Comparative Thermodynamic Analysis Research Articles

Oxidative modification of Escherichia coli glutamine synthetase. Decreases in the thermodynamic stability of protein structure and specific changes in the active site conformation.

Metal catalyzed oxidation of specific amino acid residues has been proposed to be an important physiological mechanism of marking proteins for proteolytic degradation. After initial oxidative inactivation of dodecameric Escherichia coli glutamine synthetase (GS), the integrity of the GS active site and protein structure was assessed by monitoring ATP binding, observing a susceptibility of GS to tryptic cleavage, and comparative thermodynamic analysis. The tryptic cleavage rates of an active site linked central loop were significantly accelerated for the oxidized conformer. This tryptic cleavage was essentially prevented in the presence of glutamate for native GS but not for the oxidized conformer. The integrity of the ATP binding site in the oxidized GS was substantially altered as indicated by the reduction in fluorescence enhancement associated with ATP binding. Decreases in the free energies of quaternary protein structure and subunit interactions due to oxidative modification were determined by temperature and urea induced unfolding equilibrium measurements. Comparative thermal stability measurements of a partial unfolding transition indicated that the loss in stabilization free energy for the oxidized GS conformer was 1.3 kcal/mol dodecamer. Under alkaline conditions, the urea-induced disruption of quaternary and tertiary structures of oxidized and native GS were examined. This comparative analysis revealed that the free energies of the subunit interactions and unfolding of the dissociated monomers for oxidized GS were decreased by 1.5 and 1.7 kcal/mol, respectively. Our results suggest that small free energy decreases in GS protein structural stability of only 1-2 kcal/mol may be responsible for the selective proteolytic turnover of the oxidized GS.

Choice of vapour-compression heat pump working fluids

The paper presents a comparative thermodynamic analysis of seven candidate vapour-compression heat pump working fluids, namely R-11, R-12B1, R-21, R-113, R-114, R-142b and R-216. The basis for evaluation is the temperature-boost/pressure-penalty ratio and the heat of vaporization. Four of the above refrigerants are covered under the Montreal Protocol. It is observed that R-21 and R-216 are effective alternatives for low grade heat recovery and R-142b for comfort heating. The fluids are compared for ideal operating temperature limits of each refrigerant as well as a fixed range of 30 to 100°C.

Raising the efficiency of open-throttle liquefiers

This paper makes a comparative thermodynamic analysis of certain open-throttle liquefier schemes that operate with multicomponent cryogenic agents. The most promising routes for implementing their advantages are determined. It is found that the correct choice of flow diagram and complex parameter optimization can raise the relative available energy (mass profile characteristics) of open-throttle liquefiers with mixtures to at least four times that of nitrogen installation. The most economical scheme is one that involves mixing the components in feedback, followed by double throttling of nitrogen.

Thermodynamics and mechanisms of the deformation of thermoelastoplastic block copolymers

Thermodynamic aspects and mechanisms of the deformation of thermoelastoplastic block copolymers of the styrene-diene, segmented polyurethane and polyester types are discussed. Deformation calorimetry was used to study energy and entropy effects resulting from the deformation and small angle X-ray scattering method was used to obtain information concerning structural changes during deformation. The largestrain tensile deformation of thermoelastoplastics which possess domain structure leads to a “plasticrubber” transition (stress softening effect), which under appropriate time and temperature conditions is reversible. A comparative thermodynamic analysis of the stress softening in block copolymers and filled rubbers as carried out and results dealing with recovering kinetics of initial domain morphology in stress-softened block copolymers are discussed. Particular attention is paid to the structural changes and thermodynamics of deformation of “single crystal” morphology, which can arise in diene-styrene block copolymers under appropriate thermo-mechanical treatment. The role of the chemical nature of soft and hard blocks as well as the morphology and size of hard blocks in thermodynamics and mechanisms of the deformation is analyzed. The problem of the limited extensibility of soft block polymer chains at high deformations is also discussed.

Comparative thermodynamic analysis of plasmochemical synthesis of aluminum and yttrium oxides

A thermodynamic calculation of the dependence of the end-product yield and specific enthalpy on the pressure and temperature is performed for the plasmochemical synthesis of aluminum and yttrium oxides.

A comparative thermodynamic analysis of InP and GaAs deposition

A PCl 3 transport system for epitaxial InP deposition has been considered from an equilibrium point of view and compared with a similar arsenic trichloride process for GaAs deposition. Thermochemical values for all species pertinent to these two systems were compiled and employed in conjunction with an iterative Gibbs energy minimization program to compute the gas phase equilibrium concentrations and the maximum extent of deposition. The results are presented as functions of three independent variables: the source and deposition temperatures and the initial concentration of Group III trichloride. The degree of gas phase supersaturation is presented as a function of the temperature differential between the source and deposition regions.