Ultraviolet (UV) light is able to modify the chemical composition of natural DNA inducing changes in the structure of the nucleobases. [1] In spite of the efficiency of the DNA repair machinery, [2] some photodamage persists in the DNA and can result in mutagenesis, which is one origin of skin cancer. [1b, 3] Within this context, pyrimidine (Pyr) dimers are among the most relevant DNA lesions caused by UV light exposure; they include cyclobutane pyrimidine dimers (CPDs) and the so called (6-4) photoproducts (6-4PPs). [4] CPDs arise from a formal [2+2] photocycloaddition between the C5-C6 double bonds of two adjacent Pyr nucleobases. 6-4PPs are formed by an initial Paternò-Büchi reaction between the C4 carbonyl (or imino) group of a 3' Pyr and the C5-C6 double bond of another Pyr at the 5' end, followed by oxetane (or azetidine) ring opening (see Figure 1).The UV absorption of Pyr bases displays a characteristic band centered at 260 nm, which disappears in the CPDs as a result of cyclobutane ring formation. Conversely, 6-4PPs exhibit a new UV band appearing at 320 nm associated with the presence of a 5-methyl-2-pyrimidone chromophore.Selective irradiation of this moiety triggers a 4p electrocyclization that yields a Dewar valence isomer. [5] With this background, it appears feasible that photogeneration of the pyrimidone chromophore within the double helix could actually constitute the insertion of a potential endogenous sensitizer absorbing in the UVA-UVB range. Thus, the 6-4PPs could act as Trojan horses, extending the active fraction of light and aggravating its harmful effects.To test this concept, 1-(b-d-2'-deoxyribosyl)-5-methyl-2pyrimidone (Pyo, Figure 1) was selected as a model of 6-4PPs, and its behavior as a photosensitizer was investigated. The Pyo was synthesized by a modification of a reported procedure (see Supporting Information). [6] Irradiation of Pyo was conducted upon addition of non-covalently bonded supercoiled circular DNA (pBR322), which is a useful method for detecting DNA damage. Native supercoiled form I is converted into circular form II after a single-strand break (SSB). The different electrophoretic mobility of both forms can be used to quantify the number of SSBs induced. Because CPDs cannot be directly observed as SSBs, a specific DNA repair enzyme must be added to reveal the damage. T4 endonuclease V cleaves the supercoiled DNA at the site of the dimers; this cleavage was used to monitor CPD formation mediated by Pyo (Figure 2). The agarose gel (inset) showed a consistent increase of DNA form II with irradiation time in the presence of Pyo. This trend was accentuated when the DNA and Pyo were incubated with T4 endonuclease V. SSB production without or with treatment with the enzyme were quantified by densitometry of the gel bands; the values after 30 minutes of irradiation were 20 % and 45 % of form II, respectively. Notably, the difference between these two values Figure 1. Formulas of thymidine cyclobutane dimers (Thd < >Thd), thymidine (6-4) photoproducts (6-4PPs) and 1-(b-d-2...

A strategy to light-trigger ionic cross-linking of alginates by incorporating a photosensitive Ca2+ cage (nitr-T) is presented. Upon irradiation, free Ca2+ was released, and this caused gelation of the alginate solution. Addition of Ca2+ "on-demand" allowed us to obtain homogeneous alginate (ALG) gels using concentrated initial ALG solutions (10%), not possible with other ionic gelation approaches. The cross-linking degree and derived mechanical properties of the hydrogel were modulated by the exposure dose. The light-mediated cross-linked alginate hydrogel displayed a significant improvement in the mechanical properties and homogeneity when compared to mixtures of alginate and soluble Ca2+ at comparable concentrations.

SonsVendrell Criado, V.; Rodríguez Muñiz, GM.; Lhiaubet ., VL.; Cuquerella Alabort, MC.; Miranda Alonso, MÁ. (2016). The (6-4) Dimeric Lesion as a DNA Photosensitizer. ChemPhysChem (Online). 17 (13):1979-1982. doi:10.1002/cphc.201600154 The (6-4) Dimeric Lesion as a DNA Photosensitizer Victoria Vendrell-Criado, Gemma M. Rodríguez-Muñiz, Virginie Lhiaubet-Vallet, M. Consuelo Cuquerella* and Miguel A. Miranda* AbstractBased on our previous work on the photophysical properties of the 5-methyl-2-pyrimidone (Pyo) chromophore, we have now extended our studies to the photobehavior of the dimeric (6-4) thymine photoproducts (6-4 PP) to evaluate their capability to act as an instrinsic DNA photosensitizers. The lesion presents significant absorption in the UVB/UVA region, weak fluorescence emission, a singlet excited state energy of ca. 351 kJ mol -1 , and a triplet energy of 297 kJ mol -1 . Its triplet transient absorption was observed with maximum at 420-440 nm, a lifetime of ca. 7 s, and a high formation quantum yield ISC = 0.86. This species is efficiently quenched by thymidine. Its DNA photosensitizing properties have been demonstrated by a series of experiments run on a pBR322 plasmid. It is observed that the lesion photoinduces both single strand breaks and cyclobutane thymine dimers. Altogether, these results show that, substitution of the pyrimidone ring at C4 by 5-hydroxy-5,6-dihydrothymine does not cancel out the photosensitization properties of the chromophore.

Electron transfer involving nucleic acids and their derivatives is an important field in bioorganic chemistry, specifically in connection with its role in the photo-driven DNA damage and repair. Four-membered ring heterocyclic oxetanes and azetidines have been claimed to be the intermediates involved in the repair of DNA (6-4) photoproduct by photolyase. In this context, we examine here the redox properties of the two azetidine isomers obtained from photocycloaddition between 6-aza-1,3-dimethyluracil and cyclohexene. Steady-state and time-resolved fluorescence experiments using a series of photoreductants and photooxidants have been run to evaluate the efficiency of the electron transfer process. Analysis of the obtained quenching kinetics shows that the azetidine compounds can act as electron donors. Additionally, it appears that the cis isomer is more easily oxidized than its trans counterpart. This result is in agreement with electrochemical studies performed on both azetidine derivatives.

Photocleavable oligohistidine peptides (POHP) allow in situ spatial organization of multiple His-tagged proteins onto surfaces functionalized with tris(nitrilotriacetic acid) (tris-NTA). Here, a second generation of POHPs is presented with improved photoresponse and site-specific covalent coupling is introduced for generating stable protein assemblies. POHPs with different numbers of histidine residues and a photocleavable linker based on the 4,5-dimethoxy-o-nitrophenyl ethyl chromophore are prepared. These peptides show better photosensitivity than the previously used o-nitrophenyl ethyl derivative. Efficient and stable caging of tris-NTA-functionalized surfaces by POHPs comprising 12 histidine residues is demonstrated by multiparameter solid-phase detection techniques. Laser lithographic uncaging by photofragmentation of the POHPs is possible with substantially reduced photodamage as compared to previous approaches. Thus, in situ micropatterning of His-tagged proteins under physiological conditions is demonstrated for the first time. In combination with a short peptide tag for a site-specific enzymatic coupling reaction, covalent immobilization of multiple proteins into target micropatterns is possible under physiological conditions.

Azetidines are interesting compounds in medicine and chemistry as bioactive scaffolds and synthetic intermediates. However, photochemical processes involved in the generation and fate of azetidine-derived radical ions have scarcely been reported. In this context, the photoreduction of this four-membered heterocycle might be relevant in connection with the DNA (6-4) photoproduct obtained from photolyase. Herein, a stable azabipyrimidinic azetidine (AZT ), obtained from cycloaddition between thymine and 6-azauracil units, is considered to be an interesting model of the proposed azetidine-like intermediate. Hence, its photoreduction and photo-oxidation are thoroughly investigated through a multifaceted approach, including spectroscopic, analytical, and electrochemical studies, complemented by CASPT2 and DFT calculations. Both injection and removal of an electron result in the formation of radical ions, which evolve towards repaired thymine and azauracil units. Whereas photoreduction energetics are similar to those of the cyclobutane thymine dimers, photo-oxidation is clearly more favorable in the azetidine. Ring opening occurs with relatively low activation barriers (<13 kcal mol ) and the process is clearly exergonic for photoreduction. In general, a good correlation has been observed between the experimental results and theoretical calculations, which has allowed a synergic understanding of the phenomenon.

Luminol is a prominent chemiluminescent (CL) agent, finding applications across numerous fields, including forensics, immunoassays, and imaging. Different substitution patterns on the aromatic ring can enhance or decrease its CL efficiency. We herein report a systematic study on the synthesis and photophysics of all possible 6,8-disubstituted luminol derivatives bearing H, Ph, and/or Me substituents. Their CL responses are monitored at three pH values (8, 10, and 12), thus revealing the architecture with the optimum CL efficiency. The most efficient pattern is used for the synthesis of a strongly CL luminol derivative, bearing a functional group for further, straightforward derivatization. This adduct exhibits an unprecedented increase in chemiluminescence efficiency at pH = 12, pH = 10, and especially at pH = 8 (closer to the biologically relevant conditions) compared to luminol. Complementary work on the fluorescence of the emissive species as well as quantum chemistry computations are employed for the rationalization of the observed results. Article pubs.acs.org/joc

DNA damage: The reactivity of HO(.) with silylated 2'-deoxyribonucleosides was investigated in acetonitrile by means of a time-resolved technique. The obtained rate constants were in general slightly lower than those reported for the natural nucleosides in water. Analysis of the reaction mixture by UPLC-MS revealed that HO(.) attack occurred at the nucleobase (see scheme).