This project is designed to show whether the equilibrium volume changes, deltaV, attending the sequential uptake of Ca2+ and other metal ions to calcium-binding proteins reflect primarily the coordination events to the liganding amino-acid residues in these proteins. Our previous work suggests that the sequential volume increases per Ca2+ equivalent relates to specific binding loops in certain of these proteins (calmodulin, skeletal muscle troponin-C, parvalbumin); moreover, these expansions correspond to the deltaV on complexation of Ca2+ to model compounds having similar multi-coordination environments. This program will utilize an expanded array of calcium-binding proteins, including site-directed mutants and natural variants, protein fragments, synthetic peptide analogues of the metal-binding amino-acid sequences in these proteins and additional small-molecule models. Ca2+ Cd2+, Mg2+ and the lanthanide series of metal ions will be employed in an effort to determine the deltaV contributions of charge, coordination number and stereoelectronic factors, and to isolate the electrostriction contributions of liganding donor groups from those of the test metal ions. There exists very little data base and theory on the volume property of metal-ion multicoordination (as opposed to heat transfer data), yet much of biology is concerned with metal-ion interactions and most reactions in biochemistry involve ionic changes affecting solvent volume significantly. This project, therefore, focuses upon the role of water and its propensity for relatively large volume changes in biochemical reactions. The results should also prove useful in the interpretation of extant heat-change measurements. Through the development of a proven sensitive and rapid density technique (magnetic), adapted for delivery of small volume changes (+ 0.2 nl precision in 100 ul samples), the large number of measurements required for this project can be accomplished with scarce solutes.
