Detection and mechanistic studies of multicomponent assembly by fluorescence resonance energy transfer

The kinetics and thermodynamics of multicomponent assembly in organic solut ion are investigated using fluorescence resonance energy transfer (FRET). C alix[4]arenes functionalized with ureas on their wider rims dimerize in org anic solution, via intermolecular hydrogen bonding, to form encapsulation c omplexes. When one calixarene component is outfitted with a donor fluoropho re and the other with an acceptor, dimerization brings the chromophores wit hin 20 Angstrom, a distance suitable for efficient energy transfer to occur between them. Excitation of the donor results in two colors of emitted lig ht: one fluorescence band at the donor emission wavelength, and a second at the acceptor emission wavelength signaling the noncovalent union of three species-donor, acceptor, and small-molecule guest. By monitoring these wave lengths, assembly and dissociation processes are observed in real time at n anomolar concentrations. Rate and association constants for assembly proces ses are determined for the first time, and they reveal unexpected contribut ions from the structure and concentration of the monomer. Finally, the comb ination of FRET and the molecular recognition capabilities of the encapsula tion complexes provides a sensitive and specific method for small-molecule sensing.