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

Abstract

The charge-transfer (a) force between proton donor and acceptor is known to play an important role in hydrogen bonding, in addition to the electrostatic and exchange repulsion forces. A number of theoretical studies have been carried out in the direction of the CT mechanism of hydrogen bonding [l, 21. In particular, the idea of photoinduced mutations through double proton transfer (DPT) in the base pairs of DNA has received some support from a study of hydrogen bonded complexes [3,4] and electronic transitions involving CT in these complexes have been detected [5-81. It is a well known fact that in the guanine-cytosine base pair there exist electronic transitions 7r* t 7r which can be classified as weak intermolecular CT electronic transitions producing essentially a double ionic form of the base pair [9]. On the other hand the charge distribution of hydrogen bonding in the adenine (A)-uracil (U) base pair using the all-valence-electrons bond order index (devised for nonorthogonal basis) has been studied and it was found that the HOMO and LUMO of the hydrogen-bonded system are localized on A and U, respectively, and that no important perturbations are detected when the double hydrogen bonding is formed [lo].In a previous study on the ground state [ll] the character of the localization of the 7r and 7r* MO'S in the normal and tautomeric forms of the A-thymine (T) base pair were found to be the same as that found in the isolated bases, i.e., they are localized on A (A*) or T (T*) when the DPT process is performed.The (T and (+* MO'S were found to be delocalized on both bases.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

Ab initio studies on the photophysics of the guanine–cytosine base pair

“…We have performed CIS/6-31G(d) calculations at the MP2 ground-state equilibrium geometry which predict the lowest pp*(CT) state (S 9 ) at 7.83 eV, above four pp* and three np* LE states, and one state of np*(CT) character at 7.78 eV. It is worth to mention that an early semi-empirical calculation 44 predicted the lowest pp*(CT) state of the GC base pair at 5.29 eV.…”

Sequence dependent proton‐transfer reaction in stacked GC pair I: The possibility of proton‐transfer reactions

“…He also suggested a simultaneous double proton‐transfer (DPT) in AT and GC pairs, which may cause a DNA mutation. A process of the DNA mutation may be estimated from behavior of small molecular model systems such as several DNA base pairs by means of computational chemistry 8, 9. In particular, two types of the PT reaction between GC pair have been studied extensively as illustrated in Scheme .…”