Substrate ionization

Specific base catalysis is predicted if the extent of substrate ionization is reduced from almost complete. Depends on whether an ion pair assists in removal of leaving group. 

Protonation of the leaving group anion, which prevents internal return of the ion pair to reactant, and has the effect of making substrate ionization irreversible. 

Specific base catalysis predicted if extent of substrate ionization reduced from almost complete. Effect on rate assuming no change in mechanism is caused steric factors upon substitution at C and C have not been considered. The rate predictions are geared to substituent effects such as those giving rise to Hammett reaction constants on 3- and a-aryl substitution. 

Ethylenic cation radicals are also capable of rotating around the double bond. At the same time, the main specificity of ethylenic cation radical reduction consists of the high selectivity of the reaction. One-electron oxidation was developed as a strategy for selective and efficient reduction of relatively ionizable functionalities, including conjugated dienes, styrenes, and vinyl sulfides (Mirafzal et al. 1993). Reduction is highly sensitive to substrate ionizability and permits selective reduction of the more ionizable function in a difunctional compound. 

Ileal bile salt transport in vivo studies of effect of substrate ionization on activity. Am J Physiol 219, 487-490. 

Using a familiar method of classifying the mechanisms of intermolecular electron transfers, the mechanism of substrate ionization may be of the inner sphere or the outer sphere type. Outer sphere electron transfer involves the transfer of an electron between two chemical species without the development of any significant covalent interaction between them, i.e., through space or through another molecule or mol-... 

The substrate ionization step, if it is not already reversible, often becomes reversible under these conditions, resulting in a modest to very pronounced rate diminution [40]. 

Ullrich KJ, Rumrich G. Renal contraluminal transport systems for organic anions (p-aminohippurate, p-aminohippurate) and organic cations (N1-methyl-nicotinamide, NMeN) do not see the degree of substrate ionization. Pfiugers Arch 1992 421 286-288. 

Yueh, W., Bauld, N. L. Mechanistic aspects of aminium sait-cataiyzed Dieis-Aider reactions the substrate ionization step. J. Phys. Org. Chem. 1996, 9, 529-538. 

TABLE 9 Rate Constants, fcq, Substrate Ionization Potentinls (IP), and Activation Pnra meters, A H and AS, for Quenching of OjC Ag) by Kura ns and Cyclopentadiene in Toluene... 

In general, enzymes exhibit maximum catalytic activity at a definite pH. This optimum pH is generally in the vicinity of pH 7 ( 1), although exceptions are well known. The dependence of the kinetic parameters of the fumarase reaction on pH is illustrated in Figs. 9-2 and 9-3. Several different possible effects of pH on the reaction must be distinguished. In the first place, many substrates may have ionizable groups and only one of the ionized forms of the substrate may be acted upon by the enzyme. Since substrate ionization constants can be determined quite easily and precisely,... 

Many substrates ionize in the pH range used in kinetic experiments. If the substrate ionization is possible, one ought to consider whether observed piSTa... 

As predicted by Eq. (14.65), the initial slope of the extrapolated line in Fig. 10 is 2, and the second linear segment has a slope of 1, before the pH profile levels out. Thus, in enzyme catalysis, the slope in excess of 1 usually indicates that the dissociation of a group on the substrate is involved (Schulz, 1994). However, the comphcations of several overlapping protonation ranges are increased by substrate ionizations, so it is always prudent, if possible, to use another substrates that do not ionize over the pH range under study (Tipton Dixon, 1979 Comish-Bowden, 1995). 

Terpene synthases constitute a large class of enzymes that are able to convert just a handful of linear isoprenoid precursors, made from IPP and DMAPP by prenyltransferases, into a large diversity of thousands of terpenoids. As most of the enzyme products are of (poly)cyclic nature, these enzymes are often called terpene cyclases. Two types of terpene synthases can be mechanistically distinguished by their formation of a reactive carbocationic species. Class I terpene synthases contain a highly conserved DDXXD motif as also found in a-type prenyltransferases and perform substrate ionization via metal-triggered abstraction... 

R. Krishnamoorthy, and A.K. Mitra, Complexation of weak acids and basis with cyclodex-trins Effects of substrate ionization on the estimation and interpretation of association constants, Int. J. Pharm. Adv., 1,330-343,1996. 

Transport In Vivo Studies of Effect of Substrate Ionization on Activity. 

Maugard, T., Remaud-Simeon, M. and Monsan, P. (1998) Kinetic study of chemoselective acylation of amino-alditol by immobilized lipase in organic solvent effect of substrate ionization. 

The H" -ion concentration has myriad functions within the cell. It acts, for example, as a substrate or product of dehydrogenases and as a factor affecting the ionization of groups which comprise the active site or stabilize its conformation. Substrate ionization may also be responsive to changes in A discussion of the manner in which one may analyze the effect of pH on both the apparent Km and the apparent I max is available in Dixon and Webb (1964). Recently, Knowles (1976) has evaluated the nature of the experiments that have hitherto been used to assess the relationship of pH optima to putative sites involved in the catalytic activity of a particular enzyme. His review defines limits to overambitious interpretations and extrapolations regarding participation of particular groups in the reaction mechanism. 

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