Hydrophobicity temperature dependence

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 simplicity and accuracy of such models for the hydration of small molecule solutes has been surprising, as well as extensively scrutinized (Pratt, 2002). In the context of biophysical applications, these models can be viewed as providing a basis for considering specific physical mechanisms that contribute to hydrophobicity in more complex systems. For example, a natural explanation of entropy convergence in the temperature dependence of hydrophobic hydration and the heat denaturation of proteins emerges from this model (Garde et al., 1996), as well as a mechanistic description of the pressure dependence of hydrophobic... 

The temperature dependence of solutions of an NIPAM-styrene copolymer, PNIPAM-seg-St, of Mw = 13.3 x 106gmol 1 in which hydrophobic St segments were evenly spaced along the chain was investigated under high di-... 

Additional information on the solvation layer around the polymers was obtained by PPC (Sect. 2.1), a technique that allows one to evaluate the changes in the partial volume of the polymer throughout the phase transition, and to obtain information on the temperature-dependant relative hydrophilicity/hydrophobicity of a polymer in solution [210]. Particular interest in the PPC studies was focused on the effect of the amphiphilic grafts on the volumetric properties of the polymers. 

Efforts at synthesis and studies of temperature-dependent solution behaviour of these chemically hydrophobized polyacrylamides are now in progress. However, it is reasonable to point out that in this case, contrary to the hydrophilization of the hydrophobic precursor, the problems associated with additional swelling of the globular core (as the modification proceeds) are absent however, the problem of the choice of working concentration for the precursor is still present since above the coil overlapping concentration the intermolecular aggregation processes at elevated temperatures can compete with the intramolecular formation of core-shell structures. 

Free energy variations with temperature can also be used to estimate reaction enthalpies. However, few studies devoted to the temperature dependence of adsorption phenomena have been published. In one such study of potassium octyl hydroxamate adsorption on barite, calcite and bastnaesite, it was observed that adsorption increased markedly with temperature, which suggested the enthalpies were endothermic (26). The resulting large positive entropies were attributed to loosening of ordered water structure, both at the mineral surface and in the solvent surrounding octyl hydroxamate ions during the adsorption process, as well as hydrophobic chain association effects. 

Based on the solution property of poly (DMAEMA-co-AAm) in response to temperature, the temperature dependence of equilibrium swelling of poly (DMAEMA-c6>-AAm) gel as a function of chemical composition was observed as shown in Figure 6. The transition temperature of copolymer gel between the shrunken and swollen state was shifted to the lower temperature with increases in AAm content in the gel network. This is attributed to the hydrogen bond in the copolymer gel network and its hydrophobic contribution to the LCST Copolymer II gel was selected as a model polymer network for permeation study because it showed the sharp swelling transition around 34°C. 

Hovorka, S., Dohnal, V., Roux, A.H., andRoux-Desgranges, G. Determination of temperature dependence of limiting activity coefficients for a group of moderately hydrophobic organic solutes in water. Fluid Phase Equilib., 201(1) 135-164, 2002. 

The temperature dependence (296-330 K) of the ring methyl proton resonances of these monomeric heme complexes in the hydrophobic micellar cavity shows [22] a small deviation from the Curie law as in the low-spin complexes in organic and simple aqueous solvents [1, 52]. The origin of such deviation has been variously ascribed [3, 1, 53] either to aggregation or second order Zeeman (SOZ) effect or presence of low-lying spin-quartet state. Since these low-spin hemes in micellar solutions are in deaggregated form, the deviation may be due to the SOZ and/or presence of low-lying excited state. 

Thus, the PEO segment actually becomes hydrophobic at higher temperatures. This temperature-dependent change converts the amphiphilic block copolymer to a water-insoluble hydrophobic polymer (Topp et al. 1997 Chung et al. 2000). The temperature at which the polymer exhibits this transition is called its lower critical solution temperature (LCST). In addition to PEO, substituted poly(A -isopropyl acrylamide) (PNIPAM Chart 2.1) exhibits temperature sensitivity, where the LCST can be tuned by varying the alkyl fimctionahty. The guest encapsulation combined with the temperature-sensitive precipitation of the polymers has been exploited to sequester and separate guest molecules from aqueous solutions (Fig. 2.4). 

Figure 19.1 shows the temperature-dependent volume-phase transitions of NIPA-based hydrogels. It is suggested that formation of a complex between bioactive molecules and functional groups of hydrogels is responsible for immobilization of the former. The lower critical solution temperature (LCST) of PNIPA can be tuned to the required value by introducing hydrophobic or hydrophilic fragments. 

LB films prepared from tridecylmethyl-ammonium Au-(dmit)2 and H2dmit = 4,5-dimercapto-l,3-dithiol-2-thione, transferred to hydrophobized glass substrates, and oxidized (by Br2 or electrochemicaily) Absorption spectra and temperature-dependent conductivity measurements... 

Saito and co-workers [31]. It is interesting to observe that gels swell at lower temperatures and collapse at higher temperatures. This temperature dependence, which is opposite to the transition induced by van der Waals interaction, is due to the hydrophobic interaction of the polymer network and water. At higher temperatures the polymer network shrinks and becomes more ordered, but the water molecules excluded from the polymer network become less ordered. As a whole, the gel collapse amounts to a higher entropy of the entire gel system, as should be. Detailed theory and experiments have been carried out in the literature [26-28]... 

Nemethy and Scheraga [19] quantitatively investigated the temperature dependence of the hydrophobic interaction between molecules in water, and presented the following free energy equation for the temperature range of 0-70°C ... 

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