PH jump experiments

Initial dissolution of Fe oxides can be very rapid and is then followed by a slower steady state process. The initial step often corresponds to less than 1% of the total solid (Cornell et al. 1974 Maurice et al. 1995). Samson and Eggleston (1998) subjected hematite to a pH jump experiment and found that when the pH was lowered to 1, a reservoir of dissolution active sites on the surface was depleted, but then regenerated when the pH was raised to 2. The authors suggested that these active sites, which made up -70% of a monolayer, consisted of an adsorbed nutrient Fe . 

In conclusion, the photochemical behavior is in agreement with that observed in pH-jump experiments. Although Cc is obviously the primary product of flash excitation, the observed species and their survival time (from seconds to years) before reversion to the thermodynamically stable form Ct depend on temperature and pH. 

Some dye molecules have a different x>Ka in the electronically excited state, so that they release or take up H ions when exposed to a flash of light. This can be used to bring about rapid pH changes in solution. Such pH-jump experiments can be used to follow the kinetics of biomolecular processes. Other kinds of photochemical reactions can be used to overcome mixing problems in rapid kinetic experiments. One example of this is the use of caged ATP compounds that only become available for enzyme reaction, for example, when exposed to an intense light flash. ... 

Kinetics of the alkaline transition have been characterized with pH-jump experiments. 

The two-step mechanism in Scheme 3 accounts for the pH dependence of the monoexponential rate constants observed from neutral pH to pH 10. " The first step in the alkaline isomerization, deprotonation of a titratable group in the native protein, triggers the second step, which is heme iron ligand exchange. This minimal kinetic mechanism does not consider five-coordinate intermediates or multiple alkaline conformers. Biphasic behavior observed in pH-jump experiments above pH 10 suggests the formation of a transient HS heme intermediate in ferricyt c from... 

Whenever a chemical equilibrium is subjected to a perturbation, most commonly a change in temperature, pressure, pH, or other concentrations, the system will start to relax back to a new equilibrium state. The kinetics of this relaxation can be followed. Methods for quickly inducing a perturbation followed by monitoring the relaxation are referred to as jump techniques. Changes in temperature, pH, and pressure can often be done fast enough that reactions with half-lives in the microsecond range can be followed. For example, the equilibrium positions of Bransted acid-base reactions are controlled by the pH, and therefore pH jump experiments are particularly useful with these reactions. 

Horseradish peroxidase, HRP, is a remarkably stable enz)niie it is resistant to denaturation from heat, changes in pH and photo-oxidation. With the aid of pH jump experiments, its reactions can be studied over the pH range of 3.0 to 11.5. 

Fig. 32. Double-jump experiments of unfolding and refolding. The peptide [(Ala-Gly-Pro)s]3 in 50 ml phosphate buffer (pH 7.5) was incubated at 9.2 °C and quickly unfolded by a first temperature jump from 9.2 to 30 °C. This process took 25 s, the time needed to reach the final temperature. In a first experiment (curve A), the second jump back from 30 to 9.2 °C followed immediately after complete unfolding of the peptide, i.e. 25 s after the first jump. In a second and a third experiment (curve B, C), the time lapse between the first and the second jump was 75 and 125 s, respectively...

Fig. 23 A plot of the observed pseudo-first-order rate constant for the methanolysis of 0.04mM HPNPP ( , left axis) catalyzed by 0.2mM35 2Zn(II) or 0.04mM methyl /j-nitro-phenyl phosphate (O, right axis) catalyzed by 0.4 mM 35 Zn(II) as a function of the [CH30-]/ [35 Zn(II)] ratio at 25 + 0.1 °C. Experiments done by pH jump method starting at a [CH30-]/ [35 Zn(II)] ratio of 1.0 (vertical dashed line, (pH = 9.5) and adding acid (left) or base (right). Reproduced with permission from ref. 95.

Fig. 1. Summary of chrysotile-steady state element fluxes (A) Mg and (B) Si as a function of pH. Filled and open diamonds are steady-state fluxes determined after the onset of an experiment and after a pH-jump, respectively. Circles are Bales Morgan (1985) element fluxes their rates were determined in C02-free solutions. The solid line is the linear least square fit to all the data. The stars are the range in fluxes determined from field serpentine weathering (Freyssinet Farah 2000).

Figure 4.4. Typical oscillograms of pressure-jump experiments. Relative change in conductivity for pressure-jumps of 13.1 MPa in solutions of 0.05 M InCI, pH = 3.25. (a) At 383 K showing only pressure decay, (b) At 300.5 K, r = 50 15 /us. (c) At 273.7 K. r = 215 10 jj.s. (d) Solution of 0.10 M a-ketoglutaric acid, pH 1.69. at 274 K, t = 25.8 s. [From Knoche and Wiese (1974), with permission. ...

Ikeda et al. (1984b) plotted Eq. (4.42) by determining the equilibrium concentrations from adsorption isotherms for S(H), S(NH4), and NH4, and using the pH value to determine [H+]. This plot shows good linearity (Fig. 4.11), which confirms that the mechanism hypothesized in Eq. (4.40) is operational. The kv and k- values for Eq. (4.42) can then be calculated from the slope and intercept of Fig. 4.11, and the kinetic Keq can be determined from the ratio kjk x (Table 4.2). It is important to notice that the values calculated kinetically and statically (equilibrium method) are similar, which indicates that the rate constants one calculates from p-jump experiments are chemical kinetics rate constants. These data also verify... 

Several investigations concerning the thermodynamic and kinetic aspects of the thermal reactions of flavylium-type compounds have long been in the literature,133-371 while photochemical and photophysical aspects have been systematically examined more recently.[17-19,38 31 As we shall see below, pH jump, temperature jump, and flash photolysis experiments permit measurement of the rate constants of some of the reactions involved, and steady state titration experiments (using UV/Vis and NMR techniques) allow the measurement of equilibrium constants. In order to illustrate the complex reaction network in which these systems operate, we will now focus on the behavior of the 4 -methoxyflavylium ion (Figure 2 R4 = R7=H, R4- = OCH3).[391... 

The pH-jump technique can be used in the conventional stopped-flow mode in order to elucidate the kinetics of the reaction at different pH. In such experiments, a pH-dependent equihbrium is perturbed by mixing the system equilibrated at one pH in a weak buffer with a strong buffer at a different pH. 

A limited number of commercially available hydroxyarenes, including hydroxy-pyrenetrisulfonate ( pyranine , HPTS), have somewhat higher photoacidities than naphthols and have been used as photoacids for experiments requiring instant changes in acidity, e.g., pH jump. Again, HPTS is limited to aqueous solvents. 

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