Abstract: | Differential scanning microcalorimetry was used to investigate the enthalpy (ΔHd) and the temperature (td) of thermal denaturation of normal (nondeuterated) (H-PC) and deuterated (D-PC) phycocyanins in D2O solvent. Values of td in D-PC are about 5–7°C lower than those in H-PC. The magnitudes of ΔHd in D-PC are only 21–32% of those in H-PC. During the protein unfolding, the heat-capacity changes (ΔCp) in D-PC are also lower than those in H-PC. CD was employed to evaluate the secondary structure and the urea denaturation of these proteins in D2O solvent. These proteins have about the same α-helix content. D-PC is less resistant to the denaturant urea than is H-PC. In general, the apparent free-energy change in the process of protein unfolding at zero denaturant concentration is higher in H-PC than in D-PC. Comparisons of the present results for D2O solvent with those previously reported for H2O reveal that solvent isotope effect essentially does not change the α-helix content in H-PC and D-PC. However, D-PC or H-PC has a higher random-coil content in its secondary structure in D2O than in H2O. Substitution of H2O with D2O as the solvent increases td in both D-PC and H-PC, lowers ΔHd in H-PC, and greatly lowers ΔHd in D-PC. The deuterium solvent isotope effect does not change ΔCp in H-PC but lowers ΔCp in D-PC. In the urea denaturation, the magnitudes of (Cu)1/2 in H-PC and D-PC are not affected by such a solvent effect, whereas those of ΔG are greatly increased. These results are correlated with the structure and stability of the proteins. |