Rate-limiting step types

Figure 4. DDC (A), serotonin (B), and tyrosine hydroxylase (C) immunore-activity in the posterior region of a wild-type Drosophila ventral ganglion. Tyrosine hydroxylase (TH) encodes the rate-limiting step in dopamine biosynthesis and is a marker for dopamine cells. B and C are the same CNS assayed for both serotonin and TH. M, medial dopamine neurons VL, ventrolateral serotonin neurons DL, dorsolateral dopamine neurons. Short unmarked arrows in C show vacuolated cells that do not contain DDC immunoreactivity. The immunoreactivity in these cells may represent a nonspecific cross-reactivity of the rat TH antibody. The length bar in A is 50 pM. The images are confocal projections generated on a Molecular Dynamics-2000 confocal laser scanning microscope.

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

What we shall be doing in the discussion that follows is comparing the effect that a particular Y would be expected to have on the rate of attack on positions o-/p- and m-, respectively, to the substituent Y. This assumes that the proportions of isomers formed are determined entirely by their relative rates of formation, i.e. that the control is wholly kinetic (cf. p. 163). Strictly we should seek to compare the effect of Y on the different transition states for o-, m- and p-attack, but this is not usually possible. Instead we shall use Wheland intermediates as models for the transition states that immediately precede them in the rate-limiting step, just as we have done already in discussing the individual electrophilic substitution reactions (cf. p. 136). It will be convenient to discuss several different types of Y in turn. 

Dissolution indicates the rate-limiting step for compound absorption when drugs are administered orally. The solubility of a pharmaceutical compound represents its maximum concentration in an aqueous buffer. Additional compound will not dissolve above this concentration. The solubility value is often heavily dependent upon pH and temperature and is typically measured at physiologically important pH levels and body temperature. The standards for dissolution testing are determined by the United States Pharmacopoeia (USP). Testing typically requires sampling of a solution at 15, 30, 45, and 60 min for immediate-release products. /./Pl.C is ideally suited for use in conjunction with USP apparatus types I or II and can rapidly analyze multiple time points or replicate samples. 

The membrane is the regulating barrier for exchange of chemical species between the environmental medium and cell interior. It may be practically impermeable to one type of species and highly permeable to another. In the chain of transport steps from the bulk of the medium to the cell interior, the membrane transfer step may thus vary from fully rate-limiting to apparently fast with respect to transport in the medium. The overall rate of this biouptake process is determined by mass transport either in the medium or through the membrane the actual rate-limiting step will depend on a large variety of factors. Membrane... 

It is shown that the rate-limiting step in the photoelectrochemical evolution of hydrogen in an HF electrolyte is linearly dependent on the excess electron concentration at the surface of the p-type silicon electrode. The rate of this step does not depend on the electrode potential and the H+ concentration in the solution, but is sensitive to the surface pretreatment [Sell]. The plateau in the I-V curve, slightly... 

Solvent Type Solvents Rate Limiting Step... 

Char oxidation dominates the time required for complete burnout of a coal particle. The heterogeneous reactions responsible for char oxidation are much slower than the devolatilization process and gas-phase reaction of the volatiles. Char burnout may require from 30 ms to over 1 s, depending on combustion conditions (oxygen level, temperature), and char particle size and reactivity. Char reactivity depends on parent coal type. The rate-limiting step in char burnout can be chemical reaction or gaseous diffusion. At low temperatures or for very large particles, chemical reaction is the rate-limiting step. At... 

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