Unraveling the pathways to UVA-induced DNA photodamage: (6-4) photoproduct as a potential "Trojan Horse"

DNA labelling in live cells via visible light-induced [2+2] photocycloaddition

We introduce a visible light-driven (λ_max = 451 nm) photo-chemical strategy for labelling of DNA in living HeLa cells via the [2+2] cycloaddition of a styrylquinoxaline moiety, which we incorporate into both the DNA and the fluorescent label. Our methodology offers advanced opportunities for the mild remote labelling of DNA in water while avoiding UV light activation.

Green light triggered [2+2] cycloaddition of halochromic styrylquinoxaline-controlling photoreactivity by pH

Photochemical reactions are a powerful tool in (bio)materials design due to the spatial and temporal control light can provide. To extend their applications in biological setting, the use of low-energy, long wavelength light with high penetration propertiesis required. Further regulation of the photochemical process by additional stimuli, such as pH, will open the door for construction of highly regulated systems in nanotechnology- and biology-driven applications. Here we report the green light induced [2+2] cycloaddition of a halochromic system based on a styrylquinoxaline moiety, which allows for its photo-reactivity to be switched on and off by adjusting the pH of the system. Critically, the [2+2] photocycloaddition can be activated by green light (λ up to 550 nm), which is the longest wavelength employed to date in catalyst-free photocycloadditions in solution. Importantly, the pH-dependence of the photo-reactivity was mapped by constant photon action plots. The action plots further indicate that the choice of solvent strongly impacts the system's photo-reactivity. Indeed, higher conversion and longer activation wavelengths were observed in water compared to acetonitrile under identical reaction conditions. The wider applicability of the system was demonstrated in the crosslinking of an 8-arm PEG to form hydrogels (ca. 1 cm in thickness) with a range of mechanical properties and pH responsiveness, highlighting the potential of the system in materials science.

Elucidation of the Dexter-Type Energy Transfer in DNA by Thymine-Thymine Dimer Formation Using Photosensitizers as Artificial Nucleosides

C-nucleosides of 4-methylbenzophenone, 4-methoxybenzophenone, and 2'-methoxyacetophenone were synthetically incorporated as internal photosensitizers into DNA double strands. This structurally new approach makes it possible to study the distance dependence of thymidine dimer formation because the site of photoinduced triplet energy transfer injection is clearly defined. The counterstrands to these modified strands lacked the phosphodiester bond between the two adjacent thymidines that are supposed to react with each other. Their dimerization could be evidenced by gel electrophoresis because the covalent connection by cyclobutane formation between the two thymidines changes the mobility. A shallow exponential distance dependence for the formation of thymidine dimers over up to 10 A-T base pairs was observed that agrees with a Dexter-type triplet-triplet energy transfer mechanism. Concomitantly, a significant amount of photoinduced DNA crosslinking was observed.

The (6-4) Dimeric Lesion as a DNA Photosensitizer

Based on our previous investigations into the photophysical properties of the 5-methyl-2-pyrimidone (Pyo) chromophore, we now extend our studies to the photobehavior of the dimeric (6-4) thymine photoproducts (6-4 PP) to evaluate their capability to act as instrinsic DNA photosensitizers. The lesion presents significant absorption in the UVB/UVA region, weak fluorescence emission, a singlet-excited-state energy of approximately 351 kJ mol(-1) , and a triplet-excited-state energy of 297 kJ mol(-1) . Its triplet transient absorption has a maximum at 420-440 nm, a lifetime of around 7 μs, and a high formation quantum yield, ΦISC =0.86. This species is efficiently quenched by thymidine. Its DNA photosensitizing properties are demonstrated by a series of experiments run on a pBR322 plasmid. The lesion photoinduces both single-strand breaks and the formation of cyclobutane thymine dimers. Altogether, these results show that, the substitution of the pyrimidone ring at C4 by a 5-hydroxy-5,6-dihydrothymine does not cancel out the photosensitization properties of the chromophore.