Ultrafast photo-initiated molecular quantum dynamics in the DNA dinucleotide d(ApG) revealed by broadband transient absorption spectroscopy

Abstract

The ultrafast photo-initiated quantum dynamics of the adenine-guanine dinucleotide d(ApG) in aqueous solution (pH 7) has been studied by femtosecond time-resolved spectroscopy after excitation at lambda = 260 nm. The results reveal a hierarchy of processes on time scales from tau < 100 fs to tau > 100 ps. Characteristic spectro-temporal signatures are observed indicating the transformation of the molecules in the electronic relaxation from the photo-excited state to a long-lived exciplex. In particular, broadband UV/VIS excited-state absorption (ESA) measurements detected a distinctive absorption by the excited dinucleotide around lambda = 335 nm, approximately 0.5 eV to the blue compared to the maximum of the broad and unstructured ESA spectrum after excitation of an equimolar mixture of the mononucleotides dAMP and dGMP. A similar feature has been identified as signature of the excimer in the dynamics of the adenine dinucleotide d(ApA). The lifetime of the d(ApG) exciplex was found to be tau = 124 +/- 4 ps both from the ESA decay time and from the ground-state recovery time, far longer than the sub-picosecond lifetimes of excited dAMP or dGMP. Fluorescence-time profiles measured by the up-conversion technique indicate that the exciplex state is reached around approximately 6 ps after excitation. Very weak residual fluorescence at longer times red-shifted to the emission from the photo-excited state shows that the exciplex is almost optically dark, but still has enough oscillator strength to give rise to the dual fluorescence of the dinucleotide in the static fluorescence spectrum.

Time-resolved photoelectron imaging of the isolated deprotonated nucleotides

“…16 This strongly suggests that a similar decay pathway is occurring in both environments, despite the strong effect of the solvent on the energy of the Based on the above observations, we assign the initial motion observed in the time resolved photoelectron spectrum to relaxation of the excited system away from the Franck-Condon geometry, and the slower decay to internal conversion using a 2 E CI. If this assignment is correct, then by extension we propose that the previous biexponential kinetics observed in isolated Gua also correspond to dynamics solely upon a single excited state, 9 and excited state dynamics of the purine nucleobases may follow a similar mechanism.…”

“…1 Recent measurements give the short lifetime, τ 1 = 120 fs and the longer lifetime, τ 2 = 680 fs. 16 Computational studies show that solvation dramatically blueshifts the first 1 nπ* state, to the extent where it is considered to be unlikely to have any substantial influence on the dynamics. 65 68 The dynamics then proceed purely through CIs between the 1 L a and S 0 states, although there are indications that the presence of the solvent influences which path -and ultimately which CI -is accessed to relax back to the ground electronic state.…”

“…65 In the solution phase, Gua itself is rarely studied as it has poor solubility, although dGMP -has been studied in a number of experiments with transient absorption 1,16,66 and fluorescence upconversion. 14, 16, 19, 67 14, 16, 19, 64 As for gas phase Gua excited at around 4.7 eV, solvated dGMP -is observed to relax through biexponential kinetics.…”

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

“…A similar mechanism has been proposed for aqueous dAMP -, for which there seems to be a general consensus. 2,3,32 For aqueous Ade, theoretical studies suggests a slightly different mechanism because the strong vibronic coupling in the Franck-Condon regime leads to excitation of both 1 ππ* states and the low-lying 1 nπ* state. 33,34 The first lifetime has been associated with decay from S n to S 1 and the second with decay from the S 1 state.…”

Efficient UV-induced charge separation and recombination in an 8-oxoguanine-containing dinucleotide

“…The main finding of this study is that absorption of a UVB or UVC photon by the d(OA) dinucleotide transfers an electron from O to A, producing a radical ion pair; this supports the role of O as a flavin mimic, but this study also adds to evidence that photoinduced ET occurs generally and with relatively high quantum yield between any pair of stacked nucleobases (11,(24)(25)(26). The latter conclusion rests on strong thermodynamic and kinetic similarities between CT states in other dinucleotides and in d(OA).…”