Arrhenius plot structure transitions

Effect of Solvent on Arrhenius Plots. If water is a substrate, then the presence of an organic solvent, which may disrupt the structure and/or orientation of water, may alter the Arrhenius plot. For example, a linear plot is seen with fumarate hydratase in the presence of 10% methanol. However, the plot is biphasic in the presence of 10% ethanol . See Boltzmann Distribution Collision Theory Temperature Dependency, Transition-State Theory Energy of Activation On... 

Mandelate racemase, another pertinent example, catalyzes the kinetically and thermodynamically unfavorable a-carbon proton abstraction. Bearne and Wolfenden measured deuterium incorporation rates into the a-posi-tion of mandelate and the rate of (i )-mandelate racemi-zation upon incubation at elevated temperatures. From an Arrhenius plot, they obtained a for racemization and deuterium exchange rate was estimated to be around 35 kcal/mol at 25°C under neutral conditions. The magnitude of the latter indicated mandelate racemase achieves the remarkable rate enhancement of 1.7 X 10, and a level of transition state affinity (K x = 2 X 10 M). These investigators also estimated the effective concentrations of the catalytic side chains in the native protein for Lys-166, the effective concentration was 622 M for His-297, they obtained a value 3 X 10 M and for Glu-317, the value was 3 X 10 M. The authors state that their observations are consistent with the idea that general acid-general base catalysis is efficient mode of catalysis when enzyme s structure is optimally complementary with their substrates in the transition-state. See Reference Reaction Catalytic Enhancement... 

The rate constant, k, for most elementary chemical reactions follows the Arrhenius equation, k = A exp(— EJRT), where A is a reaction-specific quantity and Ea the activation energy. Because EA is always positive, the rate constant increases with temperature and gives linear plots of In k versus 1 IT. Kinks or curvature are often found in Arrhenius plots for enzymatic reactions and are usually interpreted as resulting from complex kinetics in which there is a change in rate-determining step with temperature or a change in the structure of the protein. The Arrhenius equation is recast by transition state theory (Chapter 3, section A) to... 

In some materials, however, second-order phase transitions (or first-order transitions with very small latent heats) may occur within the same structure type. These are manifested in changing the slope ofthe Arrhenius plot (or, sometimes, in minor drops ofthe conductivity). One typical example is a distortion-type phase transition in Na3Zr2Si2POi2 from a high-temperature rhombohedral phase to low-temperature monoclinic polymorph at 180-200 °C (Figure 7.2, graph 1) established by numerous structural, calorimetric, and dilatometric studies [2, 24]. 

Microscopically, the activation energy ( ) is an energetic barrier for the elementary act of ion transport and, obviously, depends on the crystal structure, in particular, on the bottleneck size (see Section 7.2.2). Macroscopically, it is determined from the slope of the Arrhenius plot, assuming that the is independent of temperature. This assumption is not necessary true, especially in the vicinity of a phase transition when the structure changes rapidly. For example, dilatometric studies of Na3Zr2Si2. 

It is important to keep in mind that not all anomalou.. aiermal responses in cellular systems are caused by vicinal water. Many of the lipids of cell membranes also undergo abrupt structural changes, the transition temperatures of which depend on the nature of the individual lipids. This obviously complicates the task of unraveling the underlying causes of abrupt thermal anomalies. It is well to acknowledge, however, the vast number of thermal anomalies in cellular systems that do occur near vicinal water s transition temperatures reported for entirely (nonliving) lipid-free systems. An obvious approach to the study of temperature effects on any system is to establish plots of log (parameter) versus reciprocal absolute temperature. (In the case of equilibrium quantities, such a graph is referred to as a van t Hoff plot, and for rate data it is an Arrhenius plot). 

An Arrhenius-type plot of the probability constant p (= 0.693/tl/2) shows a break between —65 and —40°C. The activation energies of the a, 0 and y peaks are identical at temperatures below the structural transition of polyethylene. These activation energies and the related pre-exponential factors agree closely with those for molecular motion in polyethylene (Table 10). If oxygen is present during irradiation, the 3 and y glow peaks are completely removed at low doses and replaced by a new peak, e, at lower temperature. The thermoluminescence emission spectra of the a, y, e and probably the j3, glow peaks are identical and correspond to the emission of aliphatic aldehydes. 

In an attempt to correlate gross structure of solute-water with packing and stereochemistry, the partial molal volumes and isentropic partial molal compressibilities were measured for sugars, uronic acids, and some di- and trisaccharides in water at 25 C. Attempts to systematize the results were unsuccessful. In a different approach, the structural transitions at saturation temperature using Arrhenius plots of results obtained by conductance measurements on electrolyte-sucrose-water solution were determined. These transitions were postulated to be due to the ability of the sucrose to form intermolecular hydrogen bonds with the solvent, so that at saturation temperature it was considered to have entered the total structure of the solution. ... 

Full equilibration of ions at a known temperature in IMS allows measuring temperature-dependent rate constants for structural transitions, from which accurate activation energies and preexponential factors could be determined in an assumption-free manner using Arrhenius plots. In contrast, structural characterization techniques implemented in vacuum, such as various laser spectroscopies (threshold photoionization,photodissociation,or photoelectron spectroscopy ), MS/MS by collisional or other dissociation, or chemical reactivity studies lack a direct ion thermometer. In those methods, ion temperature is estimated as the source temperature (possibly with semiempirical adjustments) or gauged using various indirect thermometers, and vibrationally or electronically hot ions are the ever-... 

Fig. 6.5. Arrhenius plot of the characteristic times of the transition from nematic structure to smectic structure, r s, and the one from smectic structure to pre-crystalline structure, Tst...

The Arrhenius plot of these polyurethanes is representative of thermoplastics with phase-separated structure (Fig. 6.25).The data for the DC conductivity show a typicd VTFH-type behavior [Eq. (6.24)] (Tuncer et al. 2005), consistent with the coupling of the conductivity mechanism with cooperative segmental motions usually observed in linear polyurethanes and several other thermoplastics. The glass transition temperatures determined by DEA (Tg diei), DSC (TgDsc), and thermally stimulated current (Tg xsc) show very good agreement. In addition, the majority of published works on polymers [e.g., see... 

Fig. 4. Arrhenius plot of log versus l/T for plasticized PS. Three AfWs were studied at two different levels of plasticization, as shown. Slope changes indicate T>Tg transitions (see text). The structure of BzONO, the nitroxide spin probe used, is given in the inset. After Smith et al., refs. 18 and 20.

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