CONFORMATIONAL BEHAVIOR AND MAGNETIC-PROPERTIES OF ORGANIC RADICALS DERIVED FROM AMINO-ACID-RESIDUES - THE DIPEPTIDE ANALOG OF GLYCINE RADICAL

A general quantum mechanical protocol for the study of flexible open-shell systems has been applied to a simple model of the glycine radical engaged in peptide chains. The conformational freedom of the resulting dipeptide; analogue is severely restricted with respect to that of standard amino acid residues. Only planar or quasi-planar conformations are energetically accessible due to pi-electron delocalization on the whole compound. As a consequence, the structural parameters and the vibrational frequencies involving the C-alpha atom are typical of ethylenic systems. The hyperfine coupling constants of the (CH)-H-alpha moiety remain, however, similar to those of the prototype methyl radical and show the same strong dependence on out-of-plane deformations. The hyperfine coupling constants of the NH moiety are, instead, only dependent on the backbone conformation. Vibrationally averaged hyperfine coupling constants both of (CH)-H-alpha and NH moieties are in fair agreement with experimental values of glycine radicals produced in some proteins.