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Parameters of heat denaturation and intrinsic fluorescence of barnase and its close homologue, binase in the pH region 2-6 have been determined. The barnase heat denaturation (pH 2.8-5.5) proceeds according to the "all-or-none" principle. Barnase denaturation temperature is lower than that of binase and this difference increases from 2.5 degrees C at pH 5 to 7 degrees C at pH 3. Enthalpy values of barnase and binase denaturation coincide only at pH 4.5-5.5, but as far as pH decreases the barnase denaturation enthalpy decreases significantly and in this respect it differs from binase. The fluorescence and CD techniques do not reveal any distinctions in the local environment of aromatic residues in the two proteins, and the obtained difference in the parameters of intrinsic fluorescence is due to fluorescence quenching of the barnase Trp94 by the His 18 residue, absent in binase. Secondary structures of both native and denaturated proteins also do not differ. Some differences in the barnase and binase electrostatic characteristics, revealed in the character of the dipole moments distribution, have been found.

A correlation between the distribution of charged side groups in the globule of Bacillus intermedius 7P ribonuclease (binase) and the process of heat denaturation was studied at different pH values in order to estimate a relation between charge distribution in globular proteins and the character of cooperative thermodynamic transitions. As was shown by comparing the results of scanning microcalorimetric analysis of heat denaturation with the three-dimensional structure of binase, at optimal pH the molecule exists as a single cooperative system stabilized by hydrogen bonds, Van der Waals' contacts, and electrostatic interactions like salt bridges. At pH lower than 4.0 (below the physiological optimum) the cooperativity type of the system was found to change due to a reversible cooperative transition in the ternary structure of the protein globule. It has been concluded that the molecular architecture and the arrangement of atoms do not change considerably in different environments; thus the thermodynamic properties of the globule vary due to the alteration of charge distribution and the consequent changes in the size and number of cooperative regions of the globule. Thus, structural and energetic domains may be non-coincident in proteins.