Pb²⁺-Modulated G-quadruplex Formation And The Generation Of Active Oxo Species From A Ruthenium （Ⅱ） Complex

The kinetics and mechanism of the formation of Pb2+-induced G-quadruplexes from guanine-rich DNA (TBA, PS2.M, HTG and T2) and the oxidation of trans-[Ruâ…¡(TMC)(H2O)2]2+by H2O2, TBHP, NO3-and BrO3-have been studied.The kinetics of the conformational switch in the G-quadruplex of TBA induced by Pb2+ions have been investigated. Spectral analyses indicate the formation of the G-quadruplex of TBA induced by Pb2+ions, confirming the sandwich structure. Thermal denaturation profiles indicate that the Pb2+-induced intramolecular G-quadruplex is more stable than those driven by Na+or K+ions. Kinetic studies suggest that the Pb2+-induced folding G-quadruplex of TBA probably proceeds through the rapid formation of an intermediate Pb2+-TBA complex, which then isomerizes to the fully folded structure. Conformational changes transpire after the addition of Pb(NO3)2to the Na+-or K+-induced G-quadruplexes, which may be attributed to the replacement of Na+or K+ions by Pb2+ions and generation of a more compact structure of the Pb2+-TBA structure. The relaxation time, Ï„, of folding the G-quadruplex is reduced from1.05s in the presence of Pb2+ions alone to0.34s under the cooperation of initially added Na+ions, while Ï„ is increased to8.33s under the competition of initially added K+ions.The kinetics of the conformational switch in the G-quadruplex of PS2.M induced by Pb2+ions have been investigated. UV melting curves demonstrate that the Pb2++-induced G-quadruplex is formed unimolecularly and that the highest melting temperature (Tm) is72℃. The analysis of the UV titration results reveals that the binding stoichiometry of Pb2+ions to PS2.M is two, suggesting that the Pb2+ions coordinate between adjacent G-quartets. Binding of ions to G-rich DNA is a complex multiple-pathway process, which is strongly affected by the type of cation. Kinetic studies suggest that the Pb2+-induced folding of PS2.M to G-quadruplex probably proceeds through a three-step pathway involving two intermediates. Structural transition occurs after adding Pb(NO3)2to the Na+-or K+-induced G-quadruplexes, which may be attributed to the replacement of Na+or K+by Pb2+ions and the generation of a more compact Pb2+-PS2.M structure. Comparison of the relaxation times shows that the Na+â†'Pb2+exchange is more facile than the K+â†'Pb2+exchange process, and the mechanisms for these processes are proposed.The G-quadruplex formation and sequence effect on the assembly of G-rich oligonucleotides induced by Pb ions have been investigated. The analysis of optical titration data reveal that saturated Pb2+/DNA binding stoichiometries were1:1,2:1and3:1for TBA, HTG and T2, respectively. Thermal denaturation experiments were also performed to determine the structural stability. The overall results are consistent with the higher stability of T2that contains three G-quartets, as proposed to HTG (or TBA) that has only three (or two) G-quartets, respectively. The kinetic studies of the formation of G-quadruplexes of G-rich DNA induced by Pb2+ions are also presented. Binding of Pb2+ions to G-DNA is a complex multiple pathway process, which is affected by the sequence of G-DNA. The studies show that the connecting loops of the DNA play an important role in modulating the structures.The kinetics of the oxidation of trans-[Ruâ…¡(TMC)(OH2)2]2+by H2O2and TBHP have been studied in aqueous acidic solutions. Two distinct phases were observed and they were assigned to Ruâ…¡â†'Ruâ…£ and Ruâ…£â†'Ruâ…¥. The stoichiometries of the two phases can be represented as follows:trans-[Ruâ…¡(tmc)(OH2)2]2++H2O2â†'/trans-[Ruâ…£(tmc)(O)(OH2)]2++2H2O,trans-[Ruâ…£(tmc)(O)(OH2)]2++H2O2-trans-[Ruâ…¥(tmc)(O)2]2++2H2O. For the first phase, the second-order rate constants, k2, were found to be (9.82±0.17) M-1s-1at297.0K, pH=1.02and I0.1M. ΔH++and ΔS++were found to be (10.7±0.2) kcal mol-1and-(18±1) cal mol-1K-1, respectively. An oxygen-atom transfer mechanism has been proposed for both phases. Moreover, the oxidation of trans-[Ruâ…¡(tmc)(OH2)2]2+by tert-butyl hydrogen peroxide (TBHP) was studied. A similar mechanistic behavior as that of H2O2was observed.The kinetics of the oxidation of trans-[Ruâ…¡(TMC)(OH2)2]2+by NO3-have been investigated in aqueous acidic solutions. Repetitive scanning at different time scales indicated that the reaction occurred in two distinct phases. The kinetics of the first phase were monitored at350nm at pH=1.08and I=0.15M. The pseudo-first-order rate constants, kobs, increased with [NO3-]. At pH=1.08and I=0.15M, ΔH++and ΔS++were found to be (10.3±0.3) kcal mol-1and-(30.8±1.9) cal mol-1K-1. The kinetics of the second phase were monitored at290nm and350nm at pH=1.08and I=0.15M. At pH=1.08and I=0.15M, ΔH(?) and ΔS(?) were found to be (11.9±0.9) kcal mol-1and-(27.5±3.9) cal mol-1K-1.The kinetics of the oxidation of trans-[Ruâ…¡(TMC)(OH2)2]2+by BrO3-have also been examined in aqueous acidic solutions. Repetitive scanning at different time scales indicated that the oxidation of Ruâ…¡ by BrO3-occurred in three distinct phases. The kinetics of the first phase were monitored at276nm or340nm at pH=1.16and/=0.1M. The pseudo-first-order rate constants, kobs, increased with [BrO3-]. At pH=1.16and/=0.1M, ΔH++and ΔS++were found to be (5.14±0.1) kcal mol-1and-(32.3±0.9) cal mol-1K-1. The kinetics of the second phase were monitored at340nm at pH=1.16and/=0.1M. At pH=1.16and/=0.1M, ΔH++and ΔS++were found to be (7.4±0.1) kcal mol-1and-(33.0±0.9) cal mol-1K-1.