Unambiguous assignment of vibrational spectra of cyclosporins A and H.

Vibrational infrared and circular dichroism spectra of two cyclosporins A and H are predicted by density functional theory calculations using the nuclear magnetic resonance proposed structure in chloroform. Spectral signatures in the important amide II, I, and A regions are identified for typical peptide secondary structures including type II' β-turn, antiparallel β-sheet, inverse γ-turn, N-methylated peptide bond, and side-chain H-bond. Our theoretical spectra agree very well with available experimental data in nonpolar media and unambiguously clarify their controversial vibrational assignment. The new insights into the spectral signatures of secondary structures can be very useful for peptide conformation analysis in general.