Electrostatic catalysis A survey is given on my studies on the role of electrostatics, as one of the major driving forces of catalysis, conducted in the nineties. I proposed to introduce the concept of electrostatic catalysis, that takes place in a reaction if the polarity of the transition state is larger than that of the initial state, thus the dipoles of the environment stabilise it and the activation energy decreases. The reverse effect, electrostatic anticatalysis, can also be observed, an example is the Walden inversion that is faster in the gas phase than in water solution. We illustrate electrostatic catalysis on a number of examples. Maybe the simplest case is the catalytic rate acceleration of pericyclic reactions in aqueous phase in the presence of lithum ions that was shown to be mainly of electrostatic origin. Furthermore, increase of reactivity in zeolite pores can also be traced back to electrostatics. A considerable and fluctuating electrostatic field is generated by crystalline silicon surfaces allowing the stronger or weaker adsorption of ammonia molecules modelled by point dipoles. We formulated a hypothesis for the explanation of the increase of catalytic rates near surface defects, e.g. steps. The effect of steps is, at least in case of silicon surfaces, electrostatic in nature that could be proved for the decomposition of ammonia in terms of semiempirical molecular orbital calculations. Enzymatic reactions, classical examples for electrostatic catalysis, are especially important. We have shown that the electrostatic affect of the environment considerably accelerates the ring opening step in xylose isomerase reaction, thus hydride shift becomes rate limiting. In case of the latter electrostatic anticatalysis can be observed in the D254E/D256E double mutant because the negatively charged carboxylate groups get closer to the negative oxygen atom of the transition-state complex and destabilise it. The role of electrostatics is not only to influence the stability of the transition-state complex of the enzymatic reaction directly but also to exert an indirect effect, e.g. by determining the localisation of the free radical in peroxidases, thus ensuring the selectivity of various enzymes among small molecular and protein substrates. We completed our computational methods by protein crystallography that allowed us to investigate a certain phenomenon from different aspects, thus assisting in its deeper understanding.
|Number of pages||6|
|Journal||Magyar Kemiai Folyoirat, Kemiai Kozlemenyek|
|Publication status||Published - Jan 1 2000|
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