Theoretical aspects of orbital steering.

A calculation based on transition-state theory leads to the conclusion that rate accelerations of 10(3)-10(5) could be achieved in an optimally oriented reaction relative to a similar randomly oriented bimolecular reaction. This factor is obtained by the use of partition functions of simplified systems and is based on contributions to rotational and vibrational entropy from reasonable transition states. A simple harmonic oscillator calculation leads to a similar conclusion for series of intramolecular reactions. Although the uncertainty in theoretical calculations of this type is considerable, the results add support to the conclusion based on experimental studies that orientation factors can play a very significant role in the catalytic power of enzymes.