Stopped flow rapid scan

Later, Smith and coworkers succeeded in measuring rate constants of the reaction of MeLi with a carbonyl compound at various reagent concentrations with a stopped-flow/rapid scan spectroscopic method, and demonstrated that the reaction also exhibited a fractional kinetic order . Thus, the reaction of 2,4-dimethyl-4 -methylmercaptobenzophenone with MeLi in diethyl ether at 25 °C showed one-fourth order in MeLi in the concentration range of MeLi between 3.9 mM and 480 mM (Figure 1). The rate constant was 200 7 M s . Under these conditions, the monomer was considered the reactive species that exists in equilibrium with the tetramer. Addition of LiBr or Lil depressed the reaction rate but did not change the kinetic order. The same... 

The latter two processes are slow as a result of which the detection of the protonated carbonato complex, and determination of its protonation constant by stopped-flow, rapid scan spectrophotometry were possible in some cases, van Eldik et al. (83) interpreted the initial spectral changes (shift of absorption maxima and the isosbestic points) of ( -cis-[Co(edda)C03l and [Co(nta)COa] at [H+]=0.05 M due to the formation of the protonated carbonato complexes, p-cis-[Co(edda)C03H] and [Co(nta)C03H], Repetitive rapid scan spectral measurements at [H + ] = 2.0 M, however, led them to the identification... 

Figure 1.59 Scheme of a stopped-flow rapid scan measurement sequence [110] (by courtesy of RSQ. 

Problem 8.28 Consider styrene polymerization by triflic (trifluoroethanesul-fonic) acid in 1,2-dichloroethane at 20°C where is 4.2x10 mol/L (23]. For experiments performed (using stopped-flow rapid scan spectroscopy) at a styrene concentration of 0.397 M and acid concentration of 4.7x10 M at 20°C, the maximum concentration of cationic ends (both free ions and ion pairs) was found [23] to be 1.4x10 M, indicating that the initiator efficiency is 0.030. At 20°C, kf / is reported [23] to be 12. 

The optical absorption spectra of the stable Fe, Fe, and Fe NO species were determined, and stopped-flow rapid scan, flash photolysis, and low-temperature spectroscopic methods have been applied to study the reactions of Equations (7)-(9). The species P450nor(Fe Njqo) formed in reaction (7) was characterized by stopped-flow as having a diminished Soret absorbance at 434 nm and a single broad band at 558 nm. Formation of this species was complete within the dead time of the stopped-flow apparatus, even at 10 °C and [NO] of 5 pM, allowing a limit for k, the rate constant for formation of this species, to be estimated as larger than 10 s ... 

The major remaining hurdle before clinical approval of this earliest generation of HBOCs is vasoconstriction and resulting hypertension, which are presumably attributable to the high reactivity of Hb with endothelium-derived nitric oxide (NO) [1]. It has been suggested that small molecular Hbs permeate across the endothelial cell layer to the space nearby the smooth muscle, and inactivate NO. However, cellular HbV induce neither vasoconstriction nor hypertension [4]. A physicochemical analysis using stopped-flow rapid scan spectrophotometry [5] clarified that Hb encapsulation in vesicles retards NO-binding in comparison to molecular Hb because an intracellular diffusion barrier of NO is formed. The requisites for this diffusion barrier are i) a... 

The steady-state and rapid equilibrium kinetics do not give detailed information on the existence of multiple intermediates or on their lifetimes. Such information is provided by fast (or transient) kinetics. The methods can be divided in two categories rapid-mixing techniques (stopped-flow, rapid-scanning stopped-flow, quenched flow) which operate in a millisecond time scale and relaxation techniques (temperature jump, pressure jump) which monitor a transient reaction in a microsecond time scale. Most of the transient kinetic methods rely on spectrophotomet-rically observable substrate changes during the course of enzyme catalysis. 

As might be expected, the problem of obtaining spectra of a reacting system increases as the time resolution involved decreases. The spectral changes associated with a reaction may be constructed by wavelength point-by-point measurements. The method, although tedious and costly on materials, is still used. However rapid-scan spectrophotometry, linked to stopped-flow, is now more readily available and reliable. Two systems are used, shown schematically in (3.29) and (3.30). An example of its use is shown in Fig. 3.9. Rapid scan... 

Similarly, the spectrum of a mixture of Fe(tpps)H20 and Fe(tpps)(OH) can be measured by rapid scan/stopped-flow at various pH s within a few milliseconds after generation (Fig. 3.9). In this short time, dimerization is unimportant so that the spectrum of Fe(tpps)OH can be measured and the pAi of Fe(tpps)H20 estimated. 

A number of stopped-flow systems are commercially available. Three of the most used are manufactured by Atago Bussan (formerly Union Giken), Japan Dionex (formerly Durrum), USA and Hi-Tech Scientific, UK. These also manufacture rapid scan spectrophotometers, multimixer, temperature-jump and flash photolysis equipment. 

In the reverse direction, a proton may be effective by aiding ring-opening directly or via a reactive protonated species. It may intervene with the ring-opened species. A splendid example of these effects is shown in the acid hydrolysis of ferrioxamine B (9). Four stages can be separated and the kinetics and equilibria have been characterized by stopped-flow and rapid-scan spectral methods. 

The aquated iron(III) ion is an oxidant. Reaction with reducing ligands probably proceeds through complexing. Rapid scan spectrophotometry of the Fe(III)-cysteine system shows a transient blue Fe(lII)-cysteine complex and formation of Fe(II) and cystine. The reduction of Fe(lII) by hydroquinone, in concentrated solution has been probed by stopped-flow linked to x-ray absorption spectrometry. The changing charge on the iron is thereby assessed. In the reaction of Fe(III) with a number of reducing transition metal ions M in acid, the rate law... 

Optical Spectroscopy General principles and overview, 246, 13 absorption and circular dichroism spectroscopy of nucleic acid duplexes and triplexes, 246, 19 circular dichroism, 246, 34 bioinorganic spectroscopy, 246, 71 magnetic circular dichroism, 246, 110 low-temperature spectroscopy, 246, 131 rapid-scanning ultraviolet/visible spectroscopy applied in stopped-flow studies, 246, 168 transient absorption spectroscopy in the study of processes and dynamics in biology, 246, 201 hole burning spectroscopy and physics of proteins, 246, 226 ultraviolet/visible spectroelectrochemistry of redox proteins, 246, 701 diode array detection in liquid chromatography, 246, 749. 

Stop-flow, or fraction collection, also could be used to identify the presence of naphthalene in the second eluting fraction, but only at a considerable sacrifice in time relative to the rapid scanning capability of the OMA. However, as the complexity of a chromatographic separation increases, these approaches become impractical in terms of the time required to spectrally determine the peak purity of each fraction. 

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