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The formation of protein clusters or a protein-rich phase in undersaturated solutions of biopolymers is considered theoretically on the basis of phase diagrams of a water-protein-salt system. Concentrated (50-200 mg/ml of protein) water-salt solutions of human serum albumin molecules modified by a maleimide spin-label have been studied experimentally using the ESR technique to characterize the significant general features of the system behaviour suggested by the model phase diagrams. The inorganic ion content (NaCl, KSCN, MgCl(2), and CaCl(2)) was varied in the range of 10(-3)-4 M. Salt-induced changes in different experimental ESR spin-label parameters based on relations between spectral line widths and amplitudes were determined and compared with the same parameters in salt-free solution. The data on dipole-dipole interactions of spin labels obtained at 77 K and on spin exchange at normal temperatures are indicative of local protein concentration inhomogeneities. The results have been described in terms of salt-induced dissociation of stabilized supramolecular structures in protein solution-protein clusters, liquid-liquid phase transition between the hydration water of clusters and that of individual proteins, and a rise in surface tension which results in protein stabilization.

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