Antibodies Diels-alderase antibody

While it is one of the most important and versatile transformations available to organic chemists, there is no unequivocal example of a biological counterpart. Hence, attempts to generate antibodies which could catalyse this reaction were seen as an important target. The major task in producing a Diels-Alderase antibody lies in the choice of a suitable haptenic structure, because the transition state for the reaction resembles product more closely than reactants (Fig. 12). The reaction product itself is an inappropriate hapten because it is likely to result in severe product inhibition of the catalyst, thereby preventing turnover. 

Fig. 24 An alternative strategy for eliciting Diels-Alderase antibodies has employed freely rotating ferrocenes [63] and [64] as TSAs.

The development of tailored "Diels-Alderase" antibodies could therefore provide a complete control of the stereochemical outcome of this cycloaddition reaction. Another possibility would be the use of antibodies to reverse the normally observed Diels-Alder endo/exo selectivity [35]. 

Romesberg, F., Spiller, B., Schultz, P. and Stevens, R. (1998) Immunological origins of binding and catalysis in a Diels-Alderase antibody. Science, 279, 1929-1933. 

Diels-Alderase antibody 39-All minimizes product inhibition while generating a more conventional Diels-Alder product (Fig. 11) (80). Bicyclo[2.2.2]octane hapten 29 was designed to mimic the proposed boat-like transition state 30 of the [4n -F 2n] cycloaddition between diene 31 and dienophile 32. Product inhibition was circumvented by the structural disparity between the product cycloadduct and the pseudo-boat form of the hapten employed for immunization. X-ray crystallographic data of hapten 29 complexed with the 39-All Fab indicated that the diene and the dienophile are bound in a reactive conformation that reduces... 

The crystal structure of two Diels Alderase antibodies has recently been reported [75], which suggests that catalytic activity not only derives from the en-tropic effect of binding both substrates in a reactive conformation, but also from activating hydrogen bonding interactions between antibody and dienophile. 

Scheme 16. Exo- and en do-selective enantioselective Diels-Alderase antibodies...

The Diels-Alder reaction usually does not require a catalyst, and therefore, rate acceleration may be achieved by stabilizing the transition state. In fact, antibodies elicited by transition state analog are able to catalyze Diels-Alder reactions. The Diels-Alderase antibodies provide a pocket acting as an entropy trap. Using this strategy several Diels-Alderase antibodies have been created to date. " Product inhibition is an inherent problem in the use of Diels-Alderase antibodies because of the resemblance between the product and the transition state. 

To escape product inhibition, unstable boat-like haptens are used to prepare Diels-Alderase antibodies. Braisted and Schultz raised antibody 39-All with hapten 124 fixed in a boat form. The antibody 39-All catalyzed the Diels-Alder reaction between 121 and 122 to yield the chiral adduct 123 (Scheme 21). 

With hapten 134, a close analog of the transition state, Hilvert etal developed the Diels-Alderase antibody 1E9 in which elimination of the product is programmed to avoid product inhibition. In their study, the cycloaddition of reactive thiophene dioxide 130 and maleimide 131 affords the unstable adduct 132 that spontaneously eliminates SO2 followed by air oxidation to yield a stable phthalimide 133 (Scheme 23). The structure of the final product 13 3 is significantly different from that of the Diels-Alder adduct 132, and product... 

Crystal structures of three Diels-Alderase antibodies complexed with haptens (39-All and 124, 13G5 and 128b and 1E9 and 134 ) are now available. In all cases, hydrogen bonding and significant numbers of van der Waals interactions between haptens and amino acid residues in the active sites are observed. The hydrogen bonding helps to fix the orientation and conformation of the substrate but also activates the dienophile. These data provide important information on the catalytic mechanism of Diels-Alderase antibodies. 

The antibody-catalyzed Diels-Alder reaction developed by Schultz utilized a Diel-Alderase enzyme-like catalyst evolved from an antibody-combining site (Eq. 12.13). The idea is that the generation of antibodies to a structure that mimics the transition state for the Diels-Alder reaction should result in an antibody-combining site that lowers the entropy of activation by binding both the diene and dienophile in a reactive conformation. 

The library of natural catalysts has very little to offer for the catalysis of Diels-Alder (and the reverse) reactions (Diels-Alderases)[139]. For this reason one of the intriguing areas of biomimickry, namely the formation and use of antibodies exhibiting catalytic activity, has focused on [4 + 2] reactions to try to furnish proteins possessing useful catalytic properties. Thus in early studies a polyclonal catalytic antibody raised to hapten (57)[140] showed a modest rate enhancement for the reaction depicted in Scheme 48. 

A significant hurdle in the development of a Diels-Alderase catalytic antibody was minimizing product inhibition, because based on the Curtin-Hammett principle, the transition state is markedly similar to the product of a Diels-Alder reaction. A creative solution to this problem was employed in the development of antibody 1E9 (Fig. 10) (79). The cndo-hexachloronorbomene... 

Heine A, Stura EA, Yli-Kauhaluoma JT, Gao C, Deng Q, Beno 111. BR, Houk KN, Janda KD, Wilson lA. An antibody exo Diels-Alderase inhibitor complex at 1.95 angstrom resolution. Science 1998 279(5358) 1934-1940. 

---