Vibrational circular dichroism (VCD) spectroscopic features of type II β-turns were characterized previously, but, criteria for differentiation between β-turn types had not been established yet. Model tetrapeptides, cyclized through a disulfide bridge, were designed on the basis of previous experimental results and the observed incidence of amino acid residues in the i + 1 and i + 2 positions in β-turns, to determine the features of VCD spectra of type I and II β-turns. The results were correlated with electronic circular dichroism (ECD) spectra and VCD spectra calculated from conformational data obtained by molecular dynamics (MD) simulations. All cyclic tetrapeptides yielded VCD signals with a higher frequency negative and a lower frequency positive couplet with negative lobes overlapping. MD simulations confirmed the conformational homogeneity of these peptides in solution. Comparison with ECD spectroscopy, MD, and quantum chemical calculation results suggested that the low frequency component of VCD spectra originating from the tertiary amide vibrations could be used to distinguish between types of β-turn structures. On the basis of this observation, VCD spectroscopic features of type II and VIII β-turns and ECD spectroscopic properties of a type VIII β-turn were suggested. The need for independent experimental as well as theoretical investigations to obtain decisive conformational information was recognized.
All Science Journal Classification (ASJC) codes
- Organic Chemistry