|M.Sc Student||Vebenov Dafna|
|Subject||Scope of Photochemical Reactions Catalyzed by Antibody|
|Department||Department of Chemistry||Supervisor||Professor Emeritus Ehud Keinan|
Antibody 20F10, which was elicited against cis and trans oxetanes, 1 and 2, has been previously shown to catalyze the enantioselective photochemical Norrish type II cyclization to produce a cyclobutanol product. This research discovered another photochemical reaction that is catalyzed by antibody 20F10 - the photocyclization of cis-stilbenes to the corresponding substituted phenanthrenes. This reaction was investigated with the synthetic cis-stilbene substrates 3a-c. Authentic samples of all possible photoproducts were also synthesized, including the trans-stilbenes, 4a-c, and the corresponding phenanthrenes, 5a-c.
Kinetic studies showed that the methoxy substituents on the substrate play a crucial role in specific binding catalysis, as reflected by the kcat/KM values. The substrate-binding trend was found to be the opposite of what has been observed in the Norris type II cyclization reaction. These observations raised questions concerning the location of the catalytic site on the antibody surface. This issue was investigated by inhibition experiments, showing that the reaction was not inhibited by the original haptens, which are binding site inhibitors. Moreover, it was found that the reaction is catalyzed equally well not only by 20F10 but also by many other irrelevant IgG antibodies, suggesting that the catalytic site was probably located in the conserved region of the antibody. Added to the previously reported conversion of singlet molecular oxygen to hydrogen peroxide, which is catalyzed by all IgG proteins regardless of their specificity, the present observation represents the second case of a reaction that is generally catalyzed by antibodies.