14N nuclear quadrupole resonance study of the nucleotide base pair 1-methylcytosine hemihydroiodide hemihydrate

Abstract

The 14 nuclear quadrupole resonance (NQR) spectrum at 77 K of the base pair (1-methylcytosine) 2HI·H 2O was observed using a field-cycling double-resonance method. The 14N data clearly distinguish between the proton-bearing imino-type nitrogen and the counterpart nitrogen that shares in the hydrogen bonding. Although signals ascribed to all the types of nitrogen in the compound are observed, only ν + for the N-methyl nitrogen (N 1) is seen. The relatively low value of e 2Qq zz h for the nonprotonated imino nitrogen of cytosine suggests a corresponding relatively high π orbital population. Nevertheless, upon protonation, the π-electron density increases considerably; the result is that e 2Qq zz h for the protonated imino nitrogen in the base pair is quite high; the Z of the electric field gradient tensor lies normal to the plane of the molecule, along the π-orbital direction.