Large Negative Heat Capacity Research Articles

Role of the hydrophobic effect in stability of site-specific protein-DNA complexes

The site-specific binding interaction of lac repressor with a symmetric operator sequence and of EcoRI endonuclease with its specific recognition site both exhibit a characteristic dependence of equilibrium binding constant ( K obs) on temperature, in which K obs attains a relative maximum in the physiologically relevant temperature range. This behavior, which appears to be quite general for site-specific protein-DNA interactions, is indicative of a large negative standard heat capacity change ( ΔC P, obs 0) in the association process. By analogy with model compound transfer studies and protein folding data, we propose that this Δ P, obs 0 results primarily from the removal of non-polar surface from water in the association process. From ΔC P, obs 0 we obtain semiquantitative information regarding the change in water-exposed non-polar surface area ( ΔA np) and the corresponding hydrophobic driving force for association ( ΔG hyd 0): ΔG hyd 0 ≅ 8 (± 1) × 10 1 ΔC P, obs 0 ≅ −22(±5) ΔA np. We propose that removal of non-polar surface from water (the hydrophobic effect) and release of cations (the polyelectrolyte effect) drive the thermodynamically unfavorable processes (e.g. conformational distortions) necessary to achieve mutually complementary recognition surfaces (at a steric and functional-group level) in the specific complex.

Thermodynamic properties of the benzene-phenol dimer in dilute aqueous solution

Precise vapor pressure-solubility measurements have been obtained for dilute solutions of benzene and phenol in water at 15, 25, 35 and 45°C. All of the results are consistent with a mass action model that attributes deviations from ideal solution behavior to the formation of benzene dimers and hetero-dimers between benzene and phenol. The benzene-phenol dimer forms endothermically at 25°C, with a very large negative heat capacity, and a formation constant that reaches a maximum value of 0.66 l-mol−1 at approximately 35°C. Thermodynamic properties of the benzene-phenol dimer are reported and compared with those of other aggregates that are believed to be stabilized primarily by hydrophobic interactions.

Transient-phase energetics of the oxidative deamination of L-glutamate by L-glutamate dehydrogenase and NADP: a reaction with a large negative heat capacity of activation.

AbstractA study of the temperature dependence of the initial transient velocities of the oxidative deamination of L‐glutamate by glutamate dehydrogenase and NADP has permitted the determination of the enthalpies, entropies, and heat capacities of the phenomenological parameters for mechanistic steps up to and including the catalytic hydrogen‐transfer step. The most striking feature observed is the extremely large negative heat capacity of activation (−590 ± 150 cal K−1 mol−1) for the catalytic step itself. Possible sources of such heat capacities are considered, with particular focus on the effects of hidden equilibria among multiple thermodynamic states. Heat‐capacity changes of the sort observed here can be produced by shifts of such dynamic equilibria between multiple forms of the free enzyme induced by the formation of the catalytic transition state.

Thermodynamics of microtubule assembly

The temperature dependence of microtubule assembly has been the subject of much controversy. Curved van't Hoff plots obtained from birefringence data on the mitotic spindle led Inoue and Sato to conclude that the condensation mechanism did not accurately describe the assembly equilibrium in vivo. Subsequent work in vitro has established the validity of the condensation mechanism, but nonlinearity of the van't Hoff plots has been interpreted by a number of different models: (a) a contribution of microtubule nucleating species to the apparent critical concentration; (b) a gross conformational change in the microtubule lattice; and (c) a large negative heat capacity change. In this paper, data will be described which show that the curvature of the van't Hoff plot is solely a function of the microtubule disassembly reaction. The significance of this observation in terms of the mechanisms of assembly and disassembly will be discussed.

Thermodynamics of gelation of sickle cell deoxyhemoglobin

This paper describes the thermodynamic behavior of gels of deoxyhemoglobin S. The solubility of the protein with respect to assembled hemoglobin fibers has been measured using a sedimentation technique. The solubility in 0.15 m-potassium phosphate buffer (pH 7.15) is found to decrease with increasing temperature, attain a minimum value of 0.16 g cm −3 at 37 °C, and then increase at higher temperatures. The amount of polymer present at various hemoglobin concentrations and temperatures is presented as part of a phase diagram that may be useful for the calibration of other measurement techniques. The effects of varying pH and urea concentration upon the solubility have also been studied. The heat absorption accompanying gelation has been measured by scanning calorimetry. Using sedimentation data on the amount of polymer formed, molar enthalpy changes are obtained. There is a large negative heat capacity change of − 197 cal deg. mol −1 and ΔH = 0 near 37 °C. Calorimetric molar enthalpy changes are found to agree with those calculated from the temperature dependence of the solubility by the van't Hoff equation. Our previous two-phase, two-component thermodynamic model of gelation is extended to include the effects of solution non-ideality. A large contribution to the activity of the hemoglobin in the solution phase results from the geometric effect of excluded volume. Incorporating solution phase non-ideality permits the calculation of standard state thermodynamic quantities for the gelation process at 37 °C: ΔG O ≅ −3 k cal mol −1, ΔH O ~ 0, ΔS O ~ 10 cal deg. −1 mol −1. The excluded volume effect is also capable of explaining observations of the minimum gelling concentrations of hemoglobin mixtures containing deoxyhemoglobin S without requiring copolymerization of the non-S hemoglobin.