Photocontrolled Uptake and Release of Photochromic Haptens by Monoclonal Antibodies. Evidence of Photoisomerization Inside the Hapten-Binding Site

Monoclonal antibodies against a tetrapeptide carrying a photochromic azobenzene moiety (Glu–azoAla–Gly–Gly, azoAla = l–p–phenylazophenylalanine) were prepared under conditions where the azobenzene moiety was in the trans form. The binding of the hapten peptide to the antibody was investigated by fluorescence quenching of the antibodies by the hapten peptide. The results indicated that the antibodies bind the hapten peptide when the azobenzene moiety is in the trans form, but release the peptide in the cis form. The mechanism of photoreversible binding and release was studied using a pulsed laser light. Photoisomerization of the hapten peptide was found to occur inside the binding site, indicating that the latter is flexible enough to allow the trans–cis photoisomerization within a few ten picoseconds.