Arrhenius plot phase changes

They observed abrupt changes in the slope of Arrhenius plots for reactions catalyzed by NADH oxidase and p-lactate oxidase that correlate well with phase transitions detected by the ESR spectra of the nitroxide spin labels bound covalently to the enzymes (Table 5.4). 

Figure 11.5. The magnitude of ko s decreases with decreasing temperature until 170 K, whereupon it reaches a value of 3.2 x 10 s. Below this temperature, koBs remains constant. " The breakpoint in the Arrhenius plot is 180-200 K, which is in exactly the same temperature range in which the solution phase chemistry changes from the trapping of ketenimine 30 with diethylamine to the dimerization of 33t. Thus, the low-temperature data of Figure 11.5 were associated with k]sc, the rate constant for intersystem crossing of singlet to triplet phenylnitrene, and the high temperature data with k., the rate constant for rearrangement of 33t.

There is much evidence, usually based upon ESR studies with spin labels, for conformational changes in the protein during the enzyme cycle. Enzyme activity shows a discontinuity in the Arrhenius plot, once attributed to phase transitions in the membrane phospholipid, but which is similar to discontinuities in the Arrhenius plot for rotational mobility and which have been rationalized in terms of a conformational change in the ATPase.142... 

Arrhenius plots should be examined carefully because plots may be curved or show discontinuities in slope. The temperatures at which such discontinuities occur may be significant in interpreting behaviour of the reactant, e.g., there may be a phase change of the reactant within the temperature range being investigated. 

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. 

The above method is only vahd if the formulation viscosity t] follows the Arrhenius equation, which predicts a linear increase in Ini with (1/T), where T is the absolute temperature. Most practical formulations do not follow such a plot due to the possible phase changes or flocculation that may occur at high temperatures. With many surfactant systems, such phase changes may result in... 

The photo-Fries reaction occurs readily in solid polymers and is observable in phenyl esters, particularly in poly(phenyl acrylate) and poly (phenyl methacrylate) and their derivatives. The course of the reaction can be followed very easily by ultraviolet spectroscopy, since the product hydroxy ketones have strong absorbance at 260 and 320 nm (Fig. 10). Reaction occurs with equal efficiency in small model compounds in solution and in the polymers in the solid phase (44). An Arrhenius plot of the quantum yield for the para product (Fig. 11) shows a linear increase up to 294 K, above which no further change in quantum efficiency is observed, either above or below the glass transition temperature. 

J. Kumamoto, J. Raison, and J. Lyons, Temperature Breaks in Arrhenius Plots A Thermodynamic Consequence of a Phase Change, J. Theor. Biol. 31, 47-51 (1971). 

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