Journal
BIOPHYSICAL CHEMISTRY
Volume 120, Issue 2, Pages 106-113Publisher
ELSEVIER
DOI: 10.1016/j.bpc.2005.10.003
Keywords
second virial coefficients; sedimentation equilibrium; light scattering; protein crystallization; protein-protein interaction; protein-salt interaction; lysozyme; equine serum albumin
Funding
- Biotechnology and Biological Sciences Research Council [B18660] Funding Source: researchfish
- Biotechnology and Biological Sciences Research Council [B18660] Funding Source: Medline
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The effects of ammonium sulphate concentration on the osmotic second virial coefficient (B-AA/M-A) for equine serum albumin (pH 5.6, 20 degrees C) have been examined by sedimentation equilibrium. After an initial steep decrease with increasing ammonium sulphate concentration, B-AA/M-A assumes an essentially concentration-independent magnitude of 8-9 ml/g. Such behaviour conforms with the statistical-mechanical prediction that a sufficient increase in ionic strength should effectively eliminate the contributions of charge interactions to B-AA/M-A but have no effect on the covolume contribution (8.4 ml/g for serum albumin). A similar situation is shown to apply to published sedimentation equilibrium data for lysozyme (pH 4.5). Although termed osmotic second virial coefficients and designated as such (B-22), the negative values obtained in published light scattering studies of both systems have been described incorrectly because of the concomitant inclusion of the protein-salt contribution to thermodynamic nonideality of the protein. Those negative values are still valid predictors of conditions conducive to crystal growth inasmuch as they do reflect situations in which there is net attraction between protein molecules. However, the source of attraction responsible for the negative virial coefficient stems from the protein-salt rather than the protein-protein contribution, which is necessarily positive. (c) 2005 Elsevier B.V. All rights reserved.
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