News and Publications

Antibody-Catalyzed Organic and Organometallic Transformations

E. Keinan, S.C. Sinha, F. Grynszpan, D. Shabat, H. Itzhaky, A. Haskel, S. Nimri, O. Levy, S. Saphier, H. Shulman, A. Shulman, A. Brik

ORGANOMETALLIC CHEMISTRY

Organometallic transformations are attractive targets for antibody catalysis because many of these useful reactions have no enzymatic counterparts. Furthermore, the large coordination numbers and variable coordination geometries of the transition metals allow the creative design of haptens that are closely related to the postulated structure of the transition state of a given organometallic transformation. We are using antibodies elicited against a platinum complex (1 in Fig. 1) to catalyze several fundamental organotransition metal reactions, such as ligand exchange, oxidative addition, and migratory insertion.

TOTAL SYNTHESIS WITH CATALYTIC ANTIBODIES

In the past, we achieved a total synthesis of -multistriatin in which the 4 chiral centers were generated in the antibody-catalyzed step. Now we are synthesizing a number of clinically important compounds, in particular, a group of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (lovastatin, 2; pravastatin, 3; and simvastatin, 4; Fig. 1) that represent the most effective class of drugs currently available for treatment of hypercholesterolemia. We are focusing on the antibody-catalyzed synthesis of subunits B and C (Fig. 1), each of which can be used in the synthesis of pravastatin and its analogs. Antibody-catalyzed enantioselective aldol reactions will be used to generate the stereogenic carbinol centers on B and C. The catalytic antibodies obtained via reactive immunization are expected to have rate-enhanced formation of a new C-C bond, enantioselective formation of the newly formed carbinol center, and kinetic resolution of the existing stereogenic centers.

CATALYTIC ANTIBODIES IN A CONTINUOUS FLOW REACTOR

One obvious need in antibody catalysis, particularly for practical applications in organic synthesis, is to increase the cost-effectiveness of the antibodies. We reported the first successful noncovalent entrapment of catalytic antibodies in a sol-gel matrix. Antibodies entrapped directly within a tetramethoxysilane-derived glass retained their activity for long periods. This finding suggests that this approach is the method of choice for preparative-scale organic synthesis. We envisage that the catalytic reactor will allow convenient changes of reaction conditions, the substrate, and even the reaction type.

EMULATING CYTOCHROME P-450 WITH CATALYTIC ANTIBODIES

We have shown that catalytic antibodies elicited against a metalloporphyrin hapten mimic some of the enzymatic features of cytochrome P-450. We improved the haptens designed to elicit substrate-selective catalytic antibodies that function as P-450 analogs. For example, porphyrin (5 in Fig. 1) is a water-stable hapten that mimics the metalloporphyrin cofactor together with the organic substrate in the correct orientation that characterizes the transition state of several oxygenation reactions, including epoxidation, sulfoxidation, and hydroxylation.

ANTIBODY-CATALYZED DEUTERATION OF CARBONYL COMPOUNDS

The importance of selective isotopic labeling of organic compounds stems not only from the value of such labeling in elucidating chemical mechanisms and biosynthetic pathways but also from the usefulness of labeled compounds in biomedical applications. Because deuteration with deuterium oxide of ketones at the -position requires strong basic conditions, the chemical method is applicable only for very simple ketones. With polyfunctional ketones, however, occurrence of side reactions and the lack of selectivity are serious drawbacks. With aldehydes, these side reactions become dominant, prohibiting the use of basic conditions for -deuteration.

The mechanism by which catalytic lysine antibodies, such as antibody 38C2, promote the aldol condensation involves activation of the carbonyl donor in the form of an enamine intermediate. In the absence of a proper carbonyl acceptor, this enamine can only hydrolyze back to its ketone precursor. This ketone-enamine equilibrium provides the means for efficient exchange of the -hydrogens with the water hydrogens inside the binding site of the antibody. Lysine antibodies such as 38C2 can be highly selective catalysts for -deuteration of ketones and aldehydes, allowing chemoselective, regioselective, and enantioselective reactions to be carried out under neutral aqueous conditions at room temperature. Preliminary experiments with 38C2 indicate that deuterium labeling of ketones with this catalyst occurs with rate enhancements that rival those of enzyme catalysis.

PUBLICATIONS

Shabat, D., Grynszpan, F., Saphier, S., Turniansky, A., Avnir D., Keinan, E. An efficient sol-gel reactor for antibody catalyzed transformations. Chem. Mat. 9:2258, 1997.

Shabat, D., Shulman, H., Itzhaky, H., Reymond, J.L., Keinan, E. Enantioselectivity vs kinetic resolution in antibody catalysis: Formation of the (S) product despite preferential binding of the (R) intermediate. Chem. Commun., in press.

Shulman, A., Shabat, D., Barbas, C.F. III, Keinan, E. Teaching catalytic antibodies to undergraduate students: An organic chemistry lab experiment. J. Chem. Educ., in press.