Ultrafast Electronic Relaxation in Guanosine is Promoted by Hydrogen Bonding with Cytidine

Abstract

The excited electronic state lifetime of the guanosine−cytidine (G···C) Watson−Crick (WC) base pair has been directly measured in comparison to free G and C. Measurements have been carried out in solution in chloroform, where the formation of H-bonded base pairs is strongly favored, using the technique of femtosecond fluorescence up-conversion spectroscopy. The results show that the formation of the H-bonded WC pair leads to steep acceleration of the ultrafast nonradiative electronic deactivation compared to the free nucleosides, especially G, which can be explained by an intermolecular G-to-C electron-induced proton-transfer mechanism in the excited state. The results are of vital interest for bridging the huge gap between the well-known electronic properties of the isolated nucleobases and the strikingly different dynamics of DNA molecules.

Ultrafast excited-state dynamics of RNA and DNA C tracts

“…The possibility that base pairing facilitates electronic energy relaxation on an ultrafast time scale by proton or hydrogen atom transfer has been actively discussed recently [23,24,44]. By virtue of the slow energy relaxation seen in the hemiprotonated forms, the CH + ·C base pair joins the growing list of base pairs that support excited states, which are longer-lived than those of monomeric bases-at least for base pairs present in base stacks.…”

“…Markovitsi et al [21] and Buchvarov et al [9] have proposed an alternative assignment of the long-lived states to delocalized Frenkel excitons. Other researchers have emphasized the possibility of hydrogen atom or proton transfer within base pairs as a potential relaxation pathway for excess electronic energy [22][23][24]. Further experimental work on base multimers is urgently needed to better understand the effects of electronic coupling between proximal bases.…”

Two-color spectroscopy of UV excited ssDNA complex with a single-wall nanotube photoluminescence probe: Fast relaxation by nucleobase autoionization mechanism

“…The most abundant SWNT species in the CoMoCat samples are known to be (6,5) nanotubes (see Fig.7). The MD simulation was done on a fragment of (6,5) SWNT with two intertwinned ssDNA oligomers (GT) n where n = 30, as shown in Fig.6 (upper inset).…”

“…1a). Note that (6,5) is the most abundant SWNT species in our CoMoCat samples. In view of the computational resource limitations, the CNDO calculations were performed for only a part of the MD system that included a single unit cell of the SWNT (41 Å in length) and the DNA bases overlaying the unit cell with their coordinates extracted from the MD frames (see SI for detail).…”

“…While the recombination mechanisms have been studied quite extensively[1, [3][4][5], DNA autoionization [6] has received less attention, partially due to the fact that AI in solution involves charge transfer from DNA to the surrounding water solvent [7][8][9]. The irregular, dynamically changing structure of the solvent makes quantitative modeling of experiments and theoretical predictions of AI rates and mechanisms extremely complicated [10,11].…”