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Biophys J, November 2000, p. 2222-2234, Vol. 79, No. 5

Excluded Volume in Solvation: Sensitivity of Scaled-Particle Theory to Solvent Size and Density

Karen E. S. Tang and Victor A. Bloomfield

Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Saint Paul, Minnesota 55108-1022 USA

Changes in solvent environment greatly affect macromolecular structure and stability. To investigate the role of excluded volume in solvation, scaled-particle theory is often used to calculate Delta Gtrev, the excluded-volume portion of the solute transfer free energy, Delta Gtr. The inputs to SPT are the solvent radii and molarities. Real molecules are not spheres. Hence, molecular radii are not uniquely defined and vary for any given species. Since Delta Gtrev is extremely sensitive to solvent radii, uncertainty in these radii causes a large uncertainty in Delta Gtrev---several kcal/mol for amino acid solutes transferring from water to aqueous mixtures. This uncertainty is larger than the experimental Delta Gtr values. Also, Delta Gtrev can be either positive or negative. Adding neutral crowding molecules may not necessarily reduce solubility. Lastly, Delta Gtrev is very sensitive to solvent density, rho . A few percent error in rho  may even cause qualitative deviations in Delta Gtrev. For example, if rho  is calculated by assuming the hard-sphere pressure to be constant, then Delta Gtrev values and uncertainties are now only tenths of a kcal/mol and are positive. Because Delta Gtrev values calculated by scaled-particle theory are strongly sensitive to solvent radii and densities, determining the excluded-volume contribution to transfer free energies using SPT may be problematic.

Biophys J, November 2000, p. 2222-2234, Vol. 79, No. 5
© 2000 by the Biophysical Society   0006-3495/00/11/2222/13  $2.00


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