Circular dichroism of oriented α-helices. II. Electric field oriented polypeptides

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Abstract
A transition moment polarized parallel to the helical axis, as predicted to occur in an exciton split component (near 208 nm) of the peptide ππ* transition, can be exploited to study the structural orientation of the helical sections in proteins. In order to test this important feature of the exciton theory for helical polymers, many investigators have measured the polarization of electric field oriented polypeptides. The qualitative support for the theory provided by early experiments was recently overshadowed by a more extensive measurement [Yamaoka et al., J. Am. Chem. Soc. 108, 4619 (1986)] which showed the polarization of the 208 nm band not to be parallel to the orientation axis of the polypeptides. This finding is confirmed in this paper by our circular dichroism measurement of electric field oriented poly-γ-benzyl-glutamate in ethylene dichloride and in dioxane. Thus the exciton theory appears to be contradicted, as similarly concluded by Yamaoka et al., if the polypeptides are rod-like and rigid. However, our theoretical study of the bending effect on long polypeptides shows that our experiment is in fact consistent with the prediction of the exciton theory, which was unambiguously proven recently in paper I by using short membrane peptides [Olah and Huang, J. Chem. Phys. 89, 2531 (1988)].