Supramolecular host-guest interaction of acridine dye with cyclodextrin macrocycles: Photophysical, pKa shift and quenching study

Supramolecular host-guest interaction of acridine dye, a biologically important molecule, with beta and gamma cyclodextrin (beta CD and gamma CD, respectively) macrocycles has been investigated in aqueous solution using photophysical methods. Neutral form (Ac) of the dye undergoes significant interaction with beta CD and gamma CD hosts, forming mainly the 1:1 stoichiometric host-guest complex. Fluorescence intensity and lifetime of Ac reduces drastically on binding with CD hosts, ascribed to H-bonding interaction of the portal OH groups of the hosts with bound excited dye. The fraction of the free dye in these systems also shows significant reduction in fluorescence lifetime that displays liner Stern-Volmer correlation, suggesting dynamic quenching of the free dye through similar H-bonding interaction with the portal OH groups of the hosts. Present contention is support by the fluorescence intensity and lifetime reduction of the dye on using D-glucose as the quencher. Ac interacts relatively strongly with beta CD than gamma CD, attributed to tighter binding of the dye with smaller beta CD cavity than larger gamma CD cavity. The inclusion complex formation in the present Ac-CD systems is suitably supported by time-resolved fluorescence anisotropy results. Unlike neutral Ac form, the protonated AcH+ form of the dye does not show any significant interaction with either beta CD or gamma CD hosts. The differential interaction of Ac and AcH+ forms with the CDs results in the downward pK(a) shifts for the dye by similar to 0.43 and similar to 0.32 units, respectively, in the presence of 16 mM beta CD and 40 mM gamma CD (4) hosts. Since CD concentrations much higher than these could not be used due to their solubility limitations and hence the condition of complete complexation for both Ac and AcH+ forms with CD hosts could not be achieved in these solutions, the above pK(a) shifts are certainly the apparent ones, and the true pK(a) shifts for the complexed dye would have been significantly higher than the above values. In any case, the observed pK(a) shifts in the present study is the first report on the macrocyclic host induced changes in the prototropic properties of acridine dye. Observed modulations in the photophysics and acid-base properties of acridine dye reported here can have a bearing to the host assisted drug stabilization and supramolecular fluorescence probes and sensors. (c) 2013 Elsevier B.V. All rights reserved.