T-jump

Fig. XV-7. Fluorescence micrographs showing morphology of crystalline L-a-dimyris-tolphosphatidylethanolamine domains following a t jump to the plateau region of the v-a plot (a) after 2 sec b) after 1 min (c) after 20 min (d) following a second pressure jump after condition (c). (From Ref. 40.)...

Figure C3.1.3. Schematic diagram of Jouie heating T -jump apparatus for transient spectroscopy. (Adapted from French T C and Hammes G G i969 hdethods Enzymol. 16 3.)...

One can foiiow reactions of tire order of microseconds or ionger using a discharge T -jump. In a typicai exampie,... 

Fig. 2. Schematic of apparatus for temperature-jump (T-jump) measurements.

Other perturbations have been demonstrated. The pressure,, jump, similar to the T-jump in principle, is attractive for organic reactions where Joule heating may be impractical both because of the solvent being used and because concentrations might have to be measured by conductivity. Large (10 —10 kPa) pressures are needed to perturb equiUbrium constants. One approach involves pressurizing a Hquid solution until a membrane mptures and drops the pressure to ambient. Electric field perturbations affect some reactions and have also been used (2), but infrequentiy. 

FIG. 19 Scaling plot for the relaxation of the mean chain length L t) after a T-jump from Tq = 0.35 to a series of final temperatures, given as a parameter along with the respective L o s. The same Monte Carlo results [64] as in Fig. 5 are used. Full line denotes the scaling function f x = = (0.215 + 8x) . In the inset the... 

T = 0.5,0.6,0.7,0.8, and 0.9. Despite some statistical fluctuations at late times after the T-jump, it is evident from Fig. 19 that the different curves collapse onto a single one if time is scaled by a single. As for the system of rate equations, (26), we again find = (I.SSLqo) where the power 5 is determined with an accuracy of 2%. An interpolation formula for the scaling function /(jc — = (0.215 + 8jc) appears to account well... 

The most widely used transient method is the temperature-jump (T-jump) method. This is based on the van t Hoff equation, which describes the temperature dependence of the equilibrium constant. 

The sensitivity of the equilibrium constant to temperature, therefore, depends upon the enthalpy change AH . This is usually not a serious limitation, because most reaction enthalpies are sufficiently large and because we commonly require that the perturbation be a small one so that the linearization condition is valid. If AH is so small that the T-jump is ineffective, it may be possible to make use of an auxiliary reaction in the following way Suppose the reaction under study is an acid-base reaction with a small AH . We can add a buffer system having a large AH and apply the T-jump to the combined system. The T-jump will alter the Ka of the buffer reaction, resulting in a pH jump. The pH jump then acts as the forcing function on the reaction of interest. 

T-Jumps can also be produced by microwave heating and by laser pulse absorption. These methods remove the restriction to low-resistance solvents any solvent capable of absorbing energy of the applied frequency may be used. The heating time can be extremely short with laser heating. ... 

The several experimental methods allow a wide range of relaxation times to be studied. T-Jump is capable of measurements over the time range 1 to 10 s P-jump, 10 to 5 X 10" s electric field jump, 10 to 10 s and ultrasonic absorption, 10 to 10 " s. The detection method in the jump techniques depends upon the systems being studied, with spectrophotometry, fluorimetry, and conductimetry being widely used. 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

The equilibrium binding constant for this 1 1 association is Xu = ki/lLi. The Xu values were measured spectrophotometrically, and the rate constants were determined by the T-jump method (independently of the X,j values), except for substrate No. 6, which could be studied by a conventional mixing technique. Perhaps the most striking feature of these data is the great variability of the rate constants with structure compared with the relative insensitivity of the equilibrium constants. This can be accounted for if the substrate must undergo desolvation before it enters the ligand cavity and then is largely resolvated in the final inclusion complex. ... 

Other ductility behavior showed alloys with x = 1-25 and 1.54 whose ductility A (T) jumped near 300°C, passed through a maximum at about 350°C, passed through a maximum at about 350°C, and again decreased at higher test temperatures. Points in Fig. 1 correspond to temperatures of anomalous Au(T) behavior for appropriate hydrogen contents. A clear correlation is observed between the ductility anomalies and special lines in the phase diagram, it i.e., all points fall at the boundaries of the two-phase regions or at the line equidistant from these boundaries. 

A good way to think about resonance forms is to realize that a substance like the acetate ion is no different from any other. Acetate doesn t jump back and forth between two resonance forms, spending part of the time looking like one and part of the time looking like the other. Rather, acetate has a single... 

Since the pre-Socrates philosophers, who two and half thousand years ago tried to understand the world with reason, there has been a winged word, that was especially known in its Latin form Natura non saltat Leukippfith centuty BC) — nature doesn t jump. All experience has shown that in nature one thing always leads to another. Changes occur smoothly and "flow" into each other. Of course, occasionally there is some turbulence, but mostly at the edges and without any influence on the main-... 

Time-resolved IR spectra of similar peptides following a laser-excited temperature jump showed two relaxation times, unfolding 160 ns and faster components <10 ns (Williams et al., 1996). These times are very sensitive to the length, sequence, and environment of these peptides, but do show that the fundamental helix unfolding process is quite fast. These fast IR data have been contrasted with Raman and fluorescence-based T-jump experiments (Thompson et al., 1997). Raman experiments at various temperatures have suggested a folding in 1 /xs, based on an equilibrium analysis (Lednev et al., 2001). But all agree that the mechanism of helix formation is very fast. 

Much of the achieved advances result from the development and availability of instrumentation to study slow and fast reactions at pressures up to 300 MPa, including stopped-flow, T-jump, P-jump, NMR, ESR, flash-photolysis, and pulse-radiolysis instrumentation (1, 2, 4, 6, 7). Readers are advised to consult the quoted references for more detailed information, since these present a detailed account of the present instrumentation and commercial availability of such equipment. 

The strength of a solvent bond influences the rate of solvent substitution in a given compound. Kinetic measurements by means of the T-jump relaxation technique have illustrated that for the reactions of the solutions of SbCl5 with triphenylchloromethane in different solvents a relationship exists between the rate constant and the donicity of the solvent used. 

N Col/2 ch n nhch3 3.2xl06 9.lxlO6 25°, CH CN/ ch3oh Laser Raman T-Jump 43... 

Fe(6-Mepy)(py),-. 2+ z tren 4xl05 8x10 20o,H20 Laser Raman T-jump 44... 

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