Semiempirical quantum-mechanical calculation of the electronic structure of DNA. Double-minimum potential curves of the singlet and triplet excited electronic states, tunneling and tautomeric equilibrium of the NH...N and O...HN bonds in the adenine-thymine base pair

Abstract

The excited states of the double hydrogen bond in the adeninethymine nucleotide base pair has been investigated in the semiempirical CNDO/S-CI approximation. Double-minimum potential curves are obtained for several nuclear configurations characterizing simultaneous tautomeric rearrangements of the NH-N and 0-HN bonds. The energy profdes for the coupled movement of the hydrogen bonding show that the Watson-Crick configuration of the adenine-thymine base pair is the most stable for all of the excited states studied. Estimates are made within the WKB approximation of the tunneling rate and tunneling probability. The results indicate that increasing the energy of the excited states would increase the probability of double protonic transfer by tunnel effect and thus for irreversible mutation. A comparison of the composition of the potentials for the single movement of the protons with the double-minimum potential of the concerted movement shows that the potential is nonseparable. The shortcomings that follow from the WKB approximation as applied to the present problem are discussed.

Molecular Dynamics Simulation of Double Proton Transfer: Adenine–Thymine Base Pair

“…Various techniques, including molecular orbitals methods, have been used (Maran oH n et al, 1978;Grinberg et al, 1981;Maran oH n & Grinberg, 1982;Grinberg & Maran oH n 1983;Capparelli et al, 1985;Kwiatowski & Rein, 1986). In the majority of these studies, the interacting species were connected by a single hydrogen bond.…”

“…was de"ned by means of the orthogonal transformation(Maran oH n et al, 1978;Grinberg et al, 1981; Maran oH n & Grinberg, 1982; Grinberg & Maran oH n, 1983; Capparelli et al, (C2N1N16C17)o during double proton transfer from NPT form in A}T pair.FIG. 3.…”

Charge‐transfer character of the intermolecular hydrogen bond in singlet and triplet excited electronic states of the adenine–thymine base pair

“…It was found [12] that the probability of DPT by tunnel effect increases with the excitation energy and from the results presented in Table I one may conclude that the DPT is more likely in the CT triplet states where the potential barrier parameters Vl and V, are lower than those of the excited singlet and ground states. Since the tunneling time of the singlet excited state is about 100 times greater than that of the ground state [12], the conclusion that emerges is that the equilibrium distribution among the normal and tautomeric forms is lo5 times greater in the excited state than in the ground state.…”

“…This procedure has been tested yielding potential energy curves for the ground and singlet and triplet excited states of similar quality [12], in contrast with the CND0/2 scheme which fails to properly describe excited state properties. In order to determine the character of the charge-transfer electronic transitions, it is necessary to know the transition energy of the CT configuration as given by the expression [13] E~~= I; -E," +E?, where I; and E," are the vertical ionization potential of an electron donor and electron affinity of an electron acceptor, respectively; and E: T is the difference in electrostatic interaction energy between the CT and ground state configurations.…”

“…In a later work [12] a detailed study of the singlet and triplet electronic states and their correlation in DPT was performed. Since these results are crucial in the photoinduced mutation process through DPT in the base pairs of DNA, we have undertaken an analysis of the singlet and triplet electronic states of the * MCIC CONICET, Rephblica Argentina.…”