Guanine is generally accepted as the most easily oxidized DNA base when cells are subjected to ionizing radiation, photoionization or photosensitization. At pH 7, the midpoint reduction potential is on the order of 0.2 – 0.3 V higher than those of the radicals of e.g. tyrosine, tryptophan cysteine and histidine, so that the radical “repair” (or at least, a thermodynamically favorable reaction) involving these amino acids is feasible. Computational quantum studies have been done on tyrosine, tryptophan, cysteine and histidine side chains as they appear in histones. Density functional theory was employed using B3LYP/6-31G+ (d, p) basis set to study spin densities on these amino acids side chains as they pair with the guanine radical cation. The amino acid side chains are positioned so as not to disrupt the Watson-Crick base pairing. Our results indicate that, these side chains of amino acid with reducing properties can repair guanine radical cation through electron transfer coupled with proton transfer.