Temperature jump techniques

In the study of reactions of the types ether than exchange mentioned previously, the usual technique involves the spectrophotometric examination of reaction mixtures. The absorbance changes that occur, at a suitable wavelength where only one species (either reactant or product) absorbs, as the reaction proceeds are measured (manually or recorded). Treatment of the data via the Beer-Lambert law enables rate coefficients and laws to be found in the usual manner. Stopped flow and temperature jump techniques have been used for very rapid reactions. 

The first experimental data for a reaction involving proton transfer from a hydrogen-bonded acid to a series of bases which were chosen to give ApK-values each side of ApK=0 are given in Fig. 15 (Hibbert and Awwal, 1976, 1978 Hibbert, 1981). The results were obtained for proton transfer from 4-(3-nitrophenylazo)salicylate ion to a series of tertiary aliphatic amines in aqueous solution, as in (64) with R = 3-nitrophenylazo. Kinetic measurements were made using the temperature-jump technique with spectrophoto-metric detection to follow reactions with half-lives down to 5 x 10"6s. The reciprocal relaxation time (t ), which is the time constant of the exponential... 

The kinetics of proton transfer from protonated 1,8-bis(dimethyl-amino)-2,7-dimethoxynaphthalene to substituted phenolate ions (69) were studied in 70% (v/v) Me2SO—H20 using the temperature-jump technique with spectrophotometric detection to follow reactions with half-lives in the range 1-100 ms (Hibbert and Robbins, 1978). A limited... 

Results which have an important bearing on the mechanism of proton removal from intramolecular hydrogen bonds have been obtained quite recently for the second ionisation of phenylazoresorcinols (73). Kinetic measurements were made using the temperature-jump technique (Perlmutter-Hayman and Shinar, 1975 Perlmutter-Hayman et al., 1976). 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

The tautomerisation of the N(9)H- and N(7)H-isomers of adenine (93) has been studied in aqueous solution using the temperature-jump technique with Joule heating (Dreyfus et al., 1975). The equilibrium constant K = [N(7)H]/[N(9)H] has a value of 0.28 at 20°C. The reciprocal relaxation time... 

The observation of a single set of resonances in the NMR spectra of [Fe(HB(pz)3)2], spectra that are clearly obtained for a mixture of the high-spin and low-spin forms of the complex, indicates that the equilibrium between the two states is rapid on the NMR time scale [27]. Subsequent solution studies by Beattie et al. [52, 53] using both a laser temperature-jump technique and an ultrasonic relaxation technique have established that the spin-state lifetime for [Fe(HB(pz)3)2] is 3.2xl0 8 s. These studies also established... 

Preliminary results (obtained by use of the temperature jump technique) indicate that the reactions are fast (at c = 10 2 relaxation time faster than 50 /usee.) 56> and are of second order. The rate coefficients k are again a function of the donicity, whereas the rate coefficients of the reversed reactions k2 show no relation to the donicity S6 ... 

Fueno et al., 1973). The reactions were studied using the temperature-jump technique. 

Apart from the temperature-jump technique, other relaxation methods that have been used are those of ultrasonic absorption" " and electric-field pulse. Another technique that has been used for some of the more slowly included guest molecules is that of stopped-flow. ... 

Further support for the presence of a conformational change has come from the work of Orstan and Wojcik, who also studied azo dye-alpha cyclodextrin complexes by using the temperature-jump technique. The dyes that they investigated showed three different types of behavior. Certain dyes showed a limiting value of 1/t at high alpha cyclodextrin concentrations, others showed a linear dependence of 1/t on alpha cyclo-... 

Pyronine Y (PY) with beta cyclodextrin by using the temperature-jump technique. It was found that the kinetic data could be adequately explained by a mechanism involving the stepwise association of two beta cyclodextrin molecules (C) to the dye. Thus,... 

Mechanisms for the formation of [W60i9(0H)3] (paratungstate A) have been discussed. The relaxation spectra of aqueous molybdate have been determined by the temperature-jump technique at 25 °C(/i = 1.0 M, pH = 5.5— 6.8, and monomer concentration = 0.01—0.25 M). The results indicate two concentration-dependent relaxation effects, which are sensitive to heptamer, [Mo7 0 24] , and octamer, [Mog026]", formation. ... 

Raman earth halide clusters, 46 2-3 Raman laser temperature-jump technique, 32 17-18... 

For very rapid reactions such as the ionization of H2O, it is difficult to determine the rate constants using conventional methods. One often-used method is the relaxation method. The system is initially at equilibrium under a given set of conditions. The conditions are then suddenly changed so that the system is no longer at equilibrium. The system then relaxes to a new equilibrium state. The speed of relaxation is measured, usually by spectrophotometry, and the rate constants can be obtained. One technique to change the conditions is to increase temperature suddenly by the rapid discharge from a capacitor. This technique is called temperature-jump technique. 

Where there is no subsequent turnover of a substrate, such as occurs on the omission of a cosubstrate in a multisubstrate reaction, or on inhibitor binding, the temperature-jump technique is generally the most useful tool for the determination of these constants. 

A major advance in the investigation of the intramolecular dynamics of spin equilibria was the development of the Raman laser temperature-jump technique (43). This uses the power of a laser to heat a solution within the time of the laser pulse width. If the relaxation time of the spin equilibrium is longer than this pulse width the dynamics of the equilibrium can be observed spectroscopically. At the time of its development only two lasers had sufficient power to cause an adequate temperature rise, the ruby laser at 694 nm and the neodymium laser at 1060 nm. Neither of these wavelengths is absorbed by solvents. Various methods were used in attempts to absorb the laser power, with partial success for microsecond relaxation times. 

The Raman laser temperature-jump technique has been used in studies of a variety of spin-equilibrium processes. It was used in the first experiment to measure the relaxation time of an octahedral spin-equilibrium complex in solution (14). Its applications include investigations of cobalt(II), iron(II), iron(III), and nickel(II) equilibria. 

The dynamics of an octahedral spin equilibrium in solution was first reported in 1973 for an iron(II) complex with the Raman laser temperature-jump technique (14). A relaxation time of 32 10 nsec was observed. Subsequently, further studies have been reported with the use of this technique, with ultrasonic relaxation, and with photoperturbation. Selected results are presented in Table III. 

A number of observations indicate interconversion rates for some iron(III) complexes too fast to measure with existing techniques. Relaxation times less than the 30-nsec limit set by the heating rise time of the laser temperature-jump technique were observed for [Fe(benzac2trien)]+, [Fe(Salmeen)2]+, and [Fe(Me2dtc)3] (45, 128). From ultrasonic observations a limit of less than 1 nsec was placed on the relaxation time for the first of these compounds,... 

The addition and dissociation of pyridine and substituted pyridine molecules to a planar nickel(II) complex with a quadridentate N202 ligand have been studied by the microwave temperature-jump technique in chlorobenzene solvent (38). The data were interpreted with the assumption of mechanism C (Fig. 7), i.e., that k65 is the smallest rate constant. Subsequently, however, 14N NMR was used to measure the rate of pyridine exchange from the octahedral complex (138). The rates are the same for the two different experiments within a factor of two. This observation excludes mechanism B and is consistent with either mechanism A or C. The rate constants have consequently been presented in Table VI as k64. 

Holzwarth, J. F., A. Schmidt, H. Wolff, and R. Volk. 1977. Nanosecond temperature-jump technique with an iodine laserJ. Phys. Cherrfil 2300-2301. 

Robinson, B. H. (1975). The stopped-flow and temperature-jump techniques-principles and recent advances. In Chemical and Biological Applications of Relaxation Spectrometry (E. Wyn-Jones, ed.), pp. 41-48. Reidel Publ., Dordrecht, The Netherlands. 

Robinson, B. H. (1975). The stopped-flow and temperature-jump techniques—principles and... 

Temperature-jump techniques have been employed to investigate the mode of formation of polyborates (27). The reaction... 

The result of Fe +Cl O) 5(NO) is in good agreement with that determined by Kastin et al. (15) using the same experimental technique. For both Fe2+(EDTA)(NO) and Fe2+(NTA)(NO), the relaxation times due to the temperature jump were too fast to be measured. However, an upper limit of 10 /is was established for the relaxation times for both complexes. By use of this value with the equilibrium constants determined for Fe2+(EDTA)(NO) (16) and Fe2+(NTA)(NO) (10), the lower limits of formation rate constants were calculated to be 7 x 10 and 6 x 107 Z/nol -sec at 35 °C, which is in good agreement with that determined by the temperature-jump technique. From the results listed in Table I, we can conclude that the formation rate of Fe2+(EDTA)(NO) is at least 85 times faster than that of Fe2+(H20)5(N0), whereas, the dissociation rate of Fe2+(EDTA)(NO) is about 250 times slower than that of Fe2+(H20)5(N0) at 25 °C. 

T2(Oh) 1A1(Oh) spin equilibrium in iron(II) complexes based on the hy-drotris(l-pyrazolyl)borate ligand, was established for [Fe(HB(me-pz)3)2] (for abbreviations see Sect. 8.1) in the solid state 172>173 and for [Fe(HB(pz)3)2] in solution174 . Sutin et al.17S studied the dynamics of the spin interconversion in CH2C12/CH30H solutions with the laser Raman temperature-jump technique between 0 and 25 °C. The relaxation was observed to be first order with a lifetime of 32 10 ns, independent of temperature and concentration over the range studied. The ki and k j values for the process... 

Various kinetic methods in the enzyme catalysis has been described elsewhere (Likhtenshtein, 1988a Gates, 1991 Bugg, 1997 Comish-Bowden, 1995, 2001 Varfolomeev and Gurevich, 1998) Fersht,1999 Gutfreund,. 1995 Hammes, 2000 Leninger et al.1993 ). In this section we concentrate on recent developments in methods of the kinetic isotope effect, transition state analoges, and nanosecond temperature jump techniques. 

In general, cation-pseudobase equilibration is quite rapid in aqueous solution, and the use of the stopped-flow or temperature-jump techniques is usually required for the measurement of the rates of pseudobase formation from, and decomposition to, the cation. The presence of a large substituent, such as a phenyl ring, at the site of hydroxide ion addition does slow the rate of equilibration sufficiently to allow kinetic measurements by normal spectrophotometric techniques.92... 

The redox characteristics of these azaviolenes and of analogs with polyene linkages between the heterocycles have been determined by Hiinig and co-workers.462,596 The rate constant for the comproportionation reaction yielding 180 (X = CH) was evaluated as (6.5 2.0) x 108 M l sec-1 by Bennion et al.597 using the temperature-jump technique. 

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