The Volume Property: Hydration in Biosystems
This essay focuses upon the remarkable ability of liquid water to alter its molar volume substantially. These changes occur in the vicinity of solute interactions, and those local changes are so large that they affect the macroscopic volume of the system in a measurable way. The technology for such measurements is now advanced to the point where meaningful data can be acquired on systems containing less than 0.1 micromole of a reactant species or reactant moiety in 0.1 ml of solution. These macroscopic volume changes (ΔV) reflect principally the net compression or decompression of water molecules. The ΔV contributions by the solutes themselves are usually far smaller and appear not to present a significant complication except for the release of possible voids within large globular structures. The ΔV effect of hydrophobic or structural hydration is found to be somewhat smaller than that of classical electrostriction and is found to be somewhat negative; the energies for such hydration, however, may be more significant. Examples are presented to demonstrate the usefulness of direct-volume changes in the study of biosystems. The great need, however, for first carrying out volume experiments on relevant model systems in order to interpret the ΔV results from biosystems is emphasized; few model assignments exist in the literature owing to the long period of neglect in the application of the volume property. The results described here as obtained from the experiments on the calcium-binding proteins and on helical peptides suggest the following: (1) The proteins (containing 2 or more calcium-binding sites) appear to take up the calcium ion sequentially. That is, these sites having similar binding constants exhibit distinctive ΔV values per binding equivalent of Ca2+ added; (2) Similarly, the protonation of 3 aspartyl carboxylate groups on a 17-residue helical peptide also displayed a sequential uptake (of protons in this case). These examples and others with ligand-nucleic acid complexes suggest that the asymmetry in the structure of biopolymers lend a directive influence for the uptake of a common reactant to multiple sites—a phenomenon easily observed by determining the change in volume.
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Document Type: Research Article
Publication date: December 1, 2005
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