Zimm-Bragg parameters

Here k is the Boltzmann constant, T is the absolute temperature, C is the number of peptide residues per cubic centimeter of solution, / and s are the helical fraction and the Zimm-Bragg parameter for the polypeptide molecule in the absence of external field, and ft is a parameter which represents the average correlation between the helix unit at the end of a helical sequence and the random-coil unit next to it. Though a detailed account of this parameter is left for Ref. (124), we note here that / = 0 corresponds to the complete absence of correlation between these two units, while ft — 1 corresponds to the case in which the dipole moment of a random coil unit points in the same direction as the axis of the preceding helix unit. 

Since the Zimm-Bragg parameters o and s of the naturally occurring amino acids (In water) cannot be obtained from studies of the helix-coil transition in homopolymers, because of experimental difficulties, a technique Is developed to circumvent these problems. It involves the study of the thermally induced transition curves for random copolymers of "guest amino acid residues in a water-soluble host" po y(amino acid). The data may be interpreted with the aid of suitable theories for the helix-coil transition in random copolymers to obtain a and s for the "guest" residues. It is shown in this paper that, for the usual ranges of parameters found for polylamino acids), one of the two lowest order approximations (corresponding to earlier treatments by Lifson and Allegra) is completely adequate. In essence, the low-order approximations hoid if o and s for the two constituents of the copolymer do not differ appreciably from each other. 

Water-soluble random copolymers containing u-phenylalanine and A -O-hydroxypropylR-glutamine are prepared. The thermally induced helix-coil transition in these copolymers in water is studied. The incorporation of (.-phenylalanine is found to increase the helix-content of the host polymer. The Zimm-Bragg parameters cr and s for the helix-coil transition in poly(L-phenylaianine) in water are deduced from an analysis of the melting curves of the copolymers. The values of s, computed from both the Lifson and Allegra theories are (Ufson/Allegra a 0.0018) s 1.056/1.061 (273 K), 1.078/1,086 (293 K), 1.041/1.047 (313 K), 1.000/1.003 (333 K). 

Values of the Zimm-Bragg parameters s for pure poly(L-aspartlc acid)... 

The two limiting cases are also important in the kinetic description of the cooperative transition. Schwarz has shown [19] that using the Zimm-Bragg parameters the mean reciprocal relaxation time t of the conformational transition defined as... 

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