Rapid scan spectrophotometry

The enzyme horseradish peroxidase is a hemoprotein and the region of the Soret band exhibits large differences between the position and extinction coefficients of the uncombined and combined forms. Both forms were first studied by spectrophotometry, but the E—S complexes were 0 labile that they could not be examined extensively by any other spectroscopic method. Using rapid-scanning spectrophotometry and rapid mixing, Chance was able to distinguish the spectra of compound I and II and determine the various rate constants of the multistep reaction with rather poor precision. 

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... 

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... 

A kinetic smdy of the formation of zwitterionic adducts (28) from 1,3,5-trinitrobenzene and diazabicyclo derivatives indicates that reactions are surprisingly slow, with rate constants many orders of magnitude lower than those for related reactions with primary or secondary amines. The use of rapid-scan spectrophotometry was necessary to study the kinetics of reaction of 4-substimted-2,6-dinitro-A -n-butylanilines (29) with n-butylamine in DMSO the two processes observed were identified as rapid deprotonation to give the conjugate base and competitive a-adduct formation at the 3-position. The reactions of MAf-di-n-propyl-2,6-dinitro-4-trifluoromethylaniline (30), the herbicide trifluralin, and its A -ethyl-A -n-butyl analogue with deuteroxide ions and with sulfite ions in [ H6]DMS0-D20 have been investigated by H NMR spectroscopy. With deuteroxide a-adduct formation at the 3-position is followed by... 

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... 

There is information from other studies which probe the question of delineation of the electronic spectra and kinetic stability of organic radical intermediates produced by Ce(IV) oxidations. For example, Kemp et al. (1980) determined such spectra for the radical-cation and neutral radical produced in the first two kinetic steps when phenothiazine and phenoxazine are the reductants. The combination of stopped-flow and rapid-scan spectrophotometry were used to evaluate kinetic parameters for these steps. 

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... 

Acid-catalyzed aquation of [Co(NH3)sOC02], cis-j8-[Co(edda)C03] , and [Co(nta)C03] has been reinvestigated using rapid scan spectrophotometry.Direct spectral evidence for the participation of proton-ated and ring-opened carbonato species was obtained. The spectral observations are consistent with previously suggested mechanisms for the decarboxylation of monodentate and bidentate carbonato complexes. 

Rapid scan spectrophotometry has been used to study the base hydrolysis of ajS5-(salicylato)(tetraethylenepentamine) cobalt(III) (17). The instantaneous color change observed on addition of base to the complex has been attributed to formation of the phenoxide species and this point has been confirmed. Subsequent aquation and base hydrolysis of the phenoxide species then occurs with = 0.116 s and /cqh = 3.32 M s at 25"C. 

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