The concentration dependence of the pressure- and temperature-induced cloud point transition (Pc and Tc, respectively) of aqueous solutions of an elastin-like polypeptide with a repeating pentapeptide Val-Pro-Gly-Ile-Gly sequence (MGLDGSMG(VPGIG)40VPLE) was investigated by using apparent light scattering, differential scanning calorimetry, and circular dichroism methods. In addition, the effects of salts and surfactants on these properties were investigated. The Pc and Tc of the present peptide in aqueous solution were strongly concentration dependent. The calorimetric measurements showed that the enthalpy of transitions was 300-400 kJ/mol, i.e., 7-10 kJ/mol per VPGIG pentamer. The Tc of the (VPGIG)40 solution was highly affected by the addition of inert salts or SDS. The effects of salts were consistent with those observed in the lyotropic series or Hoffmeister series. The CD spectrum at low peptide concentrations indicated that the present peptide forms type II beta-turn-like structure(s) at higher temperatures, but the temperature dependence of random coil diminishment (195 nm) and beta-turn formation (210 nm) were not exactly coincident. A hypothetical mechanism of the (VPGIG)40 phase transition that could account for these observations was postulated. Observations suggest that the temperature-responsive properties of the elastin model peptides occur via a mechanism involving conformational change-association-aggregation and that the first two are strongly interactive.

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The cloud points of the aqueous solutions of poly(N-vinylisobutyramide) (PNVIBA) and poly(N-isopropylacrylamide) (PNIPAM) were measured down to a subzero temperature under elevated pressures, and the effects of ionic additives and the ionic comonomer were studied. The polymers with higher molecular weight showed lower transition pressure (P t) and smaller ellipsoids, due to larger absolute values of ΔC p and Δβ. Addition of 0.02% sodium dodecyl sulfate (SDS) into aqueous solutions of these polymers apparently extinguished the clouding phenomenon, but even under these conditions the dynamic light scattering measurement showed the chain collapse of the random coil preceding the interchain aggregations. P t and transition temperature (T t) were highly dependent on the addition of inert anions. Less lyotropic ions showed lower P t and T t, producing smaller ellipses as a result, while additions of SCN- and I- caused a slight increase in both P t and T t and therefore larger ellipses. Introduction of ionic comonomer (4-pentenoic acid) into PNIPAM (less than 20%) made the transition P−T curves pH dependent and P t was much more influenced by the charged comonomer than T t. Anionic residues less than 5% were sufficient to screen the cloud point (20 °C and <400 MPa).