ANTIBODY-CATALYZED HYDROLYSIS OF ENOL ETHERS .2. STRUCTURE OF THE ANTIBODY-TRANSITION STATE COMPLEX AND ORIGIN OF THE ENANTIOSELECTIVITY

The hydrolysis of alkyl enol ethers to their corresponding carbonyl co mpounds proceeds by acid-catalyzed, rate-determining protonation on th e beta-carbon to form an oxocarbonium ion intermediate (Kresge, A. J.; Chang, Y. J. Chem. Soc. B 1967, 53). Antibody 14D9 (anti-1) catalyzes the hydrolysis of enol ethers 4 and 5 with very high enantioselectivi ty of protonation (Reymond, J.-L.; Janda, K. D.; Lerner, R. A, J, Am, Chem. Soc, 1992, 114, 2257). Catalysis involves participation of an an tibody side chain as a general acid, as well as pyramidalization of th e enol ether's beta-carbon by hydrophobic contacts between its substit uents and the antibody (Reymond, J.-L.; Jahangiri, G. K.; Stoudt, C.; Lerner, R. A, J. Am. Chem. Soc. 1993, 115, 3909). The present study ad dresses the question of the origin of the enantioselectivity of this c atalyst. First, enantioselectivity and substrate tolerance, which are most remarkable in antibody 14D9, are shown to be recurrent features f or anti-1 or anti-2 antibodies. Four antibodies were studied, and all enantioselectively deliver a proton on the re face of enol ethers to p roduce (S)-configured carbonyl products, while stereoselectively bindi ng to analogs of the (S,S)-hapten 1. The orientation of the enol ether at the transition state relative to the hapten is then established by comparing the effect of alkyl substitutions at the beta-carbon on ant ibody catalysis with the effect of equivalent substitutions on antibod y binding to hapten analogs. For antibody 14D9 (anti-1), the results s how that the alkyl substituent of the enol ether's beta-carbon binds t o the N-methyl site of the hapten at the transition state. Substitutio n of ethyl for methyl at that position results in a 20-fold drop in tr ansition state binding and a 3-7-fold drop in affinity for inhibitors. The orientation is such that the cyclic substrates do not fit in the site complementary to the piperidine ring of the hapten at the transit ion state. The antibody-catalyzed hydrolysis of the cyclopentanone eno l ether 6, which produces exclusively (S)-7, is 40 times more efficien t than for the cyclohexanone enol ether 10. By contrast, no binding se lectivity is found for the individual enantiomers of the corresponding ketone products 7 and 18, which are neutral transition state analogs for re- or si-selective protonation of 6 or 10. The enantioselectivity of 14D9 appears only for the transition state, which suggests that it contains a dynamic component, probably the strict geometrical constra int that the enol ether be aligned with the antibody residue acting as a general acid catalyst during proton transfer. The enantioselectivit y of antibody 14D9 thus results from an unexpected combination of bind ing and catalysis. This study establishes the relationship between hap ten and transition states in unprecedented details. The observation th at the discriminating power of an antibody for enantiomeric transition states can far exceed simple binding discrimination for ground state molecules suggests a promising future for catalytic antibodies as enan tioselective catalysts.