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

The poly(Pro)II (PII) 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 PII. Although the standard exciton-based model for predicting peptide CD spectra works well for α-helices and β-sheets, it fails to reproduce the PII CD spectrum because it does not account for mixing of the nπ* and ππ* transitions with transitions in the deep UV, which is significant for the PII 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 PII are reproduced for (Ala) n conformers in the PII region of the Ramachandran map. For the canonical PII conformation from fiber diffraction of poly(Pro)II (−77, +146), the results are poor, but conformations with less negative ϕ (∼ −60°) and more positive ψ (≥160°) give spectra showing the PII characteristics. The CD of (Pro) n is not reproduced by the calculations, probably because variations in (ϕ,Ψ), ring puckering, and cis−trans isomerism are not included in the model The extended model also gives improved results for α-helical polypeptides, leading to increased amplitude for the 205-nm band and decreased amplitude for a negative band predicted near 180 nm.