Denaturation enthalpies, table

It is tempting to suppose that all the denaturation enthalpy at Tx is provided by the disruption of nonpolar contacts, i.e., actually by van der Waals interactions, but that the temperature dependence of enthalpy is determined by hydration of nonpolar groups. The latter is supported by the correlation found between AnCp and the saturation of the native structure by the contacts between the nonpolar groups (Table I). However, this simple model immediately raises two questions Why do proteins with different concentrations of nonpolar contacts have the same denaturation enthalpy values at Tx Is it reasonable to neglect the contribution of hydrogen bonds in the denaturation enthalpy ... 

It was suggested earlier that hydrogen bonds in proteins are the main contributors to the denaturation enthalpy at 7X, whereas the nonpolar contacts determine only the temperature dependence of the denaturation enthalpy (Privalov, 1979). The main argument for this was the observation that proteins that have the same enthalpies at Tx have an almost equal concentration of intramolecular hydrogen bonds, but differ in the concentration of nonpolar contacts (Table I). As is evident, the assumption of the dominant role of hydrogen bonds in the stabilization of protein structure explained the observed temperature convergence of the denaturation enthalpy, if the enthalpy of exposure of nonpolar groups to water is zero at this temperature. This assumption implied that either the enthalpy of... 

Mrevlishvili189 gave the following enthalpy differences for the helix-coil transfer of collagen fibre - coils/solution (5 kcal/mole) coils/solution - helix/solution (—5.5 kcal/mole) helix/solution - helix/fibre (0.5 kcal/mole). The following table gives the differences between the hydration water of native and denaturated protein181,188> ... 

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. 

Table I The melting temperatures (Tm) from the CD thermal denaturation studies are estimated from the temperature at which the slope of the first derivative of the uncorrected data was a minimum ty2 is the temperature of half-completion of the DSC thermal denaturation transition. A//cal is the calorimetric enthalpy. The DSC AAG°u = AG°u (mutant) - AG°u (wild-type), at the value of ti/2 for the wild-type protein (74.6°C).

Although methionine is present with relatively low frequency in naturally-occurring proteins (Klapper, 1977), in this case it appears to act as a normal hydrophobic residue. The thermal denaturations of all single variants are essentially as cooperative as wild-type with comparable enthalpies of unfolding (Table II). This is also the case for the seven- but less so for the ten-methionine variant. 

In a DSC experiment lasting about 1 h, using as little as 2 pg of material, a protein can be thermally unfolded, allowing the enthalpy and entropy of the denaturation process to be measured, as seen in Figure 16.36. The thermodynamic data collected are presented in Table 16.5. As can be seen in this table, the thermodynamic data obtained were accurate even for the 2 pg sample. 

Three inhibitors from non-food sources are included in Table II. The basic (Kunitz) trypsin inhibitor from bovine pancreas (BPTI) is a small protein that has been used extensively in protein structure studies. At neutral or acid pH, it denatures near 100°C. From the ratio of calorimetric to van t Hoff (effective) enthalpies, Privalov... 

TABLE IV. Effect of Mn2+ and Ca2+ on Denaturation Temperatures and Enthalpies of Concanavalin A. 

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