molecules Review The Two Faces of the Guanyl Radical: Molecular Context and Behavior Chryssostomos Chatgilialoglu 1,2 1 ISOF, Consiglio Nazionale delle Ricerche, 40129 Bologna, Italy;
[email protected]; Tel.: +39-051-6398309 2 Center of Advanced Technologies, Adam Mickiewicz University, 61-712 Pozna´n,Poland Abstract: The guanyl radical or neutral guanine radical G(-H)• results from the loss of a hydrogen atom (H•) or an electron/proton (e–/H+) couple from the guanine structures (G). The guanyl radical exists in two tautomeric forms. As the modes of formation of the two tautomers, their relationship and reactivity at the nucleoside level are subjects of intense research and are discussed in a holistic manner, including time-resolved spectroscopies, product studies, and relevant theoretical calculations. Particular attention is given to the one-electron oxidation of the GC pair and the complex mechanism of the deprotonation vs. hydration step of GC•+ pair. The role of the two G(-H)• tautomers in single- and double-stranded oligonucleotides and the G-quadruplex, the supramolecular arrangement that attracts interest for its biological consequences, are considered. The importance of biomarkers of guanine DNA damage is also addressed. Keywords: guanine; guanyl radical; tautomerism; guanine radical cation; oligonucleotides; DNA; G-quadruplex; time-resolved spectroscopies; reactive oxygen species (ROS); oxidation 1. The Guanine Sink Citation: Chatgilialoglu, C. The Two The free radical chemistry associated with guanine (Gua) and its derivatives, guano- Faces of the Guanyl Radical: sine (Guo), 2’-deoxyguanosine (dGuo), guanosine-50-monophosphate (GMP), and 20- Molecular Context and Behavior. deoxyguanosine-50-monophosphate (dGMP), is of particular interest due to its biological Molecules 2021, 26, 3511.