Temperature, effect hydrophobic interactions

What distinguishes water from ordinary organic solvents and justifies the term hydrophobic interaction is the molecular origin of the effect, being entropy driven in pure water at room temperature and resulting primarily from the strong water-water interactions. 

Effect of Temperature and pH. The temperature dependence of enzymes often follows the rule that a 10°C increase in temperature doubles the activity. However, this is only tme as long as the enzyme is not deactivated by the thermal denaturation characteristic for enzymes and other proteins. The three-dimensional stmcture of an enzyme molecule, which is vital for the activity of the molecule, is governed by many forces and interactions such as hydrogen bonding, hydrophobic interactions, and van der Waals forces. At low temperatures the molecule is constrained by these forces as the temperature increases, the thermal motion of the various regions of the enzyme increases until finally the molecule is no longer able to maintain its stmcture or its activity. Most enzymes have temperature optima between 40 and 60°C. However, thermostable enzymes exist with optima near 100°C. 

The EPR spectral data indicated that two cyanide anions bind to copper at low temperature where two cyanide anions and two histidines are present in the basal plane and the third histidine residue is present in the axial position. It has been proposed that the second cyanide anion displaces the coordinated water. Similarly, it has been proposed that the oxalate anion coordinated in a bidentate fashion and displaced the coordinated water. In case of sulfonamides, the coordination geometry is reported to be the same as that of ZnCa. 13C NMR spectroscopy was used to explore the location of C02 and HCO3 with respect to metal ion in CuCa (129,130,138). It indicated that HCO3 is bound directly to Cu (137). The affinity constant of C02 for CuCa is <1 M-1 but the paramagnetic effect is paradoxically high (130). These results indicated that C02 does not bind to a specific site but probably is attracted by the cavity either by hydrophobic interactions or by the metal ion or by both. 

The TEAC had little effect on K at 5 °C, but caused K to decline steadily with increasing TEAC concentration at 40 °C, above the transition temperature. This is as expected since at 5 °C, the gel is hydrophilic and hydrophobic interactions are insignificant even in the absence of TEAC. However, at 40 °C, hydrophobic interactions are strong, so the adding TEAC reduces them. In contrast, the... 

Hydrophobic interactions seem to have negligible effects on polymer com-plexation, since, in this case, this temperature-dependent solubility change shows a positive temperature dependence in optical T% changes. Therefore, hydrogen bonding forces are probably the primary intermolecular interactions. 

As is being discussed, polymers used to prepare micelles exhibit a LCST that can be deLned as the temperature at which the polymer phase separates (Heskinsand Guillet, 1968). Below the LCST, the polymer/micelle is soluble, but it precipitates at temperatures above the LCST. The diameter of these micelles rapidly rises at temperatures above the LCST, due to hydrophobic interactions that result in the aggregation of the micelles (Kohori et al., 1998). This effect of temperature on size was shown to be reversible, since the micellar architecture was maintained after lowering the temperature below the LCST (Chung et al., 1999). 

Before discussing the role of proteins in cold acclimation the physical effects caused by low temperature are briefly considered. A decrease of temperature leads to altered rates of enzymatic catalysis. Formation of hydrogen bonds and electrostatic interactions are thermodynamically more stable at a lower temperature whereas hydrophobic interactions are... 

A characteristic feature of the hydrophobic interaction is that it is dominated by entropy effects. Both the temperature dependence of alkane solubilities in water126,127) and direct calorimetric measurements128 show that Iihf is close to zero at room temperature. Some calorimetric data for heats of solution of hydrocarbons in water are shown in Table 3.2. A further noticeable feature is that Iihf is temperature dependent due to the rather large heat capacity, Cp F, associated with the hydrophobic interaction. From a systematic calorimetric study of a series of compounds with rather short alkyl chains129 it was found that... 

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