[Salt-induced critical type phase transition in a water-protein matrix of serum albumin molecules]

Effective thermodynamic parameters of activation of label transition between two microsurroundings in water-protein matrix (WPM) of spin-labelled molecules of serum albumin (HSA-SL) in 0.001 M phosphate buffer, pH 7.3 within concentration range of NaCl (m3) (10(-3) M to 2 M and concentration range of protein (m2) 50 to 200 mg/ml were determined. The phase transition (PT) between two structures of water in WPM is revealed within the m3 range 0.01 M to 0.1 M. It is close to the type I PT and more expressed at high protein concentrations. As m3 increases (to 0.2 M when m2 = 50 mg/ml and to 0.7 when m2 = 200 mg/ml) PT-I is transformed into PT of the critical type. This is indicated to zero values of the higher order derivatives of Gibbs energy of activation with respect to salt concentration, as well as by the maximum values of positive disjoining pressure in these points. The biological significance of this phenomenon is discussed.