This project is designed to show how the equilibrium volume changes per mole of reactant, deltaV, reflect principally on the differences in the molar volume of water molecules that respond to solutes undergoing chemical change. The method, by use of a sensitive density technique (magnetic suspension), responds to minor structural changes that reflect the compression of nearby water dipoles. The foci of this proposal are: 1) Calcium-protein interactions; 2) hydration of biopolymers; 3) deltaV on small models. For item 1) we determine volume-titration profiles prepared by microliter increments of Ca2+ and other metal ions added sequentially to selected intracellular calcium-binding proteins and long peptides and to mutants of each. These profiles are to be tested as aids in determining the mode of uptake of the metal ions (sequential/random/cooperative) in multi-sited proteins. These profiles should also reflect structural alterations of the metal-binding site as a result of mutations either in or distant to the site; moreover, global effects can be tested by noting if volume consequences attend the metal-ion uptake of more than one equivalent upon substitution or deletion of an amino-acid residue in or near a particular site. Comparison heat and affinity measurements will provide interpretation to the deprived entropy changes owing to the net compression or release of electrostricted water molecules. 2) A study of hydration changes will be conducted on other biopolymer systems, such as nucleic-acid models. This collaborative effort will also test the relation of deltaV to entropic changes in model systems. 3) Several small-molecule models are to be studied in order that deltaV assignments may be applied to specific chemical events--events that have application to aqueous biosystems. The volume changes to be investigated relate primarily to coulombic-type interactions (ionic and hydrogen-bond changes) involving the effect of carboxylate versus carbonyl oxygen donor atoms, other lone-pair donor/acceptor atoms and organic phosphates.
