Ultrashort Fluorescence Lifetimes of Hydrogen-Bonded Base Pairs of Guanosine and Cytidine in Solution

Abstract

The optically excited electronic states of hydrogen-bonded homo- and heterodimers of guanosine (G) and deoxycytidine (C) were investigated by femtosecond fluorescence up-conversion spectroscopy. The base pairs were prepared in CHCl(3) solution by employing tert-butyldimethylsilyl (TBDMS) groups at the OH positions of the ribose (G) or deoxyribose (C) moieties to enhance the solubilities of the nucleosides in organic solvents. The H-bonded complexes that were obtained were characterized by FTIR spectroscopy. Fluorescence lifetime measurements were performed following electronic excitation at a series of UV wavelengths from lambda(pump) = 294 nm, close to the electronic origins of the bases, to lambda(pump) = 262 nm, where significant excess vibronic energy is deposited in the molecules, at nucleoside concentrations of c(0) = 0.1 and 1.0 mM. The experimental results revealed the existence of an ultrafast deactivation pathway for the optically prepared electronically excited state(s) of the G.C Watson-Crick base pair, which was found to have a lifetime of tau(GC) = 0.30(3) ps (with 2sigma error limits) irrespective of the pump wavelength. A similar short decay time, tau(GG) = 0.32(2) ps, was observed for the respective excited G.G homodimer. In contrast, the excited G monomer displayed a significantly longer-lived and wavelength-dependent deactivation, requiring three time constants, between 0.43(6) ps < or = tau(G,1) < or = 1.2(1) ps, 4.2(8) ps < or = tau(G,2) < or = 8(1) ps, and tau(G,3) = 195(32) ps. Self-complexation of C, on the other hand, led to a longer-lived excited state with a lifetime estimated between 1 ps < or = tau(CC) < or = 10 ps, compared to the dominant initial subpicosecond decay time of the C monomer of tau(C,1) = 0.80(4) ps.

Photoreaction channels of the guanine–cytosine base pair explored by long-range corrected TDDFT calculations

“…At the TD-LC-BLYP/TZP level, the vertical excitation energies of the lowest two excited states for the FC geometry are 5.09 and 5.27 eV, respectively, and the oscillator strengths of these states are 0.080 and 0.113; see 59 These findings about the excitation energies and oscillator strengths of the GC pair are consistent with the absorption spectrum of the heterodimer of guanosine and cytidine in solution, 24 where the bands for the lowest 1 * (G → G*) and 1 * (C → C*) states are at ~280 nm and the band for the second 1 * (G → G*) state at ~250 nm exhibits more intense peak.…”

“…[11][12][13][14][15] In spectroscopic experiments, the effect of hydrogen bonding on the excited-state lifetime of the base pairs has been of great interest. [16][17][18][19][20][21][22][23][24][25][26] In the present work, excited-state potential energy profiles of the Watson-Crick form of the guaninecytosine (GC) base pair are theoretically studied for all of the potentially competing reactions mentioned above: SPT and DPT between guanine (G) and cytosine (C) as well as nonradiative decay in each single base under hydrogen bonding. One purpose of this study is to assess the favorability of the decay process in the G monomer with hydrogen-bonded to C, which was rarely discussed in previous studies.…”

Probing the excited state relaxation dynamics of pyrimidine nucleosides in chloroform solution

“…previously determined by Schwalb et al 19 The degree of association is given by the 3100 -3450 cm -1 ); in the carbonyl stretching region ( Fig. 2(a), 1600 -1800 cm -1 ), the monomer and dimer bands in the corresponding spectra overlap too much to be analysed.…”

“…for cytidine samples in other solvents or the gas phase. [15][16][17][19][20][21][22] The TEA spectra were analysed by selecting time-dependent intensity profiles at 8…”

“…19 The τ 2 value would appear to reflect the lifetime of molecules in the 1 ππ* state. It is therefore tempting to assign τ 3 to molecules that transfer from the 1 ππ* to the 1 nπ* state, but fluorescence up-conversion measurements suggested a lifetime of 21 ± 2 ps for the 1 nπ* state population, 19 which is hard to reconcile with the longer lifetime ESA component reported here. We return to this point below after consideration of the information obtained from TVAS.…”