PROTEIN SOLUTION PHASE DIAGRAMS AND PROTEIN MOLECLUE STRUCTURAL TRANSITIONS

An analysis of standard thermodynamic functions of protein structure in solution allows revealing important features of protein solution phase behavior, particularly the presence of higher and lower critical solution temperatures on phase diagram (PD). Linking the phase and the corresponding conformational transitions of protein molecules to phase transitions of the protein solution as a whole will provide consideration of the relationship between protein conformation and the structural organization of protein systems in predenaturational interval. A phase diagram (PD) for the globular protein solution is presented in the temperature-entropy coordinate plane. Solution phase states have been analyzed and a probable interrelation of topological structures arising in the supercritical region with the phase processes in low- and high-temperature range has been defined when considering PD. This shows the possibility to superpose two types of PD: for normal and retrograde protein solubility. PD in the temperature-packing density plane has been presented for a protein solution with low and high critical solution temperatures and a supercritical zone between them typical of native protein solution, and a closed zone typical of denatured protein solution in the near physiological temperature interval. Supramolecular organization of protein solution has been described on this basis including metastable phases of protein molecules in conformations altered with respect to native (N) and denatured (D) states as well as dynamical protein clusters and oligomers in the supercritical zone. The contribution of the solvent in the low- and high-temperature ranges has also been taken into account. Various PD regions have been interpreted under the assumption that molecules in conformations, corresponding to N and D states are capable of forming different nanocrystal lattices which define the topological features of the corresponding solutions in a wide temperature range. Probable significance of supercritical phase states of protein solutions for the self-regulation of the components chemical potentials at the expense of the solution supramolecular organization at changing temperature and/or salt concentration and thereby at changing environmental factors has been discussed.