Ultrafast Nuclear Dynamics of Photoexcited Guanosine-5′-Monophosphate in Three Singlet States

We report meaSuretnent of resonance Raman (RR) spectra of guanosine-5'-monophosphate (GMP), a DNA nucleotide at excitation wavelengths throughout its pi pi* absolution band (B-b) in the 210-230 nm range. From these data, we constructed wavelength-dependent Raman intensity excitation profiles (REPs) for all observed-Modes. These profiles and the absorption spectrum were then modeled using self-consistent simulations based on the time-dependent wave packet propagation formalism. We inferred the initial structural dynamics of GMP immediately after photoexcitation in terms of dimensionless displacements: The simulations also provide linewidth-broadening parameters that in turn report on the time scale of dynamics. We compared deduced structural changes in the purine ring upon photoabsorption into the B-b state with those deduced for the-two lowest lying pi pi* (L-a and L-b at 280 and 248 nm, respectively) excited states of GMP. We find that excitation to:the Lb state lengthens C-6-N-1 and C-2=N-3 bonds, which lie along the formation coordinate-of various oxidative adducts but B-b excitation does riot. We also find that photoabsorption by the B-b state weakens the C-8-N-9 bond and thus might assist imidazole ring opening via cleavage of the same bond: Electronic excitation to different pi pi* states of the,guanine chromophore results in contrasting structural changes; although-absbiption by the L-a and L-b states induces expansion Of pyrimidine and contraction of imidazole rings, excitation results-in overall Shrinkage of both the rings. Computed-ahsolute changes in internal coordinates imply that photoexcitation to any of the three/singlet states-of GMP does not lead directly to, the formation of-a cation, radical of guanine.