title: The Development of Amino Acid Sulfinate Salts as Tools for Peptide Modification
Creators: name: Hammond, Joshua
Publisher: name: The Australian National University
Year: 2024

description: Modified peptides, including compounds inspired by nature, have increasingly become a valuable area of research due to their growing prevalence in medicinal chemistry. To fully exploit the potential of peptide natural products and their derivatives, novel techniques must be developed to access these compounds in a laboratory setting. Techniques which can mimic the site selectivity of enzymatic post-translational modifications are particularly valuable. One conceivable approach is the development of novel amino acids bearing unique functional groups that impart new reactivity onto a peptide. In this thesis , the synthesis of novel amino acid sulfinate salts as valuable precursors to amino acid radicals is explored. The established radical reactivity of the sulfinate moiety is ideal as the conditions for radical formation are generally mild and selective and radical transformations can often proceed orthogonally to polar functional groups , including the diverse functionality present in peptides. This work discloses the synthesis of amino acid sulfinate salts and the optimization of a photochemically-promoted radical reaction between these novel amino acids and various radical traps, particularly disulfides. In addition, this reactivity is extended to small molecule sulfinates and disulfide-containing peptides, for the development of an umpolung approach to the modification of cysteine residues. The thesis is divided into the following chapters: Chapter one summarises the existing literature on site-specific side-chain peptide modification, the development of the sulfinate moiety as a precursor for carbon centred radicals and the subsequent use of existing sulfinate reagents in the modification of small molecules and peptides. Furthermore, a brief history of photochemistry, focussing on the use of photochemistry as a tool for peptide modification, is discussed. The specific aims of this thesis are also outlined. Chapter two details the synthesis of the first amino acid sulfinate salts derived from the native hydroxy-functionalized amino acids hydroxyproline (HPro), homoserine (HSer) and serine (Ser), and summarises efforts towards the preparation of an amino acid sulfinate salt derived from threonine (Thr). Detailed optimization of the synthetic pathways are discussed. Chapter three focusses on the optimization and subsequent diversification of the amino acid sulfinate salts. Various mild oxidative conditions were probed for their effectiveness, including chemical oxidation, electrochemistry, and photochemistry. Optimization of key photochemical conditions enabled broad application to a functionally diverse array of disulfide radical traps. Chapter four aims to extend the reactivity of disulfides to a peptide context to investigate the feasibility of the reaction as a tool for the modification of cystine residues. Suitable peptide model systems were synthesized using solid phase peptide synthesis and small molecule sulfinates ('diversinates') are probed for reactivity with peptidic disulfides. Extensive optimization was conducted to improve reaction outcomes using high-throughput experimentation. This chapter also details the incorporation of amino acid sulfinates into a peptide as an alternative strategy for peptide modification. Chapter five summarises the advances made herein towards the development of sulfinate salts as tools for the modification of peptide substrates. Future directions are also discussed, including the incorporation of the amino acid sulfinates into peptides to enable to synthesis of biologically relevant peptides, such as the lanthipeptide family of natural products . In addition, the prospect of creating tuneable protected amino acid sulfinates for one pot modification techniques is discussed. Chapter six provides the detailed experimental protocols and characterization data for all compounds described in Chapters 2-4 of this thesis.