Primary processes underlying the photostability of isolated DNA bases: Adenine

Abstract

The UV chromophores in DNA are the nucleic bases themselves, and it is their photophysics and photochemistry that govern the intrinsic photostability of DNA. Because stability is related to the conversion of dangerous electronic to less-dangerous vibrational energy, we study ultrafast electronic relaxation processes in the DNA base adenine. We excite adenine, isolated in a molecular beam, to its * state and follow its relaxation dynamics using femtosecond time-resolved photoelectron spectroscopy. To discern which processes are important on which timescales, we compare adenine with 9-methyl adenine. Methylation blocks the site of the much-discussed * state that had been thought, until now, minor. Time-resolved photoelectron spectroscopy reveals that, although adenine and 9-methyl adenine show almost identical timescales for the processes involved, the decay pathways are quite different. Importantly, we confirm that in adenine at 267-nm excitation, the * state plays a major role. We discuss these results in the context of recent experimental and theoretical studies on adenine, proposing a model that accounts for all known results, and consider the relationship between these studies and electron-induced damage in DNA.dynamics ͉ photochemistry H ow did nature protect the genetic code from damage by harmful UV radiation? Presumably, DNA itself must have inherent protection mechanisms that quickly convert dangerous electronic excitation into less-dangerous vibrational energy that subsequently cools rapidly in solution. Unfortunately, the details of these mechanisms remain obscure (1). The main UV chromophores in DNA are the nucleotide bases themselves, and therefore it is their primary photophysics and interactions, both long-and short-range, which underlie DNA photostability. The isolated DNA bases are small enough to attempt detailed quantum chemical calculations, and considerable effort has been devoted to this area (for a recent review, see ref.

On the Participation of Photoinduced N–H Bond Fission in Aqueous Adenine at 266 and 220 nm: A Combined Ultrafast Transient Electronic and Vibrational Absorption Spectroscopy Study

“…[36][37][38][39] With specific regard to the participation of the 1 πσ* state in 9H-Ade's photochemistry, a number of studies have been performed to investigate the activity of this channel. [38][39][40][41][42][43][44] Early work in both the time and frequency domains led a number of research groups to infer that ultrafast relaxation via this state may in fact be active following excitation at 267 nm; 38, 41, 42 a much longer wavelength than initially predicted by Domcke and co-workers 14 and later theoretical work. 18,21 However, high-resolution photofragment translational spectroscopy measurements by Nix et al.…”

Electronic Structure of Adenine and Thymine Base Pairs Studied by Femtosecond Electron−Ion Coincidence Spectroscopy

“…21,[24][25][26] A conical intersection to a quasi-dissociative πσ* may also play a role, 8,11 especially at higher excitation energies or in polarizable environments. 16,[27][28][29][30] Time-resolved photoelectron spectroscopy (TRPES) experiments comparing adenine with 9-methyl-substituted adenine 11 found the same biexponential time constants, 0.1 and 1.2 ps, as ion yield studies. 10,18 But different decay-associated photoelectron spectra of the 0.1 ps component were assigned to the participation of a πσ* state based on calculations of the FranckCondon spectra.…”

Mapping the Ultrafast Dynamics of Adenine onto Its Nucleotide and Oligonucleotides by Time-Resolved Photoelectron Imaging

“…4 Much of the experimental effort has been devoted to the fate of adenine (Ade) following excitation to its 1 ππ* states. Gas-phase spectroscopy [5][6][7][8][9][10][11] synergised with theoretical calculations [12][13][14][15][16][17] in particular has provided deep insight. However, there remains disagreement about the basic radiationless decay mechanism and, in particular, how the dynamics of Ade relate to those of its more biologically relevant nucleotide and oligonucleotides in aqueous solution.…”

“…2), has been performed by Stolow and co-workers and the analysis used was as done here. 5,7,8 Their study had shown that the dissociative 1 πσ* state, which is localised on the N9-H bond, may be involved in the decay dynamics of Ade but not Ade-9Me. This was discerned from the shape of the decayassociated spectra which showed additional features due to the 1 πσ* state.…”