Internucleotide J-couplings and chemical shifts of the N-H···N hydrogen-bonds in the radiation-damaged guanine-cytosine base pairs.

Internucleotide (2h)J(NN) spin-spin couplings and chemical shifts (δ((1)H) and Δδ((15)N)) of N-H···N H-bond units in the natural and radiation-damaged G-C base pairs were predicted using the appropriate density functional theory calculations with a large basis set. Four possible series of the damaged G-C pairs (viz., dehydrogenated and deprotonated G-C pairs, GC(•-) and GC(•+) radicals) were discussed carefully in this work. Computational NMR results show that radicalization and anionization of the base pairs can yield strong effect on their (2h)J(NN) spin scalar coupling constants and the corresponding chemical shifts. Thus, variations of the NMR parameters associated with the N-H···N H-bonds may be taken as an important criterion for prejudging whether the natural G-C pair is radiation-damaged or not. Analysis shows that (2h)J(NN) couplings are strongly interrelated with the energy gaps (ΔE(LP→σ*)) and the second-order interaction energies (E(2)) between the donor N lone-pair (LP(N)) and the acceptor σ*(N-H) localized NBO orbitals, and also are sensitive to the electron density distributions over the σ*(N-H) orbital, indicating that (2h)J(NN) couplings across the N-H···N H-bonds are charge-transfer-controlled. This is well supported by variation of the electrostatic potential surfaces and corresponding charge transfer amount between G and C moieties. It should be noted that although the NMR spectra for the damaged G-C pair radicals are unavailable now and the states of the radicals are usually detected by the electron spin resonance, this study provides a correlation of the properties of the damaged DNA species with some of the electronic parameters associated with the NMR spectra for the understanding of the different state character of the damaged DNA bases.