Scattering functions of poly(amino acid) in helix-coil transition

Abstract

Scattering functions of poly(amino acid) in the helix-coil transition region have been calculated on the basis of the Zimm-Bragg theory, to examine the effects of the helical content on the scattering function and on the cross-section factor, from which the radius of gyration of the cross section and the mass per unit length of the helical sequence can be obtained. The molecule is assumed to contain only one α-helical sequence of any length, leaving unfolded coils at the chain ends. The statistical mechanical averages of the interference among scattering units, apart from the interference within the helical sequence, are approximated by the expansion in terms of the even moments up to the eighth moment, which are calculated by the generator matrix method based on the conformation energies computed for poly ( l-alanine). The scattering function at intermediate angles varies systematically according to the variation of the helical content, demonstrating the sensitivity of intermediate-angle scattering to the conformational changes in the helix-coil transition. The comparison of the Guinier plots of the cross-section factors, calculated from the scattering functions at various helical contents, has indicated that the radius of gyration of the cross section of the helical sequence can be determined accurately from the plots even when the coil content is as much as 30%, and that the mass per unit length is determinable to within an error of 6% unless the coil content exceeds 10%.