Inhibiting intramolecular electron transfer in flavin adenine dinucleotide by host-guest interaction: a fluorescence study.

Modulation in the photophysical properties and intramolecular electron transfer behavior of the flavin adenine dinucleotide (FAD) molecule has been investigated in the presence of the macrocyclic hosts, alpha-, beta- and gamma-cyclodextrins (CDs), using absorption and steady-state and time-resolved fluorescence measurements. The results demonstrate that only the beta-CD host has a suitable cavity dimension to form a weak inclusion complex with FAD by encapsulating the adenine moiety, which is the preferred binding site in the large FAD molecule. Interestingly, in spite of the weak binding interaction, a significant enhancement in the fluorescence intensity of FAD is observed on complexation with beta-CD, and this has been attributed mainly to the modulation in the conformational dynamics of FAD in the presence of beta-CD. In aqueous solutions, a good fraction of FAD molecules exist in a "closed" conformation with the adenine and isoalloxazine rings stacked on each other, thus leading to very efficient fluorescence quenching due to the ultrafast intramolecular electron transfer from adenine to the isoalloxazine moiety. Complex formation with beta-CD inhibits this intramolecular electron transfer by changing the "closed" conformation of FAD to the "open" form, wherein the adenine and isoalloxazine moieties are widely separated, thus prohibiting the fluorescence quenching process. Further evidence for the conformational changes has been obtained by the observation of a long lifetime component in the fluorescence decay of FAD in the presence of beta-CD, which corresponds to the decay of the unquenched "open" form of FAD. Fluorescence up-conversion studies also indicate the absence of any ultrafast component in the fluorescence decay arising from the complexed FAD, thus supporting the formation of the "open" form in the presence of beta-CD, with no intramolecular electron transfer.