Linear response polarizability bandshape calculations of vibrational circular dichroism, vibrational absorption, and electronic circular dichroism of cyclo(Gly-Pro-Gly-D-Ala-Pro): A small cyclic pentapeptide having β- and γ-turns

Abstract

The bandshape calculations for the vibrational circular dichroism (VCD), infrared (IR) absorption, and ultraviolet CD (UVCD) of cyclo(Gly-Pro-Gly-D-Ala-Pro) have been carried out for various conformational models of an identical cyclic pentapeptide, cyclo(amide)5 by neglecting side-chain effects. In the theory of the excited states of a polymer, when the transition moments of each chromophore must be positioned at different places within the corresponding chromophore, a new formalism for multiband interactions is invoked. The previous polarizability theories developed by us cannot be applied to such cases, so that prescriptions of how to compute the polarizability tensors for representing UVCD, VCD, and IR absorption bandshapes have been presented. In the two-state exciton approximation, the electronic CD spectra obtained have been predicted well in the 200-260-nm spectral range for the various conformations computed from the polypeptide-backbone angles (φ, ψ, ω) determined by X-ray diffraction, nuclear magnetic resonance (NMR) spectroscopy, and the contact-distance calculations. However, for all thirteen conformations of cyclo (amide)5 determined by Ramakrishnan and Narasinga Rao, good UVCD spectra have been obtained only for six such conformations as C5-sym, β-A2, βγ-A2ω, γ, γγ-II, and γγγγ in terms of their notation. For the VCD calculations, the degenerate two-state exciton approximation involving both the amide I and II bands has led to quite different VCD spectra from the results of the degenerate single-state exciton approximation for the separate amide I and II bands, so that the two-state approximation for the β- and γ-turn structures may be requisite for an understanding of the region of the amide I and II bands. In this two-state approximation, when the tertiary amide-I levels were taken to be different from the secondary amide-I levels, and also the parameterization of the transition dipoles was properly done, the nondegenerate two-state approximation has improved the fit between observed and computed VCD spectra. The X-ray conformation has provided much better VCD spectra than the NMR conformation, whereas the NMR conformation has rather better predicted the observed IR absorption bands than the X-ray conformation. Also for the βγ-A2ω conformation, a good VCD spectrum has been obtained. Comparison of the UVCD and VCD spectra indicates that if the solvent effect can be neglected, the most probable conformations are the X-ray and βγ-A2ω ones. © 1996 John Wiley & Sons, Inc.