Enthalpy of conformers

Enthalpy of Conformational Transition of a Protein from Indirect Calorimetric Measurements [8]... 

The different transitions for which calorimetric data have been obtained and the relationships of the experimental A//m values to the enthalpy of transition, A//tji(T R), and to the enthalpy of binding of PALA, A// ,pala. are shown in Figure 4.1, which is a thermodynamic cycle that permits calculation of A// , for one step if all others are known. Each enthalpy AH to A//mv, in principle, contains some contribution from the enthalpy of binding and some from the enthalpy of conformational transition. It has been assumed that the enthalpy of binding per mole of ligand, AH aPALA, is identical for each of the six PALA molecules taken up sequentially by the enzyme. Thus,... 

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. 

The interpretation of these results is, however, problematic since no data on the absolute enthalpy and entropy of the respective triple helix and coiled state are available. Though it may be taken as an established fact that the entropy of conformation of a (Pro-Pro-Gly) coil is lower than in the case of a (Pro-Ala-Gly)n coil, we are not sure whether the entropy of the triple helix depends on the imino acid content. 

Table 17 contains the enthalpies of activation and reaction for the three propagation steps in the gas phase and in solution. They were calculated by using classical cations possessing all-trans conformation. 

Molecular mechanics (also known diS force-field calculations) is a method for the calculation of conformational geometries. It is used to calculate bond angles and distances, as well as total potential energies, for each conformation of a molecule. Steric enthalpy can be calculated as well. Molecular orbital calculations (p. 34) can also give such information, but molecular mechanics is generally easier, cheaper (requires less computer time), and/or more accurate. In MO calculations, positions of the nuclei of the atoms are assumed, and the wave equations take account only of... 

A molecular mechanics calculation gives the total potential energy of each conformation. If the mole fractions of all the conformations are known, or can be calculated, the enthalpy of formation of the compound can be obtained. 

Two types of situation may generally arise in respect of this equation. In the first, the enthalpy of the products exceeds that of the reactants (AH is positive), while in the second the converse happens (AH is negative). A reaction that conforms to the former situation is called an exothermic reaction and a reaction that corresponds to the latter situation is called an endothermic reaction. An exothermic reaction is accompanied by evolution of heat. An endothermic reaction, in contrast, occurs with absorption of heat. Enthalpy changes are... 

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. 

Hydration-free enthalpies of different conformers of formamide and allyl vinyl ether were studied using the ellipsoidal cavity model and the BLYP functional170,177. A very good agreement between the MP2/SCRF and the DFT(BLYP)/SCRF results was shown for the... 

Tunon et al.194 studied the water molecule in liquid water. The sample of conformations by the microscopic environment (water in this case) was obtained using Monte Carlo technique. The energy was calculated as in the approach of Stanton et al.189 i.e., using Eqs. 4.25 and 4.26. The solvent induced increase of the dipole moment amounted to 0.61 Debye in line with the results by Wei and Salahub and close to the experimental value of 0.75 Debye. The solvation enthalpy amounted —12.6 kcal/mol, while the value calculated by Salahub and Wei and the experimental ones were —10.4 kcal/mol and —9.9 kcal/mol, respectively. 

The results in the three preceding subsections conform fairly well to a consistent pattern. However, there are gaps and inconsistencies that require further thermochemical, and in some cases chemical, study. The series of solution enthalpies for the lanthanide trichlorides is satisfactory, but disagreements over the value for the enthalpy of solution of yttrium trichloride in water need resolving, and a modern value for scandium trichloride (at 25°C) would be welcome. The complete absence of enthalpies of solution of tribromides of the lanthanide elements and yttrium is regrettable, as is the lack of a value for scandium triiodide. 

Experimental Methods Measurements of specific heat and enthalpies of transition are now usually carried out on quite small samples in a Differential scanning calorimeter (DSC). DSC is applied to two different moles of analysis, of these the one is more closely related to traditional calorimetry and is described here. In DSC an average-temperature circuit measures and controls the temperature of sample and reference holders to conform to a Organisation and Qualities... 

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