Studies of chemical recognition in biology were initiated about half a century ago with the flrst kinetic data obtained on enzyme catalysis and inhibition. They led to a rather static representation of the recognition process illustrated by the lock and key model that still continues to influence our overall image of recognition and its specificity. In several cases, crystallographic studies of enzyme-substrate complexes have supported this model. Indeed, in a crystallized ligand-enzyme complex, a close fltting is observed between the active center of the enzyme and the functional groups of...