Stereoselectivity of aldolase catalytic antibodies

Abstract

Enolate- and enamine-mediated versions of the aldol condensation can be catalyzed by antibodies. Antibody 78H6, an antibody against the quaternary ammonium hapten 1, catalyzes the enolate-mediated condensation of keto-aldehyde 2 to form aldol 3a and 3b and the subsequent β-elimination to form enone 4. Catalysis of both steps is promoted by a carboxylate side-chain on the antibody acting as a general base. Antibody 78H6 catalyzes this process 10 4 times more efficiently than acetate. Despite of this efficiency, stereochemical control occurs only in the elimination step by kinetic resolution. The stereochemistry of the carbon–carbon bond forming step from 2 to 3, which is not rate-limiting in this system, cannot be controlled by the antibody. Antibody 72D4, another antibody raised against hapten 1, can be combined with primary amine 5 to form an artificial aldolase catalyzing the aldol condensation of aldehydes such as 6 with acetone to form aldols 7a/b. Catalysis occurs via a covalent enamine mechanism, whereby condensation of the amine cofactor 5 with acetone to form the corresponding enamine 9, is promoted within the hydrophobic binding pocket of the antibodies by exclusion of water. In contrast to the enolate mediated reaction, the carbon–carbon bond formation is rate-limiting in the enamine mechanism, and the antibody shows a high level of stereoselectivity in this step. Stereoselectivity is a consequence of conformational control by a hydrogen bonding group on the antibody that activates the aldehyde carbonyl. This activation furthermore allows chemoselective catalysis of aldolization over β-elimination to occur. These experiments demonstrate that reaction design has a critical influence on the stereoselectivity and outcome of antibody-catalyzed aldol condensations.