VIBRATIONAL CIRCULAR DICHROISM OF STEROIDS, PEPTIDES AND ALLENES

Narayanan, Usha

Abstract

Vibrational Circular Dichroism (VCD) was measured for several Steroids, Peptides and Allenes and attempts were made to derive correlations between structure and spectra.

Steroidal compounds studied contain two carbonyl groups on six-membered rings and measurements were made in the C=O stretching region. With one exception, the sign and shape of the spectra predicted by degenerate coupled oscillator calculations agree with the observed results in all cases. The magnitudes agree within a factor of 2, with two exceptions.

VCD of films of monodispersed, terminally blocked, linear homooligopeptides of L-Ala, L-Nva, L-Val and L-Leu up to n = 7 have been measured in the amide I and A regions. All except Leu show an increasing then decreasing (DELTA)A/A value as a function of n. This is indicative of two structural transitions, possibly the onset of (beta)-sheet structure, and the formation of a fully developed (beta)-sheet. The data imply the tendency to form a stable (beta)-sheet structure as: Ala > Val ~ Nva > Leu. VCD were also measured for L-Val and D-Ala heptamers, and L-Ala hexamer in TFE solution. The dependence of the VCD intensity on temperature and solvent (TFE/HFIP) indicates a gradual transition between two structures in solution.

Gas phase IR and liquid phase Raman spectra were measured for 1,3-dideuterioallene and fundamental bands assigned. Based on an improved force field obtained in this work for allene, VCD intensities were calculated for 1,3-dideuterioallene using Fixed Partial Charge (FPC) and Localized Molecular Orbital (LMO) theories and the results were compared.

In the case of substituted allenes, VCD observed for bands like C_X-H stretch, C=C=C asymmetric stretch and CH₃ deformations correlate well to the absolute configuration of the molecule. A valence force field was set up for 1-halo-3-methylallene and VCD intensities calculated by FPC and LMO theories. Calculated VCD were comparable to experimental VCD for isolated bands.