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

• Published in: The journal of physical chemistry. B 2009-12, Vol.113 (51), p.16365-16376
• Main Authors: Schwalb, Nina K, Michalak, Thomas, Temps, Friedrich
• Format: Article
• Language: English
• Subjects: B: Biophysical Chemistry; Base Pairing; Cytidine - chemistry; Guanosine - chemistry; Hydrogen Bonding; Solutions - chemistry; Spectrometry, Fluorescence; Spectrophotometry, Ultraviolet; Spectroscopy, Fourier Transform Infrared; Time Factors
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp904883n

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 CHCl3 solution by employing t e r t-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 λpump = 294 nm, close to the electronic origins of the bases, to λpump = 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 τGC = 0.30(3) ps (with 2σ error limits) irrespective of the pump wavelength. A similar short decay time, τGG = 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 ≤ τG,1 ≤ 1.2(1) ps, 4.2(8) ps ≤ τG,2 ≤ 8(1) ps, and τG,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 ≤ τCC ≤ 10 ps, compared to the dominant initial subpicosecond decay time of the C monomer of τC,1 = 0.80(4) ps.