Excited-state proton transfer of protonated 1-aminopyrene complexed with β-cyclodextrin

Excited-state proton transfer in inclusion complexes of 1-aminopyrene and α-naphthol with cyclodextrins was studied by time-resolved spectroscopy. We find that the kinetic studies are much more sensitive to the complexation than are steady-state spectroscopic measurements. Our data suggest two distinct binding orientations for 1-aminopyrene bound to β-cyclodextrin. The rate of proton transfer is increased by a factor of 2-3 compared to pure water with the inclusion complex resembling a water-ethanol mixture near the 75% by volume alcohol composition. Deuteration enhances the dissociation rate by a factor of 3.5 in both 80% (by volume) ethanol water mixtures and ion the complex with β-cyclodextrin. In the case of α-naphthol the rate of proton transfer slows considerably in the inclusion complex and has the rate expected for an 80% by volume ethanol-water mixture. We suggest that the water near the cavity rim of β-cyclodextrin is modified by the extensive network of OH groups in such a way as to increase its basicity. Similar effects may occur in enzymic systems.