Propinquity effects

Catalysis occurs because the catalyst in some manner increases the probability of reaction. This may result from the reactants being brought closer together [catalysis by approximation, or the propinquity effect ], or somehow assisted to achieve the necessary relative orientation for reaction. Noncovalent interactions may be responsible for the effect. Covalent bond changes may also take place in catalysis. In a formal way, the manner in which catalysis occurs can be described by schemes such as Schemes I and II. 

The proximity effect. This is the simple idea that in an intramolecular reaction the substrate function may be exposed to a larger local concentration of the reagent than in an intermolecular reaction, because the two functions are covalently constrained to occupy adjacent space. This effect has been called the approximation or propinquity effect. The proximity effect certainly seems physically reasonable and is likely to make some contribution to intramolecular reactivity, but it cannot be a major contributor when EM is large, because EM is itself a measure of a presumed local concentration, and the observed large EM values are physically impossible concentrations. The magnitude of rate enhancement achievable by prox-... 

Koshland (1962) has calculated, however, that such a propinquity effect will not explain the large rate enhancements observed with enzymes unless there are more than two functional groups involved with utilization of five functional groups (2 substrates and 3 catalytic groups) a rate increase of 10 would be possible. Such multifunctional catalysis would, of course, be impossible to demonstrate... 

This effect is also called the propinquity effect and means that the rate of a reaction between two molecules is enhanced if they are abstracted from dilute solution and held in close proximity to each other in the enzyme s active site this raises the effective concentration of the reactants. 

Propagation of errors, 40, 48, 248 Propinquity effect, 263, 365 Protol5Tsis, 147, 148 Proton inventory technique, 302 Proton transfer, 166 direct, 148 extent of, 346 fast, 97, 146, 173 isotope effect in, 296 partial, 395 Proximity effect, 365 Pseudo-first-order rate constant, 23 Pseudo-first-order reaction, 61 Pseudo-order rate constant, 23 Pseudo-order reaction, 23 Pseudo-order technique, 26, 78 Pulse NMR, 170... 

Binding groups in the active site may be positioned so as to stabilize a reactive intermediate (propinquity effect). 

The Sn2 mechanism is a second-order nucleophilic substitution whose rate depends on the concentrations of both the alkylating agent and the nucleophilic site. Busulphan 12.31), which is one of the few agents reacting by this mechanism, achieves this result by a propinquity effect without prior ionization, thus ... 

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