Temperature-dependent changes

The time/temperature-dependent change in mechanical properties results from stress relaxation and other viscoelastic phenomena that are typical of these plastics. When the change is an unwanted limitation it is called creep. When the change is skillfully adapted to use in the overall design, it is referred to as plastic memory. 

The mobility of gases in the firn column leads to temperature-dependent changes in the composition of gas trapped in ice at the base of the fim (Severinghaus et ah, 1998). If there is a rapid change in climatic temperature at the ice-sheet surface, a steep temperature gradient will temporarily exist throughout the firn. This will temporarily cause thermal fractionation of gas-... 

Diffusion-mediated release of root exudates is likely to be affected by root zone temperature due to temperature-dependent changes in the speed of diffusion processes and modifications of membrane permeability (259,260). This might explain the stimulation of root exudation in tomato and clover at high temperatures, reported by Rovira (261), and also the increase in exudation of. sugars and amino acids in maize, cucumber, and strawberry exposed to low-temperature treatments (5-10°C), which was mainly attributed to a disturbance in membrane permeability (259,262). A decrease of exudation rates at low temperatures may be predicted for exudation processes that depend on metabolic energy. This assumption is supported by the continuous decrease of phytosiderophore release in Fe-deficient barley by decreasing the temperature from 30 to 5°C (67). 

The 1,2-dimethoxynorbomyl ion studied by Nickon and Liu (1969) has also been deduced to be classical on the basis of temperature-dependent changes in the nmr spectrum. Whether the behaviour of methoxy derivatives is much related to that of alkyl derivatives is, however, not clear because much of the positive charge is expected to be delocalized to the oxygen atom of the substituent. This might be expected to provide an unusual stabilization to the classical structure. 

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

Symbol for the temperature coefficient, a quotient equal to Vt+wIvt, where Vr+io and Vj are the rates of a process (e.g., an enzyme-catalyzed reaction) at two temperatures differing by 10°C. This parameter is usually evaluated at saturating concentrations of substrate(s), so that temperature-dependent changes in Michaelis constant(s) are inconsequential. The <2io value is a characteristic property of a particular enzyme from a specific organism and cell type. For example, one cannot use the Qio value for one hexokinase from yeast to infer the temperature dependence of another hexokinase, say from rat brain. Likewise, the Qio value need not remain the same for a mutant form and a wild-type enzyme. 

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). 

In the majority of papers, the kj of enzyme at one temperature is assumed to be a measure of thermostability. Moreover, the dependence of log kj on 1/T (Arrhenius plot) is only valid over a narrow temperatnre range. In general, when temperature dependence changes, one can observe (Ugarova, Rozhkova and Berezin, 1978) ... 

Muller first noted field desorption for layers of barium on tungsten (1). He concluded correctly that tunneling could hardly be responsible for the ionization of heavy particles and assumed that the potential curve for the ad-atom substrate complex was deformed to the point where activated desorption over the barrier could take place. This view was supported by the fact that he found that the field necessary for the desorption of thorium was quite temperature dependent, changing from the (remarkably low) value of 6.7 X 10 v./cm. at room temperature to 3.5 X 10 v./cm. at 1500°K (1). Similar results were found by him for barium... 

The rate of this exchange increases with increasing temperature, and as the temperature of the compound is raised, the two separate NMR peaks gradually broaden and ultimately merge at 180°C to form a single peak. Study of this temperature-dependent change yields a rotational barrier of 23 kcal/mole. 

Temperature Effects The pH of a buffer solution is influenced by temperature. This effect is due to a temperature-dependent change of the dissociation constant (pK ) of ions in solution. The pH of the commonly used buffer Tris is greatly affected by temperature changes, with a ApKa/C° of —0.031. This means that a pH 7.0 Tris buffer made up at 4°C would have a pH of 5.95 at 37°C. The best way to avoid this problem is to prepare the buffer solution at the temperature at which it will be used and to standardize the electrode with buffers at the same temperature as the solution you wish to measure. 

Parameters in Compensation Equation Calculated by Making Appropriate Allowances for the Occurrence of Temperature-Dependent Changes in Concentration of Surface Reactants ... 

Figure 16.6 (a) The temperature-dependent change observed in the optical absorbance A (at A = 260 nm) of a nucleic acid during its structural change, and the quantities, jc and y used to calculate a from the curve (b) the temperature-dependent change in o as the reaction proceeds. Tm is the temperature at which the mid-point of the reaction is observed. 

The d(CGCGAATTCGCG)2 Duplex. When studied at comparable salt (NaCl) and duplex concentrations, the duplex of d(CGCGAATTCGCG) was found to have about the same Tm by DSC measurements (344 K) and by the temperature-dependent changes in the NMR chemical shifts of nonexchangeable GC and AT protons (345 K). At 0.01 M NaCl, the calorimetric enthalpy was 376 kJmol-1 of duplex, while the van t Hoff enthalpy evaluated from equation (16.28) was found to be 393 kJ-mol-1 of duplex. At a 10-fold higher salt concentration, the calorimetric enthalpy was 427 kJ-mol-1 of duplex, while the van t Hoff enthalpy was 310 kJ-mol-1 of duplex. From the ratio of the van t Hoff enthalpy to the calorimetric enthalpy at the higher salt concentration, it was estimated that 9 out of 12 base-pairs melt together in the cooperative unit, while all melt simultaneously at the low salt concentrations. 

An examination of the time - resolved spectra can be made, which provides additional details about the temperature - dependent changes in the nature of the hydrocarbon - bath interface during the detergency process. 

NCS- or NCSe-. The complexes were characterized, by IR and electronic spectra together with electrical conductance measurements, as having neutral (probably cis) six-coordinate structures, in which the organic ligands as well as the pseudohalide anions are coordinated via the nitrogen atoms. Four of the seven complexes under study177) show temperature dependent changes of the spin states, sT2(Oh) ... 

The authors have interpreted their data using a statistical model, which reveals that the energy gap between the 1 Aj ground state and the lowest of the spin-orbitally split ST2 levels is temperature dependent the form of this temperature dependence changes with x. The model includes spin-orbit coupling, low-symmetry field distortion, covalency, and dynamic as well as static effects of local ligand vibrations. 

Viscotropic effects in cells cultured at a single temperature may be indicative of effects that result from immediate temperature-dependent changes in lipid order and from subsequent adaptive alterations in lipid composition. Acute decreases in temperature increase static order and thereby increase resistance to conformational movements however, the compensatory decrease in intrinsic membrane order in cold-adapted or cold-acclimatized organisms (figure 7.21) resulting from changes in composi-... 

A closer look at the properties of the bilayer shows that certain characteristics of the solution bathing a membrane are likely to influence its physical state and, thereby, affect its sensitivity to temperature. Although these extrinsic effects remain much less studied than effects involving changes in lipid composition, there is increasing evidence that temperature-dependent changes in the intra- and extracellular fluids affect membranes as well as cytosolic proteins. 

PROBLEM L1.2 If you take the kT factor too seriously, then it looks as though van der Waals interactions increase linearly with absolute temperature. Show that, for contributions from a sampling-frequency range A over which A s change little, there is little change in van der Waals forces with temperature, except for temperature-dependent changes in the component e s themselves. 

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