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Proceedings of the National Academy of Sciences of the United States of America logoLink to Proceedings of the National Academy of Sciences of the United States of America
. 1971 Mar;68(3):658–661. doi: 10.1073/pnas.68.3.658

On the Concept of Orbital Steering in Catalytic Reactions

Thomas C Bruice 1, Allister Brown 1, David O Harris 1
PMCID: PMC389011  PMID: 16591915

Abstract

Angular displacement from linear overlap of but a few degrees in the transition state of the enzyme-substrate complex has been postulated to be of great kinetic significance (“orbital steering”). The concept of orbital steering is shown to have evolved from the orientation parameters of an equation previously proposed to evaluate the kinetic importance of propinquity. This equation is shown to be naive. Arguments provided against the concept of orbital steering include: (a) force constants predicted from orbital steering are about 100 times those experimentally determined from displacement of nuclei in a direction normal to the axis of a covalent bond (for example, at room temperature vibrational bending amplitudes of +5° or more are common); (b) because of the lessened directionality of orbitals containing nonbonded electron pairs, the force constants in transition states should be even smaller than in the case of a covalent bond; and (c) molecular orbital calculations predict shallow total energy minima for orbital alignment. The experimental rate data offered as a basis for the concept of orbital steering are shown to find rationalization in the previously observed dependence of ΔS‡ on kinetic order and the energy requirements for the freezing-out of single bonds in the transition state leading to the formation of medium-size ring compounds from extended ground states. It is concluded that if orbital steering does exist, experimental and theoretical evidence to support this concept have yet to be presented.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

  1. Bruice T. C., Pandit U. K. INTRAMOLECULAR MODELS DEPICTING THE KINETIC IMPORTANCE OF "FIT" IN ENZYMATIC CATALYSIS. Proc Natl Acad Sci U S A. 1960 Apr;46(4):402–404. doi: 10.1073/pnas.46.4.402. [DOI] [PMC free article] [PubMed] [Google Scholar]
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