Abstract
A detailed investigation has been carried out into the interaction of randomly coiled sodium polyglutamate with concentrated solutions of simple electrolytes in normal and heavy water. Infrared data over the range 1200-1800 cm-1 revealed a gradual shift to higher frequencies of the amide I' and of the two carboxylate stretching modes as a function of the LiBr concentration (up to 8M). The amide II' absorption shifted to a lower frequency but gained in intensity. The results from the NMR studies were as follows: the methyne (alpha-CH) resonance line showed a downfield shift with a corresponding upfield shift of the amide proton resonance, The methylene signals (beta-CH2, gammaCH2) neither broadened nor changed their positions, but at LiBr concentrations > 4M, they lost their fine structure due to spin-spin splitting. These spectral changes are interpreted in terms of: i) a symmetrical positioning of the lithium ion with respect to the two carboxylate oxygens (in the side chain), ii) a modification of the solvent-polymer interaction and iii) a binding of the lithium ion to the backbone. An extensive discussion and literature survey is presented which outlines the current thinking regarding the mechanism of denaturation of various proteins by small neutral molecules.