Calculation of Standard Reduction Potentials of Amino Acid Radicals and the Effects of Water and Incorporation into Peptides.

Guanine (Guo) is generally accepted as the most easily oxidized DNA base when cells are subjected to ionizing radiation; calculations of the standard reduction potential of the guanyl radical, Eo (Guo•+ /Guo) are within ∼0.1 V of experimental values in aqueous solution extrapolated to standard conditions. While a number of experimental studies have shown some amino acid radicals have redox properties at pH 7 which suggest or confirm a capacity for radical "repair" by electron transfer from the amino acid to Guo•+ (or its deprotonated conjugate), the redox properties of the radicals of other amino acids, including methionine, lysine and cystine, are less well characterized. In addition, the effects of incorporation of the amino acids into peptides, or the effects of water of hydration on calculated potentials, have not been extensively studied. In this work, calculations of standard reduction potentials of radicals from model amino acids as they appear in histone proteins are performed. To predict redox properties at pH 7, acid dissociation constants (pKa s) of both radical and ground state amino acids are required. In some instances these are not experimentally determined and calculated pKa s have been derived for some common amino acids and compared with experimental values.