INTERPROTON COUPLING ACROSS THE TRANS-PEPTIDE BOND ((5)J(ALPHA-ALPHA')) IN CHELATED DIPEPTIDES

Interproton coupling over five bonds of the peptide moiety of chelated dipeptides in a series of mono(dipeptidato)cobalt(III) complexes was studied in order to establish whether the coupling is analogous to hom oallylic coupling. The rather rigid conformation of chelated dipeptide s provided the reference structure for calibrating the coupling across the trans-peptide bond. The coupling constants over cis and trans pat hways are both positive. Calibration of the homoallylic type expressio n afforded results close to those obtained for linear peptides in wate r but which disagree on the effect of replacement of water with DMSO. Sensitivity of the coupling on changes in electronic pi-bond character of the peptide bond was probed by protonation of the peptide oxygen. The magnitude of the coupling was scaled up 40% upon peptide oxygen pr otonation in strongly acidic media (pD < 0.5), reaching 70% of the hom oallylic coupling in butenes. By considering the coupling in chelated dipeptides, in rigid cyclic dipeptides, and in rigid butenes, it has b een established that homoallylic coupling conformational dependence of peptides and butenes is essentially the same.