AN ANALYSIS OF THE OPTICAL ROTATORY DISPERSION OF POLYPEPTIDES AND PROTEINS, III

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Abstract
In this communication we have shown that the modified two-term Drude equation has definite advantages over methods previously available for the determination of [alpha]-helix content in solution. The origin of the extreme sensitivity of the rotation at one wavelength in the visible or near-ultraviolet to the effective dielectric constant of the solvent has been demonstrated. This sensitivity means that changes in [R[image]] in this spectral region do not necessarily arise from changes in secondary structure and therefore that [R[image]] cannot be a reliable measure of helix content. Since the one-term Drude equation and the Moffitt equation are approximate forms of the modified two-term Drude equation, they allow estimates of helix content. But the parameters Ac and [lambda]c of the one-term Drude equation can be meaningfully interpreted in terms of helix content only if they are related to the corresponding parameters of the modified two-term Drude equation. On the other hand, one can use the value of bo obtained from the Moffitt equation as a direct helix content parameter, provided that it has been calibrated for the appropriate value of [lambda]o, and the sample is known to be only [alpha]-helical and random.

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