EPR Spectroscopic Studies of the Reduction of Chromium(VI) by Methanol in the Presence of Peptides. Formation of Long-Lived Chromium(V) Peptide Complexes

The synthesis and characterization of the first Cr(V) complexes with non-sulfur-containing peptides, which may mimic the chemistry of the intermediates in the formation of Cr-induced peptide−DNA cross-links in vivo, are reported. The reduction of Cr(VI) with methanol in the presence of a number of non-sulfur-containing peptides produced relatively stable Cr(V)−peptide complexes, which were characterized by EPR spectroscopy and electrospray mass spectrometry. The reaction of Cr(VI) with methanol alone (in the absence of peptide ligands) resulted in the formation of two Cr(V)−methanol intermediates, with g_iso values of 1.9765 and 1.9687. The methanol reduction of Cr(VI) in the presence of the glycine peptides, triglycine, tetraglycine, and pentaglycine resulted in the formation of both Cr(V)−methanol and Cr(V)−peptide intermediates, while only the Cr(V)−peptide complexes were detected in the reactions with the alanine peptides trialanine, tetraalanine, and pentaalanine. Similar EPR signals were observed for all of the Cr(V)−peptide complexes with g_iso values between ∼1.986 and ∼1.979, and A_N values of (2.1−2.6) × 10^-4 cm^-1.