orcid.org/0000-0003-0484-1221

Gary A. Molander - https://orcid.org/0000-0002-9114-5584

## Data Availability Statement

All data that supports the findings of this study is available in the published article and/or the supporting information of this article.

## References

1. Fu, Z.; Li, S.; Han, S.; Shi, C.; Zhang, Y. *Signal Transduction Targeted Ther.* **2022**, *7*, 93.
doi:10.1038/s41392-022-00947-7

2. Qian, L.; Lin, X.; Gao, X.; Khan, R. U.; Liao, J.-Y.; Du, S.; Ge, J.; Zeng, S.; Yao, S. Q. *Chem. Rev.* **2023**, *123*, 7782–7853.
doi:10.1021/acs.chemrev.2c00915

3. Buttgereit, F.; Aelion, J.; Rojkovich, B.; Zubrzycka-Sienkiewicz, A.; Chen, S.; Yang, Y.; Arikan, D.; D'Cunha, R.; Pang, Y.; Kupper, H.; Radstake, T.; Amital, H. *Arthritis Rheumatol.* **2023**, *75*, 879–889.
doi:10.1002/art.42415

4. Samantasinghar, A.; Sunildutt, N. P.; Ahmed, F.; Soomro, A. M.; Salih, A. R. C.; Parihar, P.; Memon, F. H.; Kim, K. H.; Choi, K. H. *Biomed. Pharmacother.* **2023**, *161*, 114408.
doi:10.1016/j.biopha.2023.114408

5. Hobson, A. D.; Xu, J.; Marvin, C. C.; McPherson, M. J.; Hollmann, M.; Gattner, M.; Dzeyk, K.; Fettis, M. M.; Bischoff, A. K.; Wang, L.; Fitzgibbons, J.; Wang, L.; Salomon, P.; Hernandez, A., Jr.; Jia, Y.; Sarvaiya, H.; Goess, C. A.; Mathieu, S. L.; Santora, L. C. *J. Med. Chem.* **2023**, *66*, 9161–9173.
doi:10.1021/acs.jmedchem.3c00794

6. Usama, S. M.; Thapaliya, E. R.; Luciano, M. P.; Schnermann, M. J. *Curr. Opin. Chem. Biol.* **2021**, *63*, 38–45.
doi:10.1016/j.cbpa.2021.01.009

7. Lin, D.; Lechermann, L. M.; Huestis, M. P.; Marik, J.; Sap, J. B. I. *Angew. Chem., Int. Ed.* **2024**, *63*, e202317136.
doi:10.1002/anie.202317136

8. Wu, K.-L.; Yu, C.; Lee, C.; Zuo, C.; Ball, Z. T.; Xiao, H. *Bioconjugate Chem.* **2021**, *32*, 1947–1959.
doi:10.1021/acs.bioconjchem.1c00342

9. Park, J.; Lee, S.; Kim, Y.; Yoo, T. H. *Bioorg. Med. Chem.* **2021**, *30*, 115946.
doi:10.1016/j.bmc.2020.115946

10. von Witting, E.; Hober, S.; Kanje, S. *Bioconjugate Chem.* **2021**, *32*, 1515–1524.
doi:10.1021/acs.bioconjchem.1c00313

11. Walsh, S. J.; Bargh, J. D.; Dannheim, F. M.; Hanby, A. R.; Seki, H.; Counsell, A. J.; Ou, X.; Fowler, E.; Ashman, N.; Takada, Y.; Isidro-Llobet, A.; Parker, J. S.; Carroll, J. S.; Spring, D. R. *Chem. Soc. Rev.* **2021**, *50*, 1305–1353.
doi:10.1039/d0cs00310g

12. Coumans, R. G. E.; Ariaans, G. J. A.; Spijker, H. J.; Renart Verkerk, P.; Beusker, P. H.; Kokke, B. P. A.; Schouten, J.; Blomenröhrr, M.; van der Lee, M. M. C.; Groothuis, P. G.; Ubink, R.; Dokter, W. H. A.; Timmers, C. M. *Bioconjugate Chem.* **2020**, *31*, 2136–2146.
doi:10.1021/acs.bioconjchem.0c00337

13. Garbaccio, R. M. Chemistry of Antibody-Small Molecule Drug Conjugates. In *Comprehensive Organic Synthesis*, 2nd ed.; Knochel, P., Ed.; Elsevier: Amsterdam, Netherlands, 2014; Vol. 9, pp 438–462.
doi:10.1016/b978-0-08-097742-3.00942-3

14. Sochaj, A. M.; Świderska, K. W.; Otlewski, J. *Biotechnol. Adv.* **2015**, *33*, 775–784.
doi:10.1016/j.biotechadv.2015.05.001

15. Kang, M. S.; Kong, T. W. S.; Khoo, J. Y. X.; Loh, T.-P. *Chem. Sci.* **2021**, *12*, 13613–13647.
doi:10.1039/d1sc02973h

16. Lu, J.; Jiang, F.; Lu, A.; Zhang, G. *Int. J. Mol. Sci.* **2016**, *17*, 561.
doi:10.3390/ijms17040561

17. Porte, K.; Riberaud, M.; Châtre, R.; Audisio, D.; Papot, S.; Taran, F. *ChemBioChem* **2021**, *22*, 100–113.
doi:10.1002/cbic.202000525

18. Lemieux, G. A.; Bertozzi, C. R. *Trends Biotechnol. Sci. Technol.* **1998**, *16*, 506–513.
doi:10.1016/s0167-7799(98)01230-x

19. Hackenberger, C. P. R.; Schwarzer, D. *Angew. Chem., Int. Ed.* **2008**, *47*, 10030–10074.
doi:10.1002/anie.200801313

20. Stump, B. *ChemBioChem* **2022**, *23*, e202200016.
doi:10.1002/cbic.202200016

21. Haque, M.; Forte, N.; Baker, J. R. *Chem. Commun.* **2021**, *57*, 10689–10702.
doi:10.1039/d1cc03976h
22. McCombs, J. R.; Owen, S. C. *AAPS J.* **2015**, *17*, 339–351.
doi:10.1208/s12248-014-9710-8

23. Jain, N.; Smith, S. W.; Ghone, S.; Tomczuk, B. *Pharm. Res.* **2015**, *32*,
3526–3540. doi:10.1007/s11095-015-1657-7

24. Yuan, D.; Zhang, Y.; Lim, K. H.; Leung, S. K. P.; Yang, X.; Liang, Y.; Lau, W. C. Y.; Chow, K. T.; Xia, J. *J. Am. Chem. Soc.* **2022**, *144*, 18494–18503. doi:10.1021/jacs.2c07594

25. Natesan, R.; Dykstra, A. B.; Banerjee, A.; Agrawal, N. *Antibiotics* (Basel, Switz.) **2023**, *12*, 83. doi:10.3390/antib12040083

26. Alley, S. C.; Benjamin, D. R.; Jeffrey, S. C.; Okeley, N. M.; Meyer, D. L.; Sanderson, R. J.; Senter, P. D. *Bioconjugate Chem.* **2008**, *19*, 759–765. doi:10.1021/bc7004329

27. Shen, B.-Q.; Xu, K.; Liu, L.; Raab, H.; Bhakta, S.; Kenrick, M.; Parsons-Reponte, K. L.; Tien, J.; Yu, S.-F.; Mai, E.; Li, D.; Tibbitts, J.; Baudys, J.; Saad, O. M.; Scales, S. J.; McDonald, P. J.; Hass, P. E.; Eigenbrot, C.; Nguyen, T.; Solis, W. A.; Fuji, R. N.; Flagella, K. M.; Patel, D.; Spencer, S. D.; Khawli, L. A.; Ebens, A.; Wong, W. L.; Vandlen, R.; Kaur, S.; Sliwkowski, M. X.; Scheller, R. H.; Polakis, P.; Junutula, J. R. *Nat. Biotechnol.* **2012**, *30*, 184–189.
doi:10.1038/nbt.2108

28. Maruyama, K.; Ishiyama, T.; Seki, Y.; Sakai, K.; Togo, T.; Oisaki, K.; Kanai, M. *J. Am. Chem. Soc.* **2021**, *143*, 19844–19855.
doi:10.1021/jacs.1c09066

29. Xie, X.; Moon, P. J.; Crossley, S. W. M.; Bischoff, A. J.; He, D.; Li, G.; Dao, N.; Gonzalez-Valero, A.; Reeves, A. G.; McKenna, J. M.; Elledge, S. K.; Wells, J. A.; Toste, F. D.; Chang, C. J. *Nature* **2024**, *627*, 680–687. doi:10.1038/s41586-024-07140-6

30. Zhang, Y.; Tan, J.; Chen, Y. *Chem. Commun.* **2023**, *59*, 2413–2420.
doi:10.1039/d2cc06987c

31. Holland, J. P.; Gut, M.; Klingler, S.; Fay, R.; Guillou, A. *Chem. – Eur. J.* **2020**, *26*, 33–48. doi:10.1002/chem.201904059

32. Lechner, V. M.; Nappi, M.; Deneny, P. J.; Folliet, S.; Chu, J. C. K.; Gaunt, M. *J. Chem. Rev.* **2022**, *122*, 1752–1829.
doi:10.1021/acs.chemrev.1c00357

33. Aguilar Troyano, F. J.; Merkens, K.; Anwar, K.; Gómez-Suárez, A. *Angew. Chem., Int. Ed.* **2021**, *60*, 1098–1115.
doi:10.1002/anie.202010157

34. Kostova, V.; Désos, P.; Starck, J.-B.; Kotschy, A. *Pharmaceuticals* **2021**, *14*, 442. doi:10.3390/ph14050442

35. Hobson, A. D. *RSC Med. Chem.* **2024**, *15*, 809–831.
doi:10.1039/d3md00674c

36. Dudchak, R.; Podolak, M.; Holota, S.; Szewczyk-Roszczenko, O.; Roszczenko, P.; Bielawska, A.; Lesyk, R.; Bielawski, K. *Bioorg. Chem.* **2024**, *143*, 106982. doi:10.1016/j.bioorg.2023.106982

37. Ryu, K. A.; Kaszuba, C. M.; Bissonnette, N. B.; Oslund, R. C.; Fadeyi, O. O. *Nat. Rev. Chem.* **2021**, *5*, 322–337.
doi:10.1038/s41570-021-00265-6

38. Li, P.; Terrett, J. A.; Zbieg, J. R. *ACS Med. Chem. Lett.* **2020**, *11*,
2120–2130. doi:10.1021/acsmedchemlett.0c00436

39. Liu, J.-Q.; Shatskiy, A.; Matsuura, B. S.; Kärkäs, M. D. *Synthesis* **2019**, *51*, 2759–2791. doi:10.1055/s-0037-1611852

40. Bottecchia, C.; Noël, T. *Chem. – Eur. J.* **2019**, *25*, 26–42.
doi:10.1002/chem.201803074

41. Liu, Z.; Okamoto, Y.; Sato, S. *ChemCatChem* **2024**, *16*, e202301424.
doi:10.1002/cctc.202301424

42. Nakane, K.; Sato, S.; Niwa, T.; Tsushima, M.; Tomoshige, S.; Taguchi, H.; Ishikawa, M.; Nakamura, H. *J. Am. Chem. Soc.* **2021**, *143*, 7726–7731. doi:10.1021/jacs.1c01626

43. Okamoto, Y.; Mabuchi, T.; Nakane, K.; Ueno, A.; Sato, S. *ACS Catal.* **2023**, *13*, 4134–4141. doi:10.1021/acscatal.2c05946

44. Griebenow, N.; Dimaç, A. M.; Greven, S.; Bräse, S. *Bioconjugate Chem.* **2016**, *27*, 911–917.
doi:10.1021/acs.bioconjchem.5b00682

45. Bacauanu, V.; Merz, Z. N.; Hua, Z. L.; Lang, S. B. *J. Am. Chem. Soc.* **2023**, *145*, 25842–25849. doi:10.1021/jacs.3c10185

46. Lee, T.; Kim, J. H.; Kwon, S. J.; Park, S. H.; Kim, J.; Kang, H. J.; Chung, S. *J. Org. Lett.* **2020**, *22*, 8419–8423.
doi:10.1021/acs.orglett.0c03049

47. Russo, C.; Brunelli, F.; Tron, G. C.; Giustiniano, M. *J. Org. Chem.* **2023**, *88*, 6284–6293. doi:10.1021/acs.joc.2c00805

48. Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. *J. Org. Chem.* **2016**, *81*, 6898–6926. doi:10.1021/acs.joc.6b01449

49. Tower, S. J.; Hetcher, W. J.; Myers, T. E.; Kuehl, N. J.; Taylor, M. T. *J. Am. Chem. Soc.* **2020**, *142*, 9112–9118. doi:10.1021/jacs.0c03039

50. Kim, J.; Li, B. X.; Huang, R. Y.-C.; Qiao, J. X.; Ewing, W. R.; MacMillan, D. W. C. *J. Am. Chem. Soc.* **2020**, *142*, 21260–21266.
doi:10.1021/jacs.0c09926

51. Emmert, M. H.; Yang, C.; Kwan, E. E.; Chmielowski, R.; Kilgore, B.; VanAernum, Z. L.; Bottecchia, C.; Barrientos, R. C.; Haley, M.; Raymond, K.; Rauscher, M.; Dunn, Z. D.; Desai, J. *Org. Process Res. Dev.* **2025**, *29*, 79–91.
doi:10.1021/acs.oprd.4c00343

52. Kim, J.; Xu, Y.; Lim, J. H.; Lee, J. Y.; Li, M.; Fox, J. M.; Vendrell, M.; Kim, J. S. *J. Am. Chem. Soc.* **2025**, *147*, 701–712.
doi:10.1021/jacs.4c13131

53. Mato, M.; Fernández-González, X.; D'Avino, C.; Tomás-Gamasa, M.; Mascareñas, J. L. *Angew. Chem., Int. Ed.* **2024**, *63*, e202413506.
doi:10.1002/anie.202413506

54. Journeaux, T.; Bernardes, G. J. L. *Nat. Chem.* **2024**, *16*, 854–870.
doi:10.1038/s41557-024-01507-y

55. Tsuchikama, K.; Anami, Y.; Ha, S. Y. Y.; Yamazaki, C. M. *Nat. Rev. Clin. Oncol.* **2024**, *21*, 203–223.
doi:10.1038/s41571-023-00850-2

56. Journeaux, T.; Geeson, M. B.; Murray, T. V.; Papworth, M. A.; Gothard, M.; Kettle, J. G.; Vasco, A. V.; Bernardes, G. J. L. *Angew. Chem., Int. Ed.* **2025**, *64*, e202417620.
doi:10.1002/anie.202417620

57. Bell, M. R.; Engleka, M. J.; Malik, A.; Strickler, J. E. *Protein Sci.* **2013**, *22*, 1466–1477. doi:10.1002/pro.2356

58. Mahmood, T.; Shahbaz, A.; Hussain, N.; Ali, R.; Bashir, H.; Rizwan, K. *Int. J. Biol. Macromol.* **2023**, *230*, 123161.
doi:10.1016/j.ijbiomac.2023.123161

59. Vasco, A. V.; Taylor, R. J.; Méndez, Y.; Bernardes, G. J. L. *J. Am. Chem. Soc.* **2024**, *146*, 20709–20719.
doi:10.1021/jacs.4c03721

60. Cockrell, G. M.; Wolfe, M. S.; Wolfe, J. L.; Schöneich, C. *Mol. Pharmaceutics* **2015**, *12*, 1784–1797. doi:10.1021/mp5006799

61. Pattison, D. I.; Davies, M. J. Actions of ultraviolet light on cellular structures. *Cancer: Cell Structures, Carcinogens and Genomic Instability*; Experientia Supplementum, Vol. 96; Birkhäuser-Verlag: Basel, Switzerland, 2006; pp 131–157. doi:10.1007/3-7643-7378-4_6

62. Sellet, N.; Cormier, M.; Goddard, J.-P. *Org. Chem. Front.* **2021**, *8*, 6783–6790. doi:10.1039/d1qo01476e

63. Connell, T. U. *Dalton Trans.* **2022**, *51*, 13176–13188.
doi:10.1039/d2dt01491b

64. Linsley, C. S.; Quach, V. Y.; Agrawal, G.; Hartnett, E.; Wu, B. M. *Drug Delivery Transl. Res.* **2015**, *5*, 611–624.
doi:10.1007/s13346-015-0260-0
65. Ravetz, B. D.; Tay, N. E. S.; Joe, C. L.; Sezen-Edmonds, M.; Schmidt, M. A.; Tan, Y.; Janey, J. M.; Eastgate, M. D.; Rovis, T. *ACS Cent. Sci.* **2020**, *6*, 2053–2059. doi:10.1021/acscentsci.0c00948

## License and Terms

This is an open access article licensed under the terms of the Beilstein-Institut Open Access License Agreement (https://www.beilstein-journals.org/bjoc/terms), which is identical to the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0). The reuse of material under this license requires that the author(s), source and license are credited. Third-party material in this article could be subject to other licenses (typically indicated in the credit line), and in this case, users are required to obtain permission from the license holder to reuse the material.

The definitive version of this article is the electronic one which can be found at: https://doi.org/10.3762/bjoc.21.49