The adiabatic compressibility, -beta s, of 11 globular proteins in water was determined by means of sound velocity measurements at 25 degrees C. All the proteins studied except for subtilisin showed positive -beta s values, indicating the large internal compressibility of the protein molecules. The intrinsic compressibility of proteins free from the hydration effect appeared to be comparable to that of normal ice. The compressibility data for 25 proteins, including 14 reported previously [Gekko, K.,& Noguchi, H. (1979) J. Phys. Chem. 83, 2706-2714], were statistically analyzed to examine the correlation of the compressibility with some structural parameters and the amino acid compositions of proteins. It was found that -beta s increases with increasing partial specific volume and hydrophobicity of proteins. The helix element also seemed to be a dynamic domain to increase -beta s. Four amino acid residues (Leu, Glu, Phe, and His) greatly increased -beta s, and another four (Asn, Gly, Ser, and Thr) decreased it. Some empirical equations were derived for the estimation of the -beta s values of unknown proteins on the basis of their amino acid compositions. The volume fluctuations of proteins revealed by the compressibility data were in the range of 30-200 mL/mol, which corresponded to about 0.3% of the total protein volume. The conformational fluctuation seemed to enhance the thermal stability of proteins.

The adiabatic compressibility, ß3, of two native globular proteins (lysozyme and bovine serum albumin) was determined by measuring the sound velocity in aqueous solutions at various temperatures (10, 15, 25, and 40 °C). ß, was positive at room temperature, but it decreased to a negative value with decreasing temperature. The isothermal compressibility, ß , of the two proteins was estimated from the ft value by using the thermal expansion coefficient and heat capacity data. With the ß values obtained, the partial specific volumes of the two proteins were simulated as a function of temperature and pressure. The results were discussed in terms of the interdependent effects of temperature and pressure on the cavity and hydration of the protein molecules.

No abstract

Sweet aspartyl di- and tripeptide esters were extended toward the N-terminus in relation to the structural features of sweet peptides. The sweet peptides were designed on the basis of the receptor site model. It was found that an extension of the sweet aspartyl dipeptide esters by adding a small D-amino acid residue mostly gave sweet compounds (e.g., D-Ala-L-Asp-D-Ala-OMe), although this significantly decreased their sweetness potencies. Further extension at the N-terminus of the extended sweet tripeptide esters to yield the tetrapeptide esters resulted in a loss of the sweet taste. The N-terminal extension of sweet aspartyl tripeptide esters resulted in faintly sweet or nonsweet tetrapeptide esters. Interestingly, an analogous extension at the N-terminus of the sweet aminomalonyl dipeptide esters gave bitter compounds (e.g., D-Ala-DL-Ama-L-Phe-OMe). These results indicate that the receptor has a small space that can accomodate an additional small D-amino acid residue at the site facing the N-terminus of sweet aspartyl dipeptide esters.