Circular dichroism and the pH-induced ?-coil transition of poly(S-carboxymethyl-L-cysteine) and its side-chain homolog

Reversible and irreversible conversion between the intermolecular beta-structure and the disordered state of poly(S-carboxymethyl-L-cysteine) in aqueous media

Conversion between the intermolecular beta-structure and the disordered state of a fractionated low molecular weight sample of poly(S-carboxymethyl-L-cysteine) was examined mainly by the measurements of circular dichroism in the absence of salt as well as in the presence of 20 mM NaClO4 or NaCl. In 20 mM NaClO4 or NaCl solutions, the conversion was reversible. Under this condition, it was confirmed by direct and unambiguous evidence provided from the viscosity and the reduced scattering intensity that the beta-structure was formed by intermolecular association. At low degrees of neutralization, the pH increased on dilution while it remained constant over a wide range of concentration at a high degree of neutralization. In the absence of salt, the conversion was often irreversible with respect to a concentration change at a constant degree of neutralization or to a change in the degree of neutralization at a constant concentration. The extent of the irreversible conversion decreased with the amount of beta-structure in the solution. The dissociation of aggregates was very slow at low ionic strengths. It was inferred that the irreversible nature of the conversion arose from this slow dissociation of aggregates.

Beta-structure of polypeptides in non-aqueous solutions. I. Spectral characteristics of the polypeptide backbone

The optical rotatory features of the beta-structure of the polypeptides in non-aqueous solutions and films cast from these solutions have been investigated. The beta-structure of poly-S-benzyl-L-cysteine, poly-S-carbobenzoxy-L-cysteine and poly-S-benzyl-L-cysteine, poly-S-carbobenzoxy-L-cysteine and poly-O-carbo-bands of their films. The optical rotatory dispersion (ORD) and circular dichroism (CD) spectra of these polypeptides are found to be very similar in both film and solution. In solvents promoting the beta-structure, the polypeptides are characterized by CD troughs in the n-pi* transition region of the peptide chromophore. The ORD spectra are found to be positive in sign throughout the visible and accessible ultraviolet regions and are interpreted in terms of the possible existence of a relatively much larger positive pi-pi* CD bands as compared with the negative n-pi* band. The rotatory data obtained in the non-aqueous solution are compared with those obtained for other poly peptides in aqueous solutions, with respect to the type and extent of beta-structure present.