A density functional study of the effect of orientation of hydrogen bond donation on the hyperfine couplings of benzosemiquinones: relevance to semiquinone–protein hydrogen bonding interactions in vivo

The B3LYP hybrid density functional method has been used to investigate the effect of hydrogen bond orientation on spin densities and hyperfine couplings for the p-benzosemiquinone anion radical. Out-of-plane hydrogen bonding leads to a redistribution of spin density from the semiquinone carbonyl oxygen atom on to the oxygen atom of the hydrogen bonding water molecule. This leads principally to a significant anisotropic 17O hyperfine coupling for the water oxygen and a negative isotropic hyperfine coupling for the 1H hydrogen bond donor atom. The implications of these findings for the interpretation of EPR and ENDOR spectra of in vivo semiquinones are discussed.