As the orbital interaction diagram above shows, extra stabilization of a C-centred radical, called captodative stabilization, ensues if the radical is substituted by at least one p-electron donor (O, N, double bond, etc), and one p-electron acceptor (-NO2, C=O, nitrile, double bond, etc). While a normal C-H bond dissociation enthalpy (BDE) is above 400 kJ/mol (CH4 440), much lower values of BDE are measured and calculated for captodative systems:H2NCH2CH=O 320; H2NCH2BH2 333, CH2=CHCH2CH=CH2 250). For maximum stabilization, the p-systems of the donor, acceptor and C. must be coplanar. We realized in the late 1990s that the a-CH bond common to all amino acid residues and proteins may similarly be weakened by the captodative effect of the flanking amide groups, one of which presents a carbonyl group and the other of which presents the NH group. The carbonyl group of an amide is a weak p-acceptor and the NH group is a weak p-donor, so it was not clear how much captodative stabilization actually ensues. The question is important in the biological context because removal of the a-CH hydrogen atom constitutes oxidative damage to the peptide/protein. Oxidative damage is usually repaired by transfer of an H atom from glutathione (GSH), the endogenous reducing agent present in millimolar concentrations. Repair may not be possible if the a-CH bond is weaker than the S-H bond of GSH, 367 kJ/mol. The graphic at the right represents the results of a careful theoretical investigation of the BDEs of all peptidic a-CH bonds (A Rauk , D Yu, J Taylor, G V Shustov, D A Block, D A Armstrong, Biochemistry, 1999, 38, 9089-9096. doi: 10.1021/bi990249x). The figure on the right displays the -C-H bond dissociation enthalpy (BDE) of each of the amino acid residues modeled as peptides, (red dots). The black horizontal line is the experimental BDE of the S-H bond in glutathione and cysteine. With one exception, all of the a-C-H bonds are predicted to be weaker than the S-H bond!