Abstract
Following Moffitt, Fitts, Kirkwood, and Tinoco, the dipole strength and rotatory strength associated with the transition to each exciton level of an excited state have been derived for an extended polypeptide chain of finite length, correct to the zeroth order approximation of wave functions. In terms of the nearest and the first two nearest neighbor approximations, sufficiently accurate for this polymer conformation, it is shown that light absorption and circular dichroism are induced essentially at two exciton levels alone, which are polarized parallel and perpendicularly, respectively. These two levels have rotatory strengths of equal magnitude but of opposite signs, for an electrically allowed transition. This is confirmed by application of the results to the N–V1 transition of an extended polypeptide. The results are readily extended to an infinitely long extended polypeptide. From the comparison with the Loxsom’s equation applied to the two-fold helix, it is pointed out that both the cyclic boundary condition and the Rosenfeld equation are valid in calculating rotatory properties of a long extended polypeptide. Generally, circular dichroism associated with an excited state can also be approximated by an equation expanded into a Taylor series, and the condition for its applicability is also indicated.
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Ikeda, S. Conservative Circular Dichroism of an Extended Polypeptide Chain. Polym J 4, 290–300 (1973). https://doi.org/10.1295/polymj.4.290
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DOI: https://doi.org/10.1295/polymj.4.290