Temperature jump experiments

Temperature-jump experiments showed an evident increase of the rate of transition by using methanol as solvent instead of water. According to Fig. 31, this is mainly caused by the increase of the fast kinetic phase at the expense of the following slow phase. 

In this case, we point to the fact that a fast (r < 5 s) and a slow phase have been observed in temperature-jump experiments also with the peptide Col 1-3. The slow phase - as already mentioned - has been associated with the cis-trans isomerism of peptide bonds in the direct neighborhood of the helical part. Only peptide bonds to which proline or hydroxyproline contribute their secondary nitrogen are able to assume a cry-configuration at equilibrium (cis to trans ratios of 1 40 to 1 l)l45). Therefore, the fast... 

A schematic representation of temperature and concentration profiles in a temperature-jump experiment. All scales are arbitrary, and the matter to be emphasized is that the temperature jump occurs rapidly compared with the re-equilibration reaction. 

In summary, the necessary condition for temperature jump experiments is that the equilibrium for the chemical system of interest changes with a change in temperature. The advantages of temperature jump experiments are that the perturbation is achieved by a change in a property of the solvent, a fast time resolution can be achieved, as short as picoseconds when using lasers, and a time domain over more than 6 orders of magnitude can be probed with the same technique. The disadvantage of the technique... 

The dynamics of intercalation of small molecules with DNA, groove binding and binding to specific sites, such as base pair mismatches have been studied by stopped-flow,23,80 108 temperature jump experiments,26,27,94 109 120 surface plasmon resonance,121 129 NMR,86,130 135 flash photolysis,136 138 and fluorescence correlation spectroscopy.64 The application of the various techniques to study the binding dynamics of small molecules will be analyzed for specific examples of each type of binding. 

The complexity of the binding dynamics of 1 with DNA became apparent in subsequent temperature jump experiments, where three relaxation processes were observed.112 The fastest relaxation process had a small amplitude (< 14%) and its kinetics were uncoupled from the second and third relaxation processes. This fast process was assigned to the binding of 1 to a minor site with a k+ value of 1.5x10 M s and a value of 6.9 x 10 s. This assignment was problematic because of the possible interference of artifacts for temperature jump experiments when the fluorescence detection is not performed at the magic angle,29 and this kinetic component was not observed in later studies.94,120... 

The effect of the DNA sequence dependence on the binding dynamics of 5 and 6 with ct-DNA (42% GC content) and ml-DNA (72% GC content) was investigated using laser temperature jump experiments.118 Only one relaxation process was observed for both guests, but the presence of the leveling off effect at high DNA concentration was dependent on the guest and the type of DNA. No values for the rate constants were reported in this study. 

Studies on the dynamics of complexation for guests with cyclodextrins have been carried out using ultrasonic relaxation,40 151 168 temperature jump experiments,57 169 183 stopped-flow,170,178,184 197 flash photolysis,57 198 202 NMR,203 205 fluorescence correlation spectroscopy,65 phosphorescence measurements,56,206 and fluorescence methods.45,207 In contrast to the studies with DNA described above, there are only a few examples in which different techniques were employed to study the binding dynamics of the same guest with CDs. This probably reflects that the choice of technique was based on the properties of the guests. The examples below are grouped either by a type of guest or under the description of a technique. 

Compounds 30-32 formed 2 1 complexes with CDs (Scheme 13). The formation of the 1 1 complex was fast and for this reason only one relaxation process was observed. In the cases where the 2 2 complex was present its formation was also fast and only one relaxation process for the 2 1 complex was observed in the temperature jump experiments. Since the equilibria are coupled the expression for the observed rate constant includes Kt, (and K22 when the 2 2 complex is present), k21, k2, and the concentrations of guest, 1 1 complex and CD.180 182 The values for the association and dissociation rate constants and equilibrium constants were obtained from the non-linear fit of the dependence of kobs on the total concentration of CD (Table 9). 

The association rate constants were the same within experimental error. The dissociation rate constant for 31 was however an order of magnitude larger than that for 32. The association rate constants determined with fluorescence correlation spectroscopy were similar to the rate constants determined using temperature jump experiments (see above). However, a significant difference was observed for the dissociation rate constants where, for the 1 1 complex, values of 2.6 x 104 and 1.5 x 104s 1 were determined in the temperature jump experiments for 31 and 32, respectively.181,182 The reasons for this difference were not discussed by the authors of the study with fluorescence correlation spectroscopy. One possibility is that the technique is not sensitive enough to detect the presence of higher-order complexes, such as the 1 2 (31 CD) complex observed in the temperature jump experiments. One other possibility is the fact that the temperature jump experiments were performed in the presence of 1.0 M NaCl. 

Temperature jump experiments, 172-174 cyclodextrins (CD), binding dynamics of guests binding to, 208-210 DNA, binding dynamics of guests binding to, 188-191, 192, 193, 197, 203... 

In recent years, evidence has been found that both mechanisms of proton transfer can occur for certain intramolecularly hydrogen-bonded acids. Also, new kinetic behaviour has been obtained which allows a much more detailed examination of the reaction steps in (22). Kinetic data for the second ionization of substituted phenylazoresorcinols in the presence of hydroxide ions (25) were some of the first to be obtained for an intramolecularly hydrogen-bonded acid. The reciprocal relaxation time (t ) for the approach to equilibrium in a temperature-jump experiment was measured at different hydroxide-ion concentrations. A linear dependence of x on [OH] was obtained of the form of (26) (Eigen and Kruse, 1963 Inskeep et al., 1968 Rose and Stuehr, 1971). However, careful measurements at lower hydroxide-ion concentrations (Perlmutter-Hayman and Shinar, 1975 Perl-mutter-Hayman et al., 1976 Yoshida and Fujimoto, 1977) revealed that the... 

The equations may be simplified if one of the relaxation times is much faster than the other. For example, if in equation 4.71 the first step is fast, l/r2 and k2 + k-2 may be ignored in 4.69. The value of r2 may then be obtained by substituting 4.69 and 4.70. In the case of a temperature-jump experiment, this gives... 

Fig. 2. Schematic diagram of the Raman laser temperature-jump experiment.

This prediction is confirmed by observation of a very rapid relaxation of the spin equilibrium in Co(terpy)22+ in solution. A relaxation time of less than 15 nsec was observed in a Raman laser temperature-jump experiment (14). This is consistent with the absence of any relaxation of the small excess sound absorption found in ultrasonic experiments. An upper limit of 0.2 nsec for the relaxation time in water at 298 K can be calculated from the magnitude of the excess absorption, which is... 

The function describing the change in equilibrium concentration of a given species following a sudden rise in temperature (in a so-called temperature jump experiment), has two parts, corresponding to times before and after the temperature jump (Figure 2.9). 

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