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

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Abstract
In this and the previous communication we have shown that a new equation (eq. 1) describes the visible and near-ultra-violet rotatory dispersion data of [alpha]-helical and random polypeptides and proteins in organic solvents and aqueous solutions, respectively. This new equation is a modified two-term Drude equation. The two rotatory parameters A([alpha],[rho]) (193) and A([alpha],[rho])225 were shown to be [alpha]-helix parameters which are influenced appreciably only by the dielectric constant of the solvent. For a given [alpha]-helix content the variation of the rotatory parameters is small enough so that all solvents may be grouped in two categories high and low dielectric constant solvents. In each of these classes the [alpha]-helix content can be expressed by two independent linearly related parameters [eqs. (2) and (3)]. It has been shown that if a polypeptide or protein gives values of A([alpha],[rho]) (193) and A([alpha],[rho])225 which do not fit equations (2) or (3), this indicates that other structures such as extended conformations, proline helices, or other types of helices are present. We infer that the fit of A([alpha],[rho]) (193) and A([alpha],[rho])225 to these equations indicates the presence in the polypeptides or the proteins of only [alpha]-helical or random conformations within the limits of experimental error described in this communication. Finally it was found that the quantity A([alpha],[rho]) (193)-A([alpha],[rho])225 is independent of solvent to a first approximation and can therefore be used as an [alpha]-helix content parameter which is independent of solvent. An advantage of this analysis over previous types of optical rotatory dispersion analyses is that the [alpha]-helix content can be obtained independently from two rotatory dispersion parameters, thus allowing an internal check on the [alpha]-helix content of the polypeptide or protein. Comparisons of previous types of optical rotatory dispersion analyses with the present one will be discussed in a forthcoming communication.

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