Circular dichroism of oriented α helices. I. Proof of the exciton theory

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Abstract
Moffitt’s exciton theory for α helices, an important cornerstone of the circular dichroism (CD) theory for biopolymers, was recently cast in doubt by a linear dichroism measurement of electric field oriented polypeptides [Yamaoka et al., J. Am Chem. Soc. 108, 4619 (1986)]. In particular the prediction that the polarization of an exciton split component at 208 nm should be parallel to the α-helical axis was not borne out. This revealed the inadequacy of previous experiments which used long polypeptides to prove the theory. We performed two experiments to measure the effect of orientation of α helices on CD, one with a short membrane peptide, alamethicin, oriented in defect-free multibilayers and another with long polypeptides oriented with electric field. We found the result of the experiment with alamethicin to be consistent with the exciton theory. An elaborate procedure was established to measure the CD of protein molecules embedded in lipid multibilayers with light incident on bilayers at various angles (the helical sections of alamethicin are perpendicularly embedded in bilayers). Thus we are able to measure the polarization of the 208 nm band and prove that it is indeed parallel to the α-helical axis. The problem of electric field oriented polypeptides is discussed in paper II.