Enthalpy of proteins

The consistent force field (CFF) was developed to yield consistent accuracy of results for conformations, vibrational spectra, strain energy, and vibrational enthalpy of proteins. There are several variations on this, such as the Ure-Bradley version (UBCFF), a valence version (CVFF), and Lynghy CFF. The quantum mechanically parameterized force field (QMFF) was parameterized from ah initio results. CFF93 is a rescaling of QMFF to reproduce experimental results. These force fields use five to six valence terms, one of which is an electrostatic term, and four to six cross terms. 

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. 

Therefore, at the present time there is no reason for neglecting the contribution of hydrogen bonds in the enthalpy of protein denaturation. One can even expect that this contribution should increase as the temperature increases because the denatured state of the protein will make fewer hydrogen bonds with water molecules at higher temperatures. 

This argument was pointed out almost 30 years ago by Benzinger [49], in a paper which referred to some still earlier work of his, and yet its implications, even more pertinent today given the wider use of calorimetry in molecular biology, still appear to be largely ignored, an exception being Weber s work on the association enthalpy of protein subunits [50]. 

Weber G 1995 van t Hoff revisited enthalpy of association of protein subunits J. Rhys. Chem. 99 1052-9... 

FIGURE 3.3 The enthalpy change, ATT, for a reaction can be determined from the slope of a plot of R In versus l/T. To illns-trate the method, the values of the data points on either side of the 327.5 K (54.5 C) data point have been nsed to calculate ATT at 54.5 C. Regression analysis would normally be preferable. (Adapted from Brandts, ]. F., 1964. Tim thermo-dynamics of protein denatnration. I. The denatnration of ehy-motrypsinogen. om Q.7A of the American Chemical Society m 429 -430L)... 

Amino acids are the building blocks of proteins, which have long chainlike molecules. They are oxidized in the body to urea, carbon dioxide, and liquid water. Is this reaction a source of heat for the body Use the information in Appendix 2A to predict the standard enthalpy of reaction for the oxidation of the simplest amino acid, glycine (NH2CH2COOH), a solid, to solid urea (H2NCONH2), carbon dioxide gas, and liquid water ... 

Sflf-Test 6.16B Calculate the standard enthalpy of formation of urea, CO(NH2)2, a by-product of the metabolism of proteins, from the information in Tables 6.4 and 6.5. 

Procedures are now available that allow for the presence of several solvent molecules around a solute molecule. This approach takes into account the effect of molecular interactions with the solvent on properties such as the enthalpy of formation and the shape adopted by a non-rigid molecule, such as a protein or a region of DNA. These studies are important for investigating the structures and reactions of biological molecules in their natural environment. 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

This simple three-state model of protein folding, shown schematically in Figure 7, ascribes a separate force to shaping the structure of each state. Local steric interactions trap the protein chain in a large ensemble of conformations with the correct topology hydrophobic interactions drive the chain to a smaller, more compact subset of conformations then dispersion forces supply the enthalpy loss required to achieve a relatively fixed and rigid ensemble of native conformations. 

This is a deceptively simple question to ask, but one which is quite hard to answer. We have performed titration calorimetry experiments (unpublished) intended to determine the enthalpy of reaction for the reaction of aqueous formaldehyde with polypeptides or proteins, without success. In our experiments, the enthalpy of mixing was much larger than that which could be... 

The transition enthalpies of the s- and p-fractions obtained from the feed with a comonomer molar ratio of 85 15 were equal to 6 and 7 J/g, respectively, i.e. the values are very close. This, therefore, can be indicative of almost the same average length of oligoNVCl blocks. Moreover, as we have already stressed, the fractions also had virtually the same final comonomer composition. However, since the solution properties of these fractions are drastically different, one can draw the conclusion that this is apparently due to a specific distribution of hydrophobic and hydrophilic residues along the polymer chains. In turn, because of all the properties that are exhibited by the s-fraction, this fraction can be considered to be a protein-like copolymer [27]. 

As displayed in Fig. 4, for this family of proteins, the entropy of binding is either favorable or weakly unfavorable, while the enthalpy of binding is negative, resulting in micromolar affinity. The only exception is a PA-IIL ligand consisting of two fucose residues separated by a flexible linker that... 

The van t Hoff plots for thermal denaturation of proteins are linear in the transition region, thus allowing the enthalpy change (AHm) of unfolding at the transition temperature (Tm) to be estimated. Because of the change in free energy in (AG) = 0 at Tm (reversible process), the entropy of unfolding (ASm) at the transition midpoint can be calculated from ... 

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