Circular Dichroism Spectrum of Peptides in the Poly(Pro)II Conformation

The poly(Pro)II (P-II) conformation is increasingly recognized as an important element in peptide and protein conformation. Circular dichroism (CD) is one of the most useful methods for detecting and characterizing P-II. Although the standard exciton-based model for predicting peptide CD spectra works well for alpha-helices and beta-sheets, it fails to reproduce the P-II CD spectrum because it does not account for mixing of the n pi(star) and pi pi(star) transitions with transitions in the deep UV, which is significant for the P-II conformation. In this work, the exciton model is extended to include this mixing, using ab initio-derived bond polarizability tensors to calculate the contributions of the high-energy transitions. The strong negative 195-nm and weaker positive 220-nm CD bands of P-II are reproduced for (Ala)(n) conformers in the P-II region of the Ramachandran. map. For the canonical P-II conformation from fiber diffraction of poly(Pro)II (-77, +146), the results are poor, but conformations with less negative phi (similar to -60 degrees) and more positive psi (>= 160 degrees) give spectra showing the P-II characteristics. The CD of (Pro), is not reproduced by the calculations, probably because variations in ring puckering, and cis-trans isomerism are not included in the model The extended model also gives improved results for a-helical polypeptides, leading to increased amplitude for the 205-nm band and decreased amplitude for a negative band predicted near 180 nm.