C radicals are typically trigonal planar and thus achiral, regardless of whether they originate from a chiral or an achiral C-atom (e.g., C-H + (OH)-O-center dot -> C center dot + H2O). Oxidative stress could initiate radical formation in proteins when, for example, the H-atom is abstracted from the C alpha-carbon of an amino acid residue. Electronic structure calculations show that such a radical remains achiral when formed from the achiral Gly, or the chiral but small Ala residues. However, when longer side-chain containing proteogenic amino acid residues are studied (e.g., Mn), they provide radicals of axis chirality, which in turn leads to atropisomerism observed for the first time for peptides. The two enantiomeric extended backbone structures, center dot beta(L), and center dot beta(D), interconvert via a pair of enantiotopic reaction paths, monitored on a 4D Ramachandran surface, with two distinct transition states of very different Gibbs-free energies: 37.4 and 67.7 kJ/mol, respectively. This discovery requires the reassessment of our understanding on radical formation and their conformational and stereochemical behavior. Furthermore, the atropisomerism of proteogenic amino acid residues should affect our understanding on radicals in biological systems and, thus, reframes the role of the D-residues as markers of molecular aging.