VIBRATIONAL CIRCULAR DICHROISM OF SELECTED MODEL PEPTIDES

Yoder, Gorm Clifford

Abstract

A new VCD spectrometer was constructed which was optimized for VCD measurements of biological molecules over a narrow spectral region (1700 cm^-1 down to 1475 cm^-1) pertaining to the amide I and amide II vibrational modes (C=O stretching, and the CN stretching - NH bending, respectively). Its performance as compared to the other dispersive VCD instrument in the Keiderling lab will be discussed.

The first full spectroscopic study realized with this new instrument focused on the conformational properties of the C(alpha)-methylated amino acid series Z- (alphaMe-(L-Val))_n-OtBu (n = 3-8) and selected Ac- (L-(alphaMe)Val)_n-OtBu oligomers (n = 4, 6, and 8) which model a 3₁₀-helical structure. The impact of the main oligomer chain length and oligomer concentration on solution structure is discussed. Evidence for a time dependent 310-helix equilibrium is presented.

In another study the VCD, ECD, and FTIR characterization for a series of short alanine-based peptides having the general formula Ac-(AAKAA)_n-GY-NH₂ (n = 1-4) is discussed. Alanine-based peptides are known to stabilize a helical conformation. The nature of the helix within the context of thermal studies, solvent variation, and main peptide chain length is examined and further analyzed using factor analysis.

A complete spectroscopic characterization of terminally blocked (L-Pro-Aib)_n and Aib-(L-Pro-Aib)_n sequential oligopeptides known to form beta-bend ribbon spirals (a subtype of the 3₁₀-helix) was performed using VCD, ECD, and FTIR. The results for this complete spectroscopic characterization of this novel helical structure are described. The electronic CD spectra were obtained in solvents of different polarity in the 260-190 nm region. The vibrational CD and FTIR spectra were measured in deuterochloroform solution in the amide I and amide II (1750-1500 cm^-1) regions. The critical main-chain length for beta-bend ribbon spiral formation is found to be n = 2-3 or five residues.