Intramolecular Fluorescence Resonance Energy Transfer System with Coumarin Donor Included in β-Cyclodextrin

In aqueous solutions, the fluorescence of the intramolecular fluorescence resonance energy-transfer (FRET) system 1 was strongly quenched, because of close contact between the donor and acceptor moieties. FRET occurred, and the acceptor fluorescence was increased, by adding β-cyclodextrin (β-CD) to aqueous solutions of 1. Spectral analysis supported the idea that the FRET enhancement was due to the formation of an inclusion complex of the coumarin moiety in β-CD, resulting in separation of the fluorophores. On the basis of this result, we propose that covalent binding of coumarin to β-CD will provide a FRET cassette molecule. So, compound 2 bearing β-CD covalently was designed and synthesized. Fluorescence intensity of 2 was enhanced markedly compared to the intensity of 3. Applying this FRET system, various FRET probes that will be useful for ratio imaging and also the high-throughput screening will be provided.