Article pubs.acs.org/JPCB
Ultraviolet Light Makes dGMP Floppy: Femtosecond Stimulated Raman Spectroscopy of 2′-Deoxyguanosine 5′-Monophosphate Joohyun Lee, J. Reddy Challa, and David W. McCamant* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *
ABSTRACT: The ultrafast dynamics of 2′-deoxyguanosine 5′-monophosphate after excitation with ultraviolet light has been studied with femtosecond transient absorption (TA) and femtosecond stimulated Raman spectroscopy (FSRS). TA kinetics and transient anisotropy spectra reveal a rapid relaxation from the Franck−Condon region, producing an extremely red-shifted stimulated emission band at ∼440 nm that is formed after 200 fs and subsequent relaxation for 0.8− 1.5 ps, consistent with prior studies. Viscosity dependence shows that the initial relaxation, before 0.5 ps, is the same in water or viscous glycerol/water mixtures, but after 0.5 ps the dynamics significantly slow down in a viscous solution. This indicates that large amplitude structural changes occur after 0.5 ps following photoexcitation. FSRS obtained with both 480 and 600 nm Raman pump pulses observe very broad Raman peaks at 509 and 1530 cm−1, as well as a narrower peak at 1179 cm−1. All of the Raman peaks decay with 0.7−1.3 ps time constants. The 1530 cm−1 peak also shows an increasing inhomogeneous linewidth over the first 0.3 ps. Our TA and FSRS data are consistent with a structurally inhomogeneous population in the S1 (La) state and, in particular, with previous theoretical models in which out-of-plane distortion at C2 and the amine move the molecule toward a conical intersection with the ground state. These FSRS data are the first to directly observe the structural inhomogeneity imparted upon the excited-state population by the broad, flat potential energy surface of the S1 (La) state.
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INTRODUCTION The absorption of ultraviolet (UV) light by nucleic acids can cause photodamage to the genome, which in turn leads to skin cancer in humans.1 Because of this critical link, there has been a great deal of work investigating the fundamental photochemistry of DNA, culminating in the initial measurement of the sub-picosecond lifetimes of all four nucleic acid bases in 2000−2002.2−5 In those initial measurements and most experiments since then, femtosecond transient absorption (TA) and femtosecond fluorescence up-conversion (FU) have been the mainstay spectroscopic techniques, providing sensitive measurements with sufficient time resolution to observe the ultrafast relaxation from the initially excited 1ππ* state back to the ground electronic state, S0. All four DNA bases have the lowest optically active 1ππ* electronic state that dominates the absorption spectrum in the UV.6,7 It has been found that all four DNA bases relax from the 1ππ* state in less than a picosecond and that the dominant pathway is rapid direct relaxation to S0.7,8 However, the pyrimidines, thymine and cytosine, have been found to have a minor relaxation pathway to an intermediate 1nπ* state that has a much longer lifetime of 10−150 ps.9,10 Purines guanine and adenine do not access a low-lying 1nπ* state, but some measurements have found involvement with a relatively high-energy 1πσ* state in 2′-deoxyguanosine (dG) and 2′-deoxyguanosine 5′-mono© 2017 American Chemical Society
phosphate (dGMP) that is accessed primarily from higherenergy excitations and nonaqueous solvents.11 Guanine (Chart 1) and its substituted derivatives contain two nearby optically active 1ππ* excited states: a low-lying La state absorbing at 274 nm and a higher-energy Lb state absorbing at 253 nm.6,12,13 TA and FU experiments have established that there are nearly indistinguishable excited-state relaxation kinetics whether the Lb or the La state is excited, implying that Lb to La relaxation occurs in
