Steady state phase reactions

Refers to that initial period of nonhnear product formation, commencing with the initiation of the reaction and ending when the system is at steady state. Typically, the pre-steady-state phase lasts from milliseconds to a few seconds after mixing reactants. The time course of pre-steady-state rate processes often can be evaluated using stopped-flow, temperature-jump, and mix-quench methods. 

A complete description of an enzyme-catalyzed reaction requires direct measurement of the rates of individual reaction steps—for example, measurement of the association of enzyme and substrate to form the ES complex. It is during the pre-steady state that the rates of many reaction steps can be measured independently. Experimenters adjust reaction conditions so that they can observe events during reaction of a single substrate molecule. Because the pre-steady state phase is gener-... 

A traditional kinetic model of steady-state catalytic reaction assumes quasi-steady-state concentrations of intermediate species on the catalyst surface. This assumption is often invalid for unsteady-state conditions characterized by continuous changes in a fluid phase composition and temperature above the catalyst surface. Additionally, the catalyst itself can interact with the reaction mixture and can undergo significant changes, influenced by changing conditions in the gas phase. Such a modification of the catalyst can be con-... 

Identification of radical 3 as a species that is present in the steady-state phase of the reaction does not prove that it is an intermediate—it could be a species that is peripheral to the real reaction mechanism. Proof that a species is an intermediate requires a demonstration that it is kinetically competent to participate in the mechanism. In the case of a metastable radical, the usual procedure is to conduct transient kinetic studies using a rapid mixing apparatus equipped to quench samples by spraying them into liquid isopentane. The frozen aqueous samples (snows) from the timed cold quenches are then packed into EPR tubes and analyzed spectroscopically. Simple mixing of enzyme with SAM and lysine followed by freeze-quenching on the millisecond time scale does not work because the activation by SAM takes about 5 s. However, a preliminary mix of enzyme with SAM and [2- C]lysine, aging of the solution for 5 s within the apparatus. 

Dutta et al. [32] modified the pseudo-steady-state advancing reaction front model of Stroeve and Varanasi [30] by considering the polydispersity of the emulsion globules and the external phase mass transfer resistance. They also included the outer membrane film resistance in their model [5]. Their results were in good agreement with experimental data for phenol extraction. 

Choi, T., Cizmeciyan, D., Khan, S. I., Garcia Garibay, M. A., An Efficient Solid to Solid Reaction via a Steady state Phase Separation Mechanism, J. Am. Chem. Soc. 1995, 117, 12893 12894. 

If substrate is in excess, product release from E-P to regenerate E does not lead to another exponential phase. Under these conditions, product release leads to the linear steady-state phase of the reaction and the rate of release contributes to the net rate of approach to steady state. 

When superoxide dismutase purified from erythrocytes was added to the reaction mixture, either at the beginning or during the steady state phase of the reaction, almost instantaneous inhibition was observed (18). This inhibition was dose dependent and could be completely abolished by superoxide dismutase inhibitors. Furthermore, superoxide dismutase preparations from bovine erythrocytes, green peas, spinach leaves, and Esch-... 

Photometric observation of the change in concentration of spectroscopically distinct substrates, intermediates, or products during the pre-steady-state phase of enzyme reactions is the most promising procedure for obtaining detailed information about the sequence of steps in such reactions. So far we have applied this method only to enzyme-catalyzed hydrolysis reactions. This can be done in two ways in the first place, if the reaction mixture contains an indicator color, the liberation or binding of hydrogen ions during the course of the reaction can be followed. Secondly, we have studied the hydrolysis of a number of nitrophenyl esters, and we have... 

Equations (3) and (4) are based on the assumption that the reversal of the second and third steps can be neglected this is always true when initial rate measurements are used. Applications of the three-step kinetic equations to hydrolysis and acyl transfer reactions will be seen in the following sections. Further applications of this approach to many enzyme reactions are planned with the use of ultraviolet spectroscopy for the detection of intermediates during the pre-steady-state phase. 

Studies of Alcohol Oxidation. When the reaction was investigated from the direction of alcohol oxidation under pre-steady-state conditions in the presence of IBA, the time-resolved spectra obtained from RSSF measurements again show evidence for the formation of a transient intermediate in the NAD -mediated oxidation of benzyl alcohol.Data collected at pH values of 9.0, 5.6, and 4.8 are shown in Figs. 7 and 8. In the wavelength region 300 to 450 nm and at pH 9.0, the time-resolved spectra are characterized by a fast, pre-steady-state (exponential) phase dominated by the appearance of bound NADH. This process is followed by an approximately zeroth-order (steady-state) phase in which free NADH is generated by multiple turnovers. The difference spectra in Fig. 7C,D compare the changes which occur in the pre-steady-state phase with those in the steady... 

Fig. 7. Time-resolved spectra and difference spectra for the pre-steady state and steady-state phases of the Co(II)E-catalyzed oxidation of benzyl alcohol by NAD at pH 9 (A, C) and pH 4.8 (B, D) and 25° for the wavelength range 300-450 nm. Scanning was carried out as described in the caption to Fig. 5. Difference spectra in (C) and (D) were calculated by subtracting the last spectrum collected in the pre-steady-state phase [respectively spectrum 8 of (A) and spectrum 10 of (B)] from all other spectra in the set. Single-wavelength time courses are shown in insets a and b of (A) and (B). Conditions after mixing were as follows (A, C) [Co(II)E], 29 fiN [NAD+], 1.4 mM [benzyl alcohol], 2 mM [IBA], 50 mAf 50 mM glycine and 50 mM Bis-Tris, final pH 9.0 (B, D) [Co(II)E], 29 fiN [NAD ], 3.5 mM [benzyl alcohol], 4 mM [IBA], 50 mM 10 mM H2SO4 and 50 mM Bis-Tris, final pH 4.8. Reaction was initiated by mixing enzyme in Bis-Tris buffer (pH 6.5) with NAD, benzyl alcohol, and IBA preincubated in the above-indicated solutions. (From Sartorius et al. with permission.) Copyright 1987 American Chemical Society.

In accordance with Laidler it follows from this expression that for large values of t the exponential term approaches e ", which is almost zero. Thus, the remaining term responds to the amount of intermediate [EO] available during the steady-state phase. It becomes apparent that the ratios of substrates together with the ratio of rate constants are responsible for the percentage of [EO] proportional to the total enzyme concentration deployed. Due to the fact that the maximum product rate can only be reached when [EO] equals [E]o it can happen that a reaction system with k. - has a slower overall rate than the opposite case. However, the rate of product formation (or the equal rate of substrate consumption) for this system is now defined... 

Under steady state, the reaction rate in wetted zone of pellet is equal to the diffusion flux in the gas phase through the external surface of pellet Fr and is also equal to the diffusion flux of components into wetted globules Fc-... 

E]=10 moll and [S] = 10 rrx)ll The simulation shows three distinct phases to the reaction time-course, an initial transient phase that lasts for about a millisecond followed by a longer steady-state phase of 30 min when [ES] remains constant but only a small portion of the substrate reacts. This is followed by the final phase taking 6h during which the substrate is completely converted to product. (Reproduced with permission from Martin F Chaplin Enzyme Technology.)... 

Discrimination between the oxygen adsorption and oxide phase formation theories has important implications in electrocatalysis. If the adsorption theory is correct, then Pt metal should be considered the electrode material and the adsorbed oxygen-containing species as possible intermediates in the steady state electrode reactions of molecular oxygen or a competition with intermediates for adsorption sites. If the phase oxide theory is correct, then Pt oxide has to be considered as the electrode material with obvious consequences concerning the energies of adsorption of intermediates on Pt oxides and not on the bare metal. 

Therefore, a proteolytic reaction that is a mixed chain reaction system cannot enter a steady-state phase. Therefore, steady state does not exist in proteolytic reaction systems. 

The indices a and b with a refer to the respective interfaces of the A- and -carrying phases with the continuous phase. Note that in the steady state both reaction rates are equal (stoechiometry constants are all assumed to be unity), so that we have three equations with three unknowns. The specific surface areas and... 

One of the characteristics of the lipoxygenase-catalyzed dioxygenation reaction is the existence of a finite induction (lag) period before the onset of the steady-state phase. The magnitude of this "lag" is a function of both the substrate and product (hydroperoxide)... 

The reaction mixture studied passes through a column containing substance of a suitable steady state phase. The phenomena of adsorption, desorption or dissolution at the column exit delay the various components of the mixture, which are thus separated. It is then, therefore, possible to assay these components. 

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