Vibrational CD studies of the solution conformation of N-urethanyl-L-amino acid derivatives

The solution conformations of several N-urethanyl-L -amino acids in 0.2–0.002M chloroform and 0.2M dimethylsulfoxide have been investigated by using vibrational circular dichroism (VCD), ir absorption, and ¹³C-nmr spectroscopies. Both the N-carbobenzoxy (N-CBZ) and N-t-butoxycarbonyl derivatives of L -alanine, L -proline, L -valine, and L -phenylalanine, and N-CBZ-serine were studied. The ¹³C-nmr results indicate that the molecules occur predominantly as the cis isomer about the C? N peptide bond at room temperature. The interpretation of the strong VCD couplet in the carbonyl-stretching region using the degenerate coupled oscillator model is consistent with the conformational range of ? = ?60° to ?90° and ψ = 30° to 90° for molecules forming acid dimers. Conformations with an intramolecularly hydrogen-bonded C₇ ring are also present, which give rise to biased VCD features in the NH-, CH-, and C?O-stretching regions that can arise due to vibrationally generated ring currents.