Influence of hydrogen bonds on the electronic g-tensor and 13C-hyperfine tensors of 13C-labeled ubiquinones -: EPR and ENDOR study

Selectively C-13-labeled ubiquinone anion radicals in protic and aprotic solvents are investigated by EPR acid ENDOR spectroscopy, yielding information about the effect of hydrogen bonds on the electronic g-tensor and the carbonyl carbon C-13-hf tensors. Formation of the hydrogen bonds alter the g-tensor significantly to lower values and increases the A(zz) component of the C-13-hf tensor. Both effects can be explained by electrostatic interactions between the positively charged hydrogen and the electrons at the carbonyl oxygen leading to a redistribution of charge and pi-spin density. Two different hydrogen bonds were obtained for UQ(0)(-.) which are in agreement with the results of DFT (density functional theory) calculations.