Chiral recognition in dipeptides containing 1-aminocyclopropane carboxylic acid or α-aminoisobutyric acid: NMR studies in solution

Protected dipeptides containing 1-aminocyclopropane carboxylic acid (Ac₃c) or α-aminoisobutyric acid (Aib) residues at the C-terminus and Phe, Val or Ala residues at the N-terminus displayed different proton NMR spectra for the pure enantiomers and the racemic mixtures in deuterochloroform (CDCl₃) solution. An unequal mixture of enantiomers showed two sets of resonances (NMR nonequivalence), one corresponding to major and the other to minor enantiomer. The NMR nonequivalence was originated by the presence of the C-terminal Ac₃c or Aib residues, which have been known for their unique spatial preferences in avoiding an extended (C₅) conformation. When a C₅ conformation favoring residue such as glycine was incorporated in place of Ac₃c or Aib, negligible NMR nonequivalence was observed. The magnitude of the NMR nonequivalence depended on the side chain as well as on the protecting groups at N-terminus α-amino acid. For the same peptide, the magnitude of nonequivalence increased with increasing solution concentration and/or with decreasing the solution temperature. The NMR nonequivalence disappeared in polar solvent-like deuterated dimethylsulfoxide (DMSO-d₆). A preference for hetero-chiral recognition leading to dimeric association under fast exchange conditions had been invoked to explain the observed phenomenon. The dipeptides thus prepared could well serve as ‘model peptides’ for the evaluation of any preparative methods.