Photochemistry and Photobiology, 2022, 98: 662–670 Special Issue Research Article Interstrand Crosslinking Involving Guanine: A New Major UVC Laser- Induced Biphotonic Oxidatively Generated DNA Damage † Dimitar Angelov 1,2 * , Imtiaz Nisar Lone 2 , Herv  e Menoni 3 and Jean Cadet 4 * 1 Ecole Normale Sup erieure de Lyon, CNRS, Laboratoire de Biologie et de Mod elisation de la Cellule LBMC, Universit e de Lyon, Lyon, France 2 Izmir Biomedicine and Genome Center IBG, Dokuz Eylul University Health Campus, Balc ßova, Izmir, Turkey 3 CNRS UMR 5309, INSERM U1209, Institute for Advanced Biosciences IAB, Universit e Grenoble Alpes, La Tronche, France 4 D epartement de M edecine nucl eaire et Radiobiologie, Facult e de M edecine, Universit  e de Sherbrooke, Sherbrooke, QC, Canada Received 17 October 2021, accepted 29 November 2021, DOI: 10.1111/php.13587 ABSTRACT Several classes of oxidatively generated DNA damage includ- ing oxidized purine and pyrimidine bases, interstrand base crosslinks and DNA-protein crosslinks have been previously shown to be generated in both isolated DNA and cellular DNA upon exposure to either 266-nm laser irradiation or one-electron oxidants. In this study, we provide evidence that biphotonic ionization of guanine bases by UVC laser irradia- tion of double-stranded deoxyoligonucleotides in aerated aqueous solutions induces the formation of interstrand cross- links (ICLs). This is supported by various experiments including sequencing gel analyses of formed photoproducts and effects of UVC laser intensity on their formation. This constitutes a novel example of the diversity of reactions of guanine radical cation that can be generated by various one- electron oxidants including UVC laser biphotonic ionization, direct effect of ionization radiation and type I photosensitiz- ers. However, the exact structure of the interstrand base adducts that is a challenging analytical issue remains to be further established. Examples of relevant biochemical/struc- tural applications of biphotonic induction of ICLs in DNA samples by high-intensity UVC laser pulses are provided. INTRODUCTION Cellular DNA is subject to a wide variety of endogenously and environmentally induced modifications including base/sugar oxi- dation, hydrolytic deamination and base release (1–4) together with the formation of interstrand crosslinks (ICLs) and DNA- protein adducts (5–9). ICLs that covalently link the two comple- mentary DNA strands together constitute a serious problem to the cell because they prevent DNA strand separation and thus interfere with DNA transcription and replication. Several classes of ICLs have been characterized in model studies and also detected in the cells. For example, it has been shown that hydro- xyl radical ( • OH)-mediated oxidation of the 2-deoxyribose at C4’ leads to the generation of a highly reactive a,ß-unsaturated keto-aldehyde intermediate that efficiently covalently adds to either cytosine (10) or adenine (11) on the opposite DNA strand. The resulting slow forming ICLs that involve cytosine have been detected in cellular DNA exposed to either gamma rays or bleomycin (12). Also, C1’ abasic sites that are intermediates of the base excision repair of oxidized and alkylated bases have shown their ability to create ICLs (13). Furthermore, endogenous oxidation of lipids produces unsaturated aldehydes such as acrolein, crotonaldehyde or 4-hydroxynonenal that act as bifunc- tional alkylating agents, thus crosslinking guanine residues in DNA (14,15). Acetaldehyde and malonaldehyde, two endoge- nously formed highly reactive aldehydes, are also efficient ICL agents (7). In addition, several classes of chemical drugs includ- ing nitrogen mustards, platinums and activatable mytomycins also have the capability of generating deleterious ICLs (8). UVA irradiation of DNA in the presence of intercalating bifunc- tional psoralen derivatives including 8-methoxypsoralen, 5- methoxypsoralen and 4,5 0 ,8-trimethylpsoralen (16–18) leads to the formation of ICLs that present a real challenge for DNA repair machinery (19). Poorly reactive octahedral Pt (IV) anti- cancer complexes have also been shown to generate thymine-Pt- guanine crosslinks upon blue light irradiation (20). Exposure of DNA to high intensity 266-nm laser pulses triggers the forma- tion of specific photolesions including predominant oxidized bases (21,22) together with DNA-protein crosslinks (DPCs) (23,24) and minor GT intrastrand lesions (25), as the result of biphotonic ionization of the nucleobases. Interestingly, under these conditions of irradiation, DNA ICLs are also formed with an enhanced quantum yield by almost one order of magnitude with respect to the conventional UVC lamp irradiation (for a review, see (26)). This study focuses on the formation and par- tial characterization of ICLs in aerated aqueous solutions of DNA duplexes consisting of 20-mer deoxyoligonucleotides fol- lowing exposure to high intensity 266-nm laser pulses. *Corresponding authors email: dimitar.anguelov@ens-lyon.fr (Dimitar Angelov) and Jean.Cadet@USherbrooke.ca (Jean Cadet) † This article is part of a Special Issue celebrating the achievements of Prof. Jean Cadet. © 2021 American Society for Photobiology 662