Nanoparticles are important catalysts for petroleum processing, energy conversion, and pollutant removal. As compared to their bulk counterparts, their often superior or new catalytic properties result from their nanometer size, which gives them increased surface-to-volume ratios and chemical potentials. The size of nanoparticles is thus pivotal for their catalytic properties. Here, we use single-molecule fluorescence microscopy to study the size-dependent catalytic activity and dynamics of spherical Au-nanoparticles under ambient solution conditions. By monitoring the catalysis of individual Au-nanoparticles of three different sizes in real time with single-turnover resolution, we observe clear size-dependent activities in both the catalytic product formation reaction and the product dissociation reaction. Within a model of classical thermodynamics, these size-dependent activities of Au-nanoparticles can be accounted for by the changes in the adsorption free energies of the substrate resazurin and the product resorufin because of the nanosize effect. We also observe size-dependent differential selectivity of the Au-nanoparticles between two parallel product dissociation pathways, with larger nanoparticles less selective between the two pathways. The particle size also strongly influences the surface-restructuring-coupled catalytic dynamics; both the catalysis-induced and the spontaneous dynamic surface restructuring occur more readily for smaller Au-nanoparticles due to their higher surface energies. Using a simple thermodynamic model, we analyze the catalysis- and size-dependent dynamic surface restructuring quantitatively; the results provide estimates on the activation energies and time scales of spontaneous dynamic surface restructuring that are fundamental to heterogeneous catalysis in both the nano- and the macro-scale. This study further exemplifies the power of the single-molecule approach in probing the intricate workings of nanoscale catalysts.

Metalloregulators regulate transcription in response to metal ions. Many studies have provided insights into how transcription is activated upon metal binding by MerR-family metalloregulators. In contrast, how transcription is turned off after activation is unclear. Turning off transcription promptly is important, however, as the cells would not want to continue expressing metal resistance genes and thus waste energy after metal stress is relieved. Using single-molecule FRET measurements we studied the dynamic interactions of the copper efflux regulator (CueR), a Cu þ -responsive MerR-family metalloregulator, with DNA. Besides quantifying its DNA binding and unbinding kinetics, we discovered that CueR spontaneously flips its binding orientation at the recognition site. CueR also has two different binding modes, corresponding to interactions with specific and nonspecific DNA sequences, which would facilitate recognition localization. Most strikingly, a CueR molecule coming from solution can directly substitute for a DNA-bound CueR or assist the dissociation of the incumbent CueR, both of which are unique examples for any DNA-binding protein. The kinetics of the direct protein substitution and assisted dissociation reactions indicate that these two unique processes can provide efficient pathways to replace a DNA-bound holo-CueR with apo-CueR, thus turning off transcription promptly and facilely.single-molecule imaging | protein-DNA interaction dynamics B acteria often dwell in environments with high concentrations of metals. Some of these metals are essential, but many are toxic. Even the essential metals, for example iron and copper, can become detrimental above a certain concentration inside cells. Many biological processes are thus present to regulate and maintain intracellular metal homeostasis (1-9). One of them is through metalloregulators, which respond to metal ions and regulate the transcription of genes that protect the bacteria from metal-induced stress (5-7, 10). The MerR-family metalloregulators respond to many metal ions with high selectivity and sensitivity, such as Hg 2þ and Cu 2þ (5,11,12).All MerR-family metalloregulators are homodimeric proteins. They regulate transcription via a DNA distortion mechanism (5, 13-16). They recognize specific dyad-symmetric DNA sequences within a promoter, and both their apo and holo forms bind DNA tightly. In the absence of metal, the metalloregulator bends the DNA; in this configuration RNA polymerase (RNAp) cannot interact with both −10 and −35 sequences properly and transcription is repressed. Upon binding metal, the metalloregulator changes its conformation and further unwinds the DNA slightly to allow proper RNAp interactions with the −10 and −35 sequences; transcription is then activated.Although the mechanisms of transcription activation by MerRfamily metalloregulators are well-studied (5, 13-16), little is yet known about how transcription activation is reversed. Turning off transcription promptly is important, however, as the cells would not want to conti...

Excited-state proton transfer (ESPT) of 2-(2'-pyridyl)benzimidazole (2PBI) in reverse micelles has been studied by steady-state and time-resolved fluorescence spectroscopy. The nanometer sized water pool in the n-heptane/Aerosol OT (AOT)/water microemulsion is found to promote tautomer emission of this probe, as is evident from the emergence of a Stokes shifted band at 450 nm at the expense of the normal emission band on increasing the water content of the system. In the nonaquous microemulsion with a methanol core, the normal emission is quenched but no tautomer emission is obtained. With an acetonitrile core, there is no change in emission properties. Similarly, there is no evidence of ESPT in Triton X-100 reverse micelles. This indicates the requirement of ESPT to occur in microheterogeneous media; the medium should be a ternary system comprised of water and a hydrophobic phase separated by a negatively charged interface. In the microemulsions with an aqueous core, the fluorescence decays of 2PBI at the red end exhibit rise times of 0.8 ns and the time-resolved area-normalized emission spectra (TRANES) exhibit an isoemissive point, indicating slow dynamics of the two-state ESPT of 2PBI in aqueous AOT reverse micelles. The origin of the selective enhancement in AOT microemulsions as well as the slow dynamics is explored using fluorescence spectroscopic techniques, with support from quantum chemical calculation.

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The extents of fluorescence enhancement of epicocconone are found to be different in the micelles of the surfactants sodium dodecyl sulfate (SDS) and Triton X100 (TX 100). A decrease in fluorescence, observed in the cationic cetyltrimethylammonium bromide (CTAB) micelles, is rationalized by the formation of anions of the fluorophore at the Stern layer. To understand the difference in the effects of SDS and TX 100, the nature of the excited-state process in the fluorophore has been investigated by fluorescence spectroscopy, supported by complementary quantum chemical calculations. The excited-state dynamics of epicocconone is found to depend on polarity and viscosity of the medium, with a more pronounced dependence on viscosity. An inspection of the molecular orbitals involved in the electronic absorption of the molecule reveals the possibility of photoisomerization, which conforms to the observed solvent dependence of the fluorescence spectral properties. An apparent mismatch between trends observed in steady-state spectra and those in temporal decays indicates a significant contribution of an ultrafast component, which cannot be detected in the time resolution of our instrument. The viscosity dependence of the fluorescence quantum yields provides an explanation for the difference in the extents of fluorescence enhancement in the two micelles, in the light of location of the fluorophore at different depths of the micelle. The enhancement of fluorescence, with an unchanged fluorescence maximum, opens up the possibility that the fluorophore could be a useful dual emitting marker for fluorescence microscopy of heterogeneous systems, as the fluorescence of protein-bound epicocconone has been previously reported to be significantly red-shifted.