Group increments heat capacity

The transition of a protein or a single cooperative domain from the native to the denatured state is always accompanied by a significant increase of its partial heat capacity (see, for reviews, Sturtevant, 1977 Privalov, 1979). The denaturationaJ increment of heat capacity A JCP = C° Cp amounts to 25-50% of the partial heat capacity of the native protein and does not depend noticeably on the environmental conditions under which denaturation proceeds (Fig. 1) or on the method of denaturation. However, it is different foi different proteins and seems to correlate with the number of contacts between nonpolar groups in native proteins (Table I). On the other hand, the partial specific heat capacities of denatured states of different proteins appear to be rather similar (Tiktopulo et... 

The most plausible explanation for the significant denaturational increment of the protein heat capacity is that it is due to water that comes in contact with the protein nonpolar groups exposed upon denaturation... 

Brandts, 1967 Privalov and Khechinashvili, 1974 Sturtevant, 1977). It is assumed that water ordering increases in the vicinity of nonpolar groups (Kauzmann, 1959). If the order of the water molecules surrounding nonpolar groups decreases faster than that of bulk water as the temperature rises, one will observe the gradual melting of ordered water as the increment of the partial heat capacity of protein in water media. 

Comparison of the enthalpy of protein denaturation (Table I) with the enthalpy of solution of liquid hydrocarbons at Ts (Table II) shows also a great difference in their values the enthalpy of protein denaturation at Ts is about 6 kJ per mole of amino acid residues with an average molecular weight of 1 IS the enthalpy of solution of hydrocarbons of comparable size (ethylbenzene, Afw = 106) is almost five times larger at this temperature. For denaturation of solutions of proteins in water AnCp(25°C) is about 70 J K-1 per mole of amino acid residues, whereas A"Cp(250C) for ethylbenzene is 318 J K-1 mol-1. However, this difference in the enthalpy and heat capacity increment is quite understandable, as not all of the groups in a protein are nonpolar, not all are screened from water in the native state, and not all are in contact with water in the denatured state. 

Thermochemical data were required for the estimation of ground state strain. Heats of formation ( 0.5 kcal mol-1) were obtained by the experimental determination of heats of combustion 25 -27) using either a stirred liquid calorimeter 25) or an aneroid microcalorimeter 26) heats of fusion and heat capacities were measured by differential scanning calorimetry (DSC), heats of vaporization 21, 25, 27) by several transport methods, or they were calculated from increments 28). For the definition of the strain enthalpies Schleyer s single conformation increments 29) were used and complemented by increments for other groups containing phenyl30) and cyano substituents. 

A method to estimate thermochemical properties for radicals from the corresponding properties of the parent and of derivation of hydrogen bond increment (HBI) groups, is described by Lay et al. [25] and Sun and Bozzelli [133]. The method uses the bond energy (298.K) for loss of a hydrogen on the central atom for the enthalpy term, the difference between the radical and the parent for the heat capacity (Cp(T)) term and the intrinsic entropy difference for the term. 

These examples only hint at the analysis of heats of formation of organic compounds that is possible. Benson and co-workers summarized the methods and data for calculations for the major functional groups in organic chemistry. ° In addition, the data allow calculation of heat capacities and entropies of these compounds in the same marmer in which heats of formation are determined. Heats of formation are valuable reference points in discussing the stabilities of various isomers or products of reactions, whether they are calculated by bond increments or group increments or are derived as part of a theoretical calculation. 

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