# Ru(II)-Dppz Derivatives and Their Interactions with DNA: Thirty Years and Counting
## 3 DNA Binding
### Few Examples of Applications: Cellular Imaging, Photodynamic Therapy (PDT), and Sensing  
Studies carried out on cells, including cytoetric and imaging studies [155], showed that cellular uptake is generally a function of the integrity of the cell membrane and the nuclear membrane [156]. Species based on dppz were studied from a photophysical point of view in a variety of solvents, in the presence and absence of DNA [157; 158]. Studies were also carried out on systems embedded in phospholipid vesicles [159; 160], and in general, it was confirmed in the biological environment that these properties of Ru(II)-based complexes were highly dependent on the environment [161]. Recently, [Ru(bpy)\({}_{2}\)(dppz)]\({}^{2+}\) was studied for the first time in cells, using localized pump-probe spectroscopy, showing that the ultrafast dynamics of this species is a function of the position where the complex was internalized. The results obtained on a sample of fixed HepG2 cells clearly showed that it was necessary to have a good understanding of the dynamics of photoactive complexes in the actual target environment of the drug, so that the properties of the photodrug could be unambiguously correlated with its in vitro activity. The DNA light-switching effect of the Ru-dppz complexes offers potential for use in fluorescent cellular imaging, particularly for nuclear staining with high-density DNA. One of the key parameters for the use of these species at the cellular level is to optimize the cellular uptake process, especially, the crossing of the nuclear membrane [162]. In particular, research by Professor [PERSON]'s group showed that a key parameter was the level of lipophilicity of the Ru-dppz photo-switch derivative obtained using different accessory ligands (bpy, phen, CO\({}_{2}\)Et-bpy,...) [163; 164; 165]. However, this did not constrain absorption in the nuclear zone. For example, in the case of the [Ru(DIP)\({}_{2}\)(dppz)]\({}^{2+}\) complex, confocal microscopy images of HeLa cells showed that this species is able to localize in the mitochondria or in the endoplasmic reticulum region, but not in the nuclear one [149]. When the lipophilicity of Ru-dppz complexes was modulated by the use of aliphatic chains on the ancillary ligands, cell imaging of fixed CHO-K1 cells revealed that the less lipophilic complex was preferentially localized within the nucleus, whereas, when the degree of lipophilicity became more pronounced, the light-switch essentially localised in the cell membrane. It was clear therefore that in a complex system such as the cellular one, it was difficult to find a univocal behavior, but the structural properties of these complexes must be dosed appropriately in order to convey them univocally [166; 167; 168]. One way to achieve this was to conjugate Ru-dppz like complexes with a nuclear targeting fragment such as the peptide D-octaarginine (D-R8) or the tetrapeptide RrRK, in which case nuclear localization was facilitated [124; 125]. It is known that one of the key steps to increase the efficiency of the photosensizers in DNA photocleavage is to bind, more or less strongly, to the DNA. Therefore, the use of intercalators was particularly studied in the field of PDT. However, [Ru(bpy)\({}_{2}\)(dppz)]\({}^{2+}\) showed negligible photocytotoxicity since, despite its high affinity for DNA, the singlet O\({}_{2}\) production efficiency was very low [169]. Figure 7: Molecular structure of [M(tpy)(pyridppz)]\({}^{2+}\). In order to intervene in this direction and improve the photosensizing properties of Ru-dppz derivatives, [PERSON] designed a series of derivatives containing functional subunits on the dppz ligand [170; 171], by maintaining the affinity for DNA, on the one hand but improving the photocytotoxicity values as PDT agents (improving the performance of the \({}^{3}\)MLCT state), on the other hand.