Excited-State Properties and Transitions of Fluorescent 8-Vinyl Adenosine in DNA

Abstract

8-vinyl-adenosine (8VA) is a new fluorescent nucleoside analogue with improved spectroscopic properties as compared to 2-aminopurine (2AP). To further understand its photophysics, we analyzed by the time-dependent density functional theory and the configuration interaction single method, the electronic properties, and transitions of 8VA in its free form and stacked with one or two flanking bases. For free 8VA, the predicted excited-state energy gaps, absorbance peak position, and oscillator strength were found to be in excellent agreement with the experimentally determined ones. Moreover, its high fluorescence quantum yield was found to be associated with the dipole-allowed S1 --> S0 transition. Stacking of 8VA with C, T, or A in dimers or trimers resulted in fluorescence quenching through mechanisms that depend on the nature and relative orientation of the flanking base(s). When 8VA is stacked with T, quenching mainly results from nonradiative relaxation to low-lying dark excited state(s) that do not exist in free 8VA. When 8VA is stacked with A, quenching results mainly from mixing of the molecular orbitals in the ground state. Both types of quenching are thought to accompany the stacking of 8VA with C. In addition, the C-8VA-C trimer was found to exhibit a low-lying S1 emissive state that may generate an increased fluorescence quantum yield and lifetime. The predicted photophysical properties of the trimers are highly consistent with the spectroscopic data of a series of 15-mer oligonucleotides differing only by the nature of the residues flanking the central 8VA.