CONFORMATIONAL STUDY OF LINEAR ALTERNATING AND MIXED D-PROLINE AND L-PROLINE OLIGOMERS USING ELECTRONIC AND VIBRATIONAL CD AND FOURIER-TRANSFORM-IR

Vibrational CD (VCD) spectra of a series of blocked linear, alternatin g D- and L-proline containing oligopeptides, dissolved in D2O and in C DCl3, are reported For the BoC-LDL-Pro(3) to BOC-DLDLDLDL-Pro(8) oligo mers, the VCD spectra in the amide I band is a positive couplet, oppos ite in sense to that obtained for (L-Pro)(n) oligomers. While this adm its the possibility of their favoring a right-handed helical chain con formation, the amide I ir spectra for these DL oligomers in D2O indica te 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 AA/A) with an increa se in chain length. In CDCl3 a similar pattern of positive VCD coupler s decreasing in intensity with length was seen, but the iv spectra are narrower. Their electronic CD (ECD), in the uv, also indicates a loss in intensity with increasing length. Oligomers with odd or even numbe rs 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 couplin g of the amides in the helical structure. All the spectra are consiste nt 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. (C) 1995 John Wiley & Sons, Inc.