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Protein Interactions in Undersaturated and Supersaturated Solutions: A Study Using Light and X-Ray Scattering

Janaky Narayanan^* and X. Y. Liu

^* Department of Physics, R. J. College, Ghatkopar (W), Mumbai 400 086, India and Department of Physics, Blk. S12, Faculty of Science, National University of Singapore, Singapore 117542

Correspondence: Address reprint requests to Janaky Narayanan, R. J. College, Ghatkopar (W), 2/78 Krishna Mahal Pestom Sagar, Tilak Nagar P.O., Mumbai, Ma, India 400 089. Tel.: +91-22-522-4413; Fax: +91-22-522-4413; E-mail: janaky{at}vsnl.com.

Protein interactions in undersaturated and supersaturated solutions were investigated using static and dynamic light scattering and small angle x-ray scattering. A morphodrom of lysozyme crystals determined at 35°C and pH = 4.6 was used as a guideline in selecting the protein and precipitant concentrations. The osmotic second virial coefficient, B₂₂, was determined by static and dynamic light scattering. At low ionic strengths for which no crystals were formed, B₂₂ was positive indicating repulsive interactions between the protein molecules. Negative B₂₂ at higher ionic strengths corresponds to attractive interactions where crystallization becomes possible. At two extreme salt concentrations, small angle x-ray scattering data were collected and fitted with a statistical mechanical model based on Derjaguin–Landau–Verwey–Overbeek potential using Random Phase Approximation. This model accounted well for the small angle x-ray scattering data at undersaturated condition with constant potential parameters. At very high salt concentration corresponding to supersaturated solution this model seems to fail, possibly due to the presence of non-Derjaguin–Landau–Verwey–Overbeek hydration repulsion between the molecules.

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