The mobility of water in water/Aerosol OT (AOT)/isooctane reverse micelles has been investigated via polar solvation dynamics measurements using ultrafast, time-resolved fluorescence-upconversion spectroscopy. For the smallest reverse micelles studied, w 0 ) 1.1, no solvation dynamics are observed on the time scale of the instrument, suggesting that the water is essentially frozen. As w 0 increases, the mobility of water increases, and the time-correlation functions exhibit multiexponential decays. A subpicosecond component is observed on a time scale similar to that of diffusive bulk-water motion. 1 The other components decay on time scales of a few picoseconds to hundreds of picoseconds. The amplitude of the solvent response increases as w 0 increases, which is attributed to increased mobility of the water inside the reverse micelles. Because the micellar interior is highly ionic, the solvation dynamics of coumarin 343 in a 1 M Na + (Na 2 SO 4 ) solution were also investigated to compare the dynamics of water in the reverse micelles with the dynamics of bulk water in an ionic solution.

Specific binding of biotinilated bovine serum albumin (bBSA) and tetramethylrhodamine-labeled streptavidin (SAv−TMR) was observed by conjugating bBSA to CdSe−ZnS core−shell quantum dots (QDs) and observing enhanced TMR fluorescence caused by fluorescence resonance energy transfer (FRET) from the QD donors to the TMR acceptors. Because of the broad absorption spectrum of the QDs, efficient donor excitation could occur at a wavelength that was well resolved from the absorption spectrum of the acceptor, thereby minimizing direct acceptor excitation. Appreciable overlap of the donor emission and acceptor absorption spectra was achieved by size-tuning the QD emission spectrum into resonance with the acceptor absorption spectrum, and cross-talk between the donor and acceptor emission was minimized because of the narrow, symmetrically shaped QD emission spectrum. Evidence for an additional, nonspecific QD−TMR energy transfer mechanism that caused quenching of the QD emission without a corresponding TMR fluorescence enhancement was also observed.Fluorescence resonance energy transfer (FRET) is a process whereby the electronic excitation energy of a donor chromophore is nonradiatively transferred to a nearby acceptor molecule via a through-space dipole-dipole interaction between the donor-acceptor pair. 1-5 FRET occurs when there is appreciable overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. The strong distance dependence of the FRET efficiency has been widely exploited in studying the structure and dynamics of proteins and nucleic acids, in the detection and visualization of intermolecular association, and in the development of intermolecular binding assays. 6 FRET-based studies involving pairs of organic dye molecules as the donoracceptor complexes are often limited by cross-talk caused by spectral overlap of the donor and acceptor emission. The need for significant overlap between the emission and absorption spectra of the donor and acceptor, coupled with the narrow absorption spectrum of conventional organic dye molecules, makes it difficult to avoid direct excitation of the acceptor molecules at the excitation wavelength needed to efficiently excite the donor. In addition, the broad emission spectrum of the donor, with its long red tail, can often overlap significantly with the emission spectrum of the acceptor. Several recent reports have confirmed that luminescent semiconductor quantum dots (QDs), such as CdSe and CdTe, are able to participate in resonance energy transfer processes analogous to FRET, 7-10 which makes these materials good candidates to overcome some of the limitations associated with conventional organic dye molecules in FRET-based studies of biomolecular structure, ligand-receptor binding, etc.Semiconductor QDs are currently being investigated for their use as luminescent biological probes because of their high photostability relative to organic dye molecules and their unique, size tunable spectral properties. 11-14 QDs possessing high luminescence qu...

The solvation dynamics of water in lecithin/cyclohexane reverse micelles have been determined via ultrafast time-resolved fluorescence studies. At hydration levels w 0 ≤ 4.8, the reverse micellar samples are nonviscous. Here a single relaxation time is observed that is much longer than the response of free or bulk water. In contrast, small additions of water to the samples produces a viscous gel, referred to by others as an organogel or “living polymer”. At hydration levels of w 0 ≥ 5.8, three relaxation times are observed with approximate time constants of 0.5, 15, and 200 ps, the shortest of which correlates to free water motion. The dynamics reveal no evidence of micelle crossover or branch points associated with gel formation. In comparison to Aerosol OT reverse micelles of similar hydration, the water in the lecithin reverse micelles is significantly more restricted. It is proposed that lecithin sequesters significantly more water than previously predicted precluding formation of distinct core water pools in the micelles. The results are also compared to models for aqueous structure and dynamics near phospholipid membranes and to bulk water dynamics.

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H-D exchange reactions of methanol-d, with protonated amino acids were performed in an external-source Fourier transform mass spectrometer. Absolute rate constants were determined for the group which included glycine, alanine, valine, leucine, isoleucine and proline. By comparing reactivities with selected methyl esters, it was found that exchange on the carboxylic acid occurs 3-10 times faster than exchange on the amino group. No simple correlation is observed between the rates of H-D exchange on the acid group and the size of the alkyl group. However, the rates of exchange on the amine decrease with increasing gas-phase basicity. Glycine, the least basic amino acid, exchanges its amine hydrogens the fastest. These results are useful for determining the interaction of methanol with protonated amino acids and can provide insight into the H-D exchange reactions observed with polyprotonated proteins produced by electrospray ionization.