title: Direct observation of guanine radical cation deprotonation in G-quadruplex DNA.
abstract: Although numerous studies have been devoted to the charge transfer through double-stranded DNA (dsDNA), one of the major problems that hinder their potential applications in molecular electronics is the fast deprotonation of guanine cation (G(+•)) to form a neutral radical that can cause the termination of hole transfer. It is thus of critical importance to explore other DNA structures, among which G-quadruplexes are an emerging topic. By nanosecond laser flash photolysis, we report here the direct observation and findings of the unusual deprotonation behavior (loss of amino proton N2-H instead of imino proton N1-H) and slower (1-2 orders of magnitude) deprotonation rate of G(+•) within G-quadruplexes, compared to the case in the free base dG or dsDNA. Four G-quadruplexes AG3(T2AG3)3, (G4T4G4)2, (TG4T)4, and G2T2G2TGTG2T2G2 (TBA) are measured systematically to examine the relationship of deprotonation with the hydrogen-bonding surroundings. Combined with in depth kinetic isotope experiments and pKa analysis, mechanistic insights have been further achieved, showing that it should be the non-hydrogen-bonded free proton to be released during deprotonation in G-quadruplexes, which is the N2-H exposed to solvent for G bases in G-quartets or the free N1-H for G base in the loop. The slower N2-H deprotonation rate can thus ensure less interruption of the hole transfer. The unique deprotonation features observed here for G-quadruplexes open possibilities for their interesting applications as molecular electronic devices, while the elucidated mechanisms can provide illuminations for the rational design of G-quadruplex structures toward such applications and enrich the fundamental understandings of DNA radical chemistry.
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences , Beijing 100190, China.
authors.forename: Lidan
authors.initials: L
authors.lastname: Wu
authors.forename: Kunhui
authors.initials: K
authors.lastname: Liu
authors.forename: Jialong
authors.initials: J
authors.lastname: Jie
authors.forename: Di
authors.initials: D
authors.lastname: Song
authors.forename: Hongmei
authors.initials: H
authors.lastname: Su
mesh_terms: D002412:Cations / Q000737:chemistry; D004247:DNA / Q000737:chemistry*; D005609:Free Radicals / Q000737:chemistry; D054856:G-Quadruplexes; D006147:Guanine / Q000737:chemistry*; D011522:Protons
chemical_list: D002412:Cations; D005609:Free Radicals; D011522:Protons; D006147:Guanine; D004247:DNA