Distribution of charges in Bacillus intermedius 7P ribonuclease determines the number of cooperatively melting regions of the globule.

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.

[Heat denaturation of crystalline and dissolved leghemoglobin]. / Teplovaia denaturatsiia leggemoglobina v kristalle i rastvore.

Heat transitions in crystals of leghemoglobin (LH) are studied by means of scanning microcalorimetry and microscopy. It has been found that LH crystals do not melt and their loss of crystal lattice is due to the denaturation of protein globules inside the crystal. Peculiarities of the crystal state (as compared to the solution) are shown in an increase in the cooperative character of heat transition and relaxation time of the system. Subsequent consideration of different variants of correlation of two stages of heat absorption by LH crystals made it possible to determine the type of physical process proceeding in the object by the shape of calorimetric curve. Both observed peaks of heat absorption were grouped with intramolecular processes of different thermodynamic properties. The first peak of heat absorption is a manifestation of intramolecular mobility, both of individual protein segments in relation to each other and of individual segments of alpha-helical regions. Thus microcalorimetry allows a study of peculiar intramolecular dynamics of globular proteins precisely in the crystal state, because the crystal as if synchronizes the movement of individual molecules at the expense of the unification of their kinetic energy, surroundings and mutual orientation.

[The compact form of DNA in solution. V. The heat effect preceding compactization of double-chained DNA in PEG containing water-salt solutions]. / Kompaktnaia forma DNK v rastvore. V. Teplovoi éffekt, predshestvuiushchii kompaktizatsii dvukhtsepochechnoi DNK v soderzhashchikh PEG vodno-solevykh rastvorakh

DNA-dependent heat effects accompanying mixing of water-salt (0.3 M NaCl) solutions of PEG and DNA within the range of PEG 10-50 mg/ml at 25 degrees C were determined by the method of difference microcalorimetry. It was found that, unlike optical and hydrodynamical methods, microcalorimetry makes it possible to detect some changes of the DNA-PEG system preceding formation of compact particles of DNA. In the studied range of DNA concentrations (up to 50 X 10(-3) MG/ML) the specific DNA-dependent heat effect is essentially independent of DNA concentration. It is negative and its absolute value increases from 0 to 5 cal/g of DNA in the PEG concentration range from 0 to 35-40 mg/ml after that the rate of its increase raises greatly and at PEG concentration of 50 mg/ml it is equal to 35 cal/g of DNA. It is suggested that the studied DNA dependent heat effects at low concentrations of PEG (less than 40 mg/ml) are caused by dehydration of DNA preceding its compactization.