Conformational study of linear alternating and mixed D- and L-proline oligomers using electronic and vibrational CD...

Vibrational CD (VCD) spectra of a series of blocked linear, alternating D- and L-proline containing oligopeptides, dissolved in D2O and in CDCl3, are reported. For the Boc-LDL-Pro3 to Boc-DLDLDLDL-Pro8 oligomers, the VCD spectra in the amide I band is a positive couplet, opposite in sense to that obtained for (L-Pro)n oligomers. While this admits the possibility of their favoring a right-handed helical chain conformation, the amide I ir spectra for these DL oligomers in D2O indicate a mixed, apparently alternate, cis-trans conformation that prevents a simple conclusion. Their VCD in D2O evidence no narrowing and has a progressive loss in intensity (measured as delta A/A) with an increase in chain length. In CDCl3 a similar pattern of positive VCD couplets decreasing in intensity with length was seen, but the ir spectra are narrower. Their electronic CD (ECD), in the uv, also indicates a loss in intensity with increasing length. Oligomers with odd or even numbers of Pro residues have different ECD patterns, indicating that those spectra are strongly influenced by local contributions arising in the N-terminal groups. The VCD arises from dipolar and vibrational coupling of the amides in the helical structure. All the spectra are consistent with the chiral end groups leading to formation of an excess of one helical handedness. With an increase in length, the influence of this selectiveness is less and the overall CD measured decreases.