Monoclonal antibodies (mAbs) were generated against the coplanar transition state (TS double dagger) analogue 1 and assayed for their ability to catalyze the isomerization of bridged biphenyls 4, 6, and 7. This is a relatively simple unimolecular reaction whose activation barrier arises from unfavorable steric interactions between the two benzylic methylene groups and strain in the bridging ring system. Seven mAbs were found that catalyzed the isomerization of 4 to 6; the most efficient (mAb 64D8E10) has k(cat) and K-M values of 4.3 x 10(-5) s(-1) and 420 mu M, respectively. This corresponds to a rate enhancement over the unimolecular uncatalyzed reaction (k(cat)/k(uncat)) of 2900. The dissociation constant for the TS double dagger analogue, K-d, was determined to be 210 nM. For both the antibody (64D8E10) catalyzed and uncatalyzed reactions, the free energy of activation (Delta G double dagger) is comprised largely of the enthalpy term; the antibody decreases the enthalpy of activation by 5 kcal/mol. Despite relatively large differences in the values of k(cat)/k(uncat) for the five antibodies, the ratios of K-d to K-M(4) are very similar. It is likely that the antibodies catalyze this reaction by reducing both ring strain and nonbonded steric interactions in the transition state.