Photoinduced Dynamics of Guanosine Monophosphate in Water from Broad-Band Transient Absorption Spectroscopy and Quantum-Chemical Calculations

Abstract

Guanosine monophosphate (GMP) in aqueous solutions has been studied with femtosecond broad-band transient absorption spectroscopy and by quantum-mechanical calculations. The sample was excited at 267 or 287 nm and probed between 270 and 1000 nm with 100 fs resolution, for various pH values between 2 and 7. At pH 2, when the guanine ring is ground-state protonated (GMPH(+)), we observe isosbestic behavior indicating state-to-state relaxation. The relaxation is biexponential, tau(1) = 0.4 ps, tau(2) = 2.2 ps, and followed by slower internal conversion with tau(3) = 167 ps. For nonprotonated GMP in the pH range 7-4, we find biexponential decay in the region 400-900 nm (tau(1) = 0.22 ps, tau(2) = 0.9 ps), whereas, between 270 and 400 nm, the behavior is triexponential with one growing, tau(1) = 0.25 ps, and two decaying, tau(2) = 1.0 ps, tau(3) = 2.5 ps, components. The excited-state evolution is interpreted with the help of quantum-chemical calculations, performed at the time-dependent PBE0 level accounting for bulk solvent effects and specific solvation. The computed dynamics involves L(a) and L(b) bright excited states, whereas the n(0)pi* and pisigma* dark excited states play a minor role. Independent of the pH, the photoinduced evolution involves ultrafast L(b)-->L(a) conversion (tau(ba) << 100 fs) and exhibits the presence of a wide planar plateau on L(a). For neutral GMP a barrierless path connects this region to a conical intersection (CI) with the ground state, giving an account of the ultrafast decay of this species. For protonated GMPH(+) the system evolves into a stable minimum L(a min) characterized by out-of-plane displacement of NH and CH groups, which explains the longer (167 ps) fluorescence lifetime.

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…There is still debate regarding the specific assignment of the multiexponential decay components. Some authors have assigned the relaxation pathways to nonadiabatic transitions among multiple electronic states [1,4,109], whereas others have assigned them to diabatic relaxation pathways occurring on a single potential energy surface [82,89,[110][111][112]. Another general observation is that conical intersections, i.e., points of degeneracy between two or more potential energy surfaces in the nuclear coordinate space of the molecule [113][114][115][116], play a key role in the ultrafast deactivation of the excited-state electronic energy in all the nucleic acid monomers [109,110,[116][117][118][119][120][121][122][123][124].…”

Time-resolved photoelectron imaging of the isolated deprotonated nucleotides

“…65 In the solution phase, Gua itself is rarely studied as it has poor solubility, although dGMP -has been studied in a number of experiments with transient absorption 1,16,66 and fluorescence upconversion. 14, 16, 19, 67 14, 16, 19, 64 As for gas phase Gua excited at around 4.7 eV, solvated dGMP -is observed to relax through biexponential kinetics.…”

DNA/RNA: Building Blocks of Life Under UV Irradiation

“…Furthermore, resonance Raman experiments have shown shown that uracil-like molecules keep a planar geometry during the first femtoseconds after the excitation of S π ,. 13 Triggered by the increasing amount of experimental findings, quantum chemistry calculations at various levels were dedicated to the study of the excited state behavior of the NABs and have provided a very rich source for understanding the mechanisms at the origin for their exceedingly short lifetimes 7,10,[14][15][16][17][18][19][20][21][22] The picture emerging from these experimental and theoretical studies is that the ultrafast decay of the first singlet excited state S * is due to highly efficient conical intersections (CI) between this state and the ground state (S 0 ). Very loosely speaking, for the pyrimidines, the internal conversion through the CI involves a pyramidalization of C 5 and/or a torsion of the C 5 C 6 bond accompanied by an out-of-plane motion of the 5-substituent (see Figure 1 for atom labelling).…”

“…We underline that such non-exponential decays are not necessarily due to the involvement of several electronically excited states. Two detailed studies of guanosine 5'-monophosphate (GMP) 7 and 2'-deoxyguanosine 5'-monophosphate (dGMP) 26 show clearly that the correspondence "one time constant -one electronic state"…”