Compressibility and Thermal Expansion

Although the values of protein thermal expansion coefficients appear to be reasonable in relation to those of comparable polymers and small molecules of similar nature, protein compressibilities are much smaller than would be expected if proteins are impermeable to solvent [75]. Because of their hardness, knots can be expected to have low compressibilities, but the considerable empty space in matrices and surfaces should produce high values if the interior of proteins were mechanically isolated from the environment through which the pressure is applied. Thus far it is found that the hemoglobins have isothermal or adiabatic compressibilities in the normal range for liquids, but most other proteins studied had values characteristic of soft solids. Apparent compressibilities would be consistent with 100% knot and no free volume. The explanation appears to be that although the true low knot compressibilities contribute correctly to the apparent compressibilities, the matrices are permeable to water and thus mechanically transparent. The matrices contribute only relaxation contributions from conformational readjustments under changing pressure and the flow of water between bulk phase and protein interior. The few relevant experiments indicate that application of pressure forces water into protein matrices. The interior of the protein is in mechanical equi librium with the solvent medium so it is the primary-bonded structure that provides the true phase boundary. Examining the x-ray results for lysozyme at pressures of 1000 bar determined by Kundrow and Richards [76] shows that only the knots are undistorted.

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