Direct Observation of Guanine Radical Cation Deprotonation in G-Quadruplex DNA

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(+center dot)) 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 AG(3)(T(2)AG(3))(3), (G(4)T(4)G(4))(2), (TG(4)T)(4), and G(2)T(2)G(2)TGTG(2)T(2)G(2) (TBA) are measured systematically to examine the relationship of deprotonation with the hydrogen-bonding surroundings. Combined with in depth kinetic isotope experiments and pK(a) 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 N-2-H exposed to solvent for G bases in G-quartets or the free N-1-H for G base in the loop. The slower N-2-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.