Interproton coupling across the trans-peptide bond (⁵J_αα′) 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 homoallylic coupling. The rather rigid conformation of chelated dipeptides provided the reference structure for calibrating the coupling across the trans-peptide bond. The coupling constants over cis and trans pathways are both positive. Calibration of the homoallylic type expression afforded results close to those obtained for linear peptides in water but which disagree on the effect of replacement of water with DMSO. Sensitivity of the coupling on changes in electronic π-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 protonation in strongly acidic media (pD < 0.5), reaching 70% of the homoallylic coupling in butenes. By considering the coupling in chelated dipeptides, in rigid cyclic dipeptides, and in rigid butenes, it has been established that homoallylic coupling conformational dependence of peptides and butenes is essentially the same.