Kinetic relaxation measurements

Young RC, Meyer TJ, Whitten DG (1975) Kinetic relaxation measurement of rapid electron transfer reactions by flash photolysis. The conversion of light energy into chemical energy using the Ru(bipy)3 A Ru(bipy)3 couple. J Am Chem Soc 97 4781-4783... 

This model does not say anything about the mechanism of triple-helix formation, because even in the case of an AON mechanism, nucleation may take place at many positions of the chains and may lead to products the chains of which are staggered. The AON model is based on the assumption that these products are too instable to exist in measurable concentration. As already mentioned, Weidner and Engel142 succeeded in proving by relaxation measurements of al CB2 that the kinetics of in vitro triple-helix formation is governed by more than one relaxation time. This rules out an AON mechanism, but the fitting to the experimentally found equilibrium transition curves nevertheless showed good accommodation and AH° computed from these curves could be confirmed by calorimetric measurement. 

Transient measnrements (relaxation measurements) are made before transitory processes have ended, hence the current in the system consists of faradaic and non-faradaic components. Such measurements are made to determine the kinetic parameters of fast electrochemical reactions (by measuring the kinetic currents under conditions when the contribution of concentration polarization still is small) and also to determine the properties of electrode surfaces, in particular the EDL capacitance (by measuring the nonfaradaic current). In 1940, A. N. Frumkin, B. V. Ershler, and P. I. Dolin were the first to use a relaxation method for the study of fast kinetics when they used impedance measurements to study the kinetics of the hydrogen discharge on a platinum electrode. 

Researchers have accumulated a large body of thermodynamic and kinetic data to assess these effects, and many of these results are included in the tables of reference 101. Qualitatively, one concludes that for small molecule Gd(III) complexes—those of molecular weight<1000 Da—a high relaxivity, measured in mM 1 s 1, will approach an upper limit of 5 mM-1s-1. Some data are collected in Table 7.3. Newer macromolecular conjugate-Gd(III) complex systems, also discussed below, may approach relaxivities five to six times larger per Gd(III) ion. 

For the present model the equilibrium state corresponds to complete conversion. It is also relatively easy to show that the final approach of each of the concentrations to this state is given by a sum of three exponentially decaying terms exp( — k0t), exp( — kj), and exp( — k2t) (this forms the basis of the relaxation technique in chemical kinetics for measuring the rate constants for fast reactions). 

Studies on the kinetic behaviour of nucleoside and nucleotide complexes are less common than those on structural aspects. This arises because of the rapid rates of the formation and dissociation reactions, requiring NMR or temperature-jump relaxation measurements. The number of species that can coexist in solution also hinders interpretation. The earlier kinetic studies have been reviewed by Frey and Stuehr.127 Two important biological reactions of the nucleotides are phosphoryl and nucleotidyl group transfers. Both reactions are catalytic nucleophilic reactions and they both require the presence of a divalent metal ion, in particular Mg2+. Consequently, one of the main interests has been in understanding the catalytic mechanism of the metal ion involvement. This has mainly involved studies on related non-enzymic reactions.128... 

The measurement of the exchange time xm may provide useful kinetic information on the system. Kinetic parameters for the dissociation process may be obtained by performing relaxation measurements as a function of temperature. If it is assumed that the dissociation of the ligand from the paramagnetic site is a first order kinetic process, the dissociation rate constant r 1 is given by the Eyring relationship... 

Carbon-13 relaxation measurements have been covered in a number of texts (3, 23—25) and reviews. (26—30) For steroids it is clearly established that 13C relaxation is almost entirely due to interactions (dipole-dipole) with attached or nearby protons. (31-33) The relaxation process is kinetically first-order, characterized by a rate constant Tf1. For steroids, at useful concentrations, 13C T, values range from 0-02 to 5 s. Since Tt is markedly affected by molecular tumbling, it is necessary carefully to control and state solvents, concentrations, and temperatures in acquiring and reporting Tx values. Given all of these variables, Tx reproducibilities are usually found to be around 10%, while reports from different laboratories not infrequently differ by 20%. 

Methods such as nuclear magnetic resonance (NMR), electron spectroscopy for chemical analysis (ESCA), electron spin resonance (ESR), infrared (IR), and laser raman spectroscopy could be used in conjunction with rate studies to define mechanisms. Another alternative would be to use fast kinetic techniques such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4), where chemical kinetics are measured and mechanisms can be definitively established. 

The above studies have clearly shown that p-jump relaxation measures chemical kinetics and thus one derives the actual rate constants. The implications these types of measurements have for ascertaining mechanisms of ion exchange and of catalytic reactions on soil constituents is tremendous. The application of p-jump relaxation to studying ion exchange kinetics of NH on zeolite (Ikeda et al., 1984b) and the adsorption/ desorption of Pb2+ on y-Al203 (Hachiya et al., 1979) is presented below,... 

Schwarz, G. Seelig, J. Kinetic properties and electric field effect of the helix-coil transition of poly(gamma-benzyl 1-glutamate) determined from dielectric relaxation measurements. Biopolymers 1968, 6, 1263-1277. 

Perkin Elmer MPF-3 spectrofluorometer. X- and Q-band measurements of EPR spectra were carried out at liquid nitrogen and liquid helium temperatures. Microcalorimetric measurements were performed on a LKB 10700 batch microcalorimeter. Temperature-jump relaxation kinetics were measured using a double beam instrument (18) with a cell adapted for anaerobic work. The relaxation signals were fed into an H.P. 2100 computer and analyzed as described in Ref. 7. The pulse radiolysis exepriments were carried out on the 5-MeV linear accelerator at the Hebrew University. Details of the system have been published previously (19). 

NMR relaxation methods discussed herein can be applied to obtain kinetic parameters, discriminate regions of flexibility within bound ligands, and determine the structure of bound ligands. As with all NMR relaxation measurements, structure information and dynamic information are interwoven, and the goal is to develop methods to obtain structural and dynamic information separately. Two approaches will be discussed in this paper the first focuses on dynamie information and the second on structural information. These methods... 

EPR signals for both the flavosemiquinone radical and the low-spin ferric heme have been reported (65, 78-82). The flavosemiquinone signal, which is easily observed at 123 K, shows a typical g value of 2.0039 0.002 (65). The bandwidth, which is around 15 G, is very like that of an anionic, or red, semiquinone (65). The EPR signal of the low-spin ferric heme can be observed at low temperatures ( 28 K) and shows g values of 2.99, 2.22, and 1,47 (65), which are similar to those found for cytochrome 65 (81). EPR rapid freezing studies have allowed the amounts of semiquinone and ferric heme to be monitored during reduction of the enzyme by L-lactate (66). This has proved to be extremely useful in the development of kinetic schemes to describe the flow of electrons in the enzyme. The distance between the prosthetic groups in H. anomala flavocytochrome 62 has been estimated from EPR experiments and spin-lattice relaxation measurements (82). Pyruvate was used to stablize the flavosemiquinone and the effect on the signal of this species from oxidized and reduced heme was measured. The results indicated a minimum intercenter distance of 18-20 A (82). 

This review has focused on recent research directed toward characterization of the active sites for water-gas shift over magnetite-based catalysts. The reaction can be described by a regenerative mechanism wherein gas phase or weakly adsorbed CO reduces anion sites and steam oxidizes the resultant surface oxygen vacancies. Kinetic relaxation techniques indicate this to be a primary pathway. The sites which participate in this reaction comprise only about 10% of the BET monolayer, and these sites can be titrated using CO/CO2 adsorption at 663 K. In contrast, the total cation site density is effectively titrated with NO at 273 K. In fact, the ratio of the extent of CO/CO2 adsorption to the extent of NO adsorption provides a measure of the fraction of the magnetite surface which is active for water-gas shift. 

The most powerful technique for studying molecular motions in protein-water systems below 0°C is magnetic resonance. Dielectric relaxation measurements can be used, but these measurements are more suitable at higher temperatures in homogenous solutions (13). Recently, the frequency dependence of the mehcanical properties of biopolymers has been shown to yield considerable kinetic information (14). I will limit discussion to the salient results attainable from these techniques. 

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