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

• Published in: Journal of the American Chemical Society 2009-04, Vol.131 (16), p.5839-5850
• Main Authors: Karunakaran, Venugopal, Kleinermanns, K, Improta, R, Kovalenko, S. A
• Format: Article
• Language: English
• Subjects: Computer Simulation; Guanosine Monophosphate - chemistry; Hydrogen-Ion Concentration; Models, Molecular; Photochemistry; Quantum Theory; Spectrophotometry; Water - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja810092k

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, τ1 = 0.4 ps, τ2 = 2.2 ps, and followed by slower internal conversion with τ3 = 167 ps. For nonprotonated GMP in the pH range 7−4, we find biexponential decay in the region 400−900 nm (τ1 = 0.22 ps, τ2 = 0.9 ps), whereas, between 270 and 400 nm, the behavior is triexponential with one growing, τ1 = 0.25 ps, and two decaying, τ2 = 1.0 ps, τ3 = 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 n0π* and πσ* dark excited states play a minor role. Independent of the pH, the photoinduced evolution involves ultrafast L b →L a conversion (τ 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.