Fig. 1. The concept of allosteric reaction control and its implementation.

a A substrate reaction is accelerated allosterically if it is coupled to a remote receptor site such that binding of another molecule (effector) to this site suitably strains the substrate (in this example by imposing a compressive load, represented by a pair of square arrows, on the substrate that contracts during the reaction). This strain comes at the expense of reduced affinity of the effector for the receptor. b, c Here, allosteric acceleration of E-stiff stilbene isomerization (red) by binding of PtCl₂ moiety to a bidentate phosphine receptor (blue) is demonstrated using a series of strained macrocycles, E(m,n). These are synthesized by photoisomerization of strain-free Z(m,n) analogs (b); the two diastereomers of Z macrocycles are in rapid equilibrium and only the S configuration of the stiff stilbene is shown; conversely, epimerization of E-stiff stilbene in macrocycles is negligibly slow. Photoisomerization of Z(3,3) yields a separable mixture of (S,S)-E(3,3) and (S,R)-E(3,3) diastereomers which differ in the configuration of E-stiff stilbene; (S,R)-E(2,3) and (S,R)-E(2,2) are formed as single diastereomers. In the presence of a source of the effector (nbd = norbornadiene), rapid coordination of PtCl₂ to the phosphine on the opposite side of E-stiff stilbene accelerate its thermal isomerization of the Z isomer (c), as illustrated on the example of the S,S-diastereomer.