Ultrafast structural dynamics of photoexcited adenine.

We report deep UV initiated excited state dynamics of the canonical nucleobase adenine (Ade) through Resonance Raman (RR) intensity analysis. RR spectra of Ade at excitation wavelengths throughout the B absorption band in the 210-230 nm wavelength range are measured and subsequently converted to scattering cross-sections. The time-dependent wave packet (TDWP) formalism has been employed for self-consistent simulations of the resulting wavelength dependent Raman excitation profiles (REP) and absorption spectrum of Ade. These simulations yield instantaneous nuclear dynamics of Ade within tens of femtoseconds (fs) of photoabsorption as structural distortions, linewidth broadening and solvation parameters. The instantaneous geometrical distortions of the purine ring following photoexcitation into the B state are analyzed vis-à-vis the low energy L state (∼260 nm) of Ade. We find that while photoabsorption by the L state causes major distortions of the imidazole ring, pyrimidine ring suffers maximal changes following B excitation. Seven in-plane stretching vibrations out of fifteen resonantly enhanced modes of Ade are found to contribute 76% of the total internal reorganization energy (981 cm) in the B excited state. In addition, the inertial response of the solvation shell to photoexcitation is found to be of 1190 cm in magnitude, and with a relaxation time of 26.5 fs. A parallel comparison is drawn between the UV-C initiated photodynamics of Ade (6-aminopurine) with that of two substituted purines, viz., 6-chloroguanine (6-ClG or 2-amino-6-chloropurine) and guanine (2-amino-6-oxo-purie) which were reported earlier.