Observing the Reversible Single Molecule Electrochemistry of Alexa Fluor 647 Dyes by Total Internal Reflection Fluorescence Microscopy.

Alexa Fluor 647 is a widely used fluorescent probe for cell bioimaging and super-resolution microscopy. Herein, the reversible fluorescence switching of Alexa Fluor 647 conjugated to bovine serum albumin (BSA) and adsorbed onto indium tin oxide (ITO) electrodes under electrochemical potential control at the level of single protein molecules is reported. The modulation of the fluorescence as a function of potential was observed using total internal reflectance fluorescence (TIRF) microscopy. The fluorescence intensity of the Alexa Fluor 647 decreased, or reached background levels, at reducing potentials but returned to normal levels at oxidizing potentials. These electrochemically induced changes in fluorescence were sensitive to pH despite that BSA-Alexa Fluor 647 fluorescence without applied potential is insensitive to pH between values of 4-10. The observed pH dependence indicated the involvement of electron and proton transfer in the fluorescence switching mechanism.

Understanding the performance of a paper-based UV exposure sensor: The photodegradation mechanism of brilliant blue FCF in the presence of TiO2 photocatalysts in both the solid state and solution.

RATIONALE: The decolouration of brilliant blue FCF by the action of titanium dioxide (TiO2 ) under ultraviolet (UV) exposure has been recently reported as the basis of a paper-based sensor for monitoring UV sun exposure. The mechanism of brilliant blue FCF photodegradation in the presence of the photocatalyst and the resulting photoproducts are thus far unknown. METHODS: The UV-initiated photodegradation of brilliant blue FCF in the presence of TiO2 for both the aqueous and the solid state was investigated. Degradation in the solid state was observed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS). Decomposition of the dye in the aqueous state was analyzed using liquid chromatography/mass spectrometry (LC/MS) and ultraviolet-visible (UV-Vis) spectroscopy. RESULTS: After UV radiation exposure, the brilliant blue FCF base peak [M1 + NH4 ]+ (m/z calc. 766.194 found 766.194) decreased in the LC/MS chromatogram with a concomitant appearance of BB-FCF decomposition products involving the sequential loss of the N-ethyl and N-methylbenzene sulfonate (MBSA) groups, assigned as [M2 + H]+ (-MBSA, calc. 579.163 found 579.162), [M3 + H]+ (-MBSA, -Et, calc. 551.131 found 551.131), [M4 + H]+ (-2MBSA, calc. 409.158 found 409.158), [M5 + H]+ (-2MBSA, -Et, calc. 381.127 found 381.127). Ions [M2 + H]+ and [M3 + H]+ were also identified in the photodegradation products using MALDI-MS. Observation by UV-Vis indicated a decrease in the solution absorbance maxima and an associated blue-shift upon UV exposure in solution. CONCLUSIONS: The LC/MS analysis indicated two main oxidation processes. The most obvious was attack of the N-methylene, eliminating either ethyl or MBSA groups. The presence of the hydroxylated decomposition product M13 ([M13 + H]+ , calc. 595.157 found 595.157) supported this assignment. In addition, the detection of photoproduct M8, proposed to be 3-((ethylamino)methyl)benzenesulfonic acid ([M8 + H]+ , calc. 216.069 found 216.069), indicates an aryl-oxidative elimination. The absence of the aryl-hydroxy products normally expected to accompany the formation of M8 is proposed to be due to TiO2 -binding catechol-like derivatives, which are then removed upon filtration.

Cyclic peptide unguisin A is an anion receptor with high affinity for phosphate and pyrophosphate.

Unguisin A (1) is a marine-derived, GABA-containing cyclic heptapeptide. The biological function of this flexible macrocycle is obscure. Here we show that compound 1 lacks any detectable activity in antimicrobial growth inhibition assays, a result that runs contrary to a previous report. However, we find that 1 functions as a promiscuous host molecule in a variety of anion-binding interactions, with high affinity particularly for phosphate and pyrophosphate. We also show that a series of rigidified, backbone-fluorinated analogues of 1 displays altered affinity for chloride ions.

Quantifying highly efficient incoherent energy transfer in perylene-based multichromophore arrays.

Multichromophore perylene arrays were designed and synthesized to have extremely efficient resonance energy transfer. Using broadband ultrafast photoluminescence and transient absorption spectroscopies, transfer timescales of approximately 1 picosecond were resolved, corresponding to efficiencies of up to 99.98%. The broadband measurements also revealed spectra corresponding to incoherent transfer between localized states. Polarization resolved spectroscopy was used to measure the dipolar angles between donor and acceptor chromophores, thereby enabling geometric factors to be fixed when assessing the validity of Förster theory in this regime. Förster theory was found to predict the correct magnitude of transfer rates, with measured ∼2-fold deviations consistent with the breakdown of the point-dipole approximation at close approach. The materials presented, along with the novel methods for quantifying ultrahigh energy transfer efficiencies, will be valuable for applications demanding extremely efficient energy transfer, including fluorescent solar concentrators, optical gain, and photonic logic devices.

Thermodynamic factors impacting the peptide-driven self-assembly of perylene diimide nanofibers.

Synthetic peptides offer enormous potential to encode the assembly of molecular electronic components, provided that the complex range of interactions is distilled into simple design rules. Here, we report a spectroscopic investigation of aggregation in an extensive series of peptide-perylene diiimide conjugates designed to interrogate the effect of structural variations. By fitting different contributions to temperature dependent optical absorption spectra, we quantify both the thermodynamics and the nature of aggregation for peptides by incrementally varying hydrophobicity, charge density, length, as well as asymmetric substitution with a hexyl chain, and stereocenter inversion. We find that coarse effects like hydrophobicity and hexyl substitution have the greatest impact on aggregation thermodynamics, which are separated into enthalpic and entropic contributions. Moreover, significant peptide packing effects are resolved via stereocenter inversion studies, particularly when examining the nature of aggregates formed and the coupling between π electronic orbitals. Our results develop a quantitative framework for establishing structure-function relationships that will underpin the design of self-assembling peptide electronic materials.

Pyromellitamide gelators: exponential rate of aggregation, hierarchical assembly, and their viscoelastic response to anions.

The gelation and aggregation properties of a newly synthesized structurally simplified tetrahexyl pyromellitamide 2 have been studied and compared to the previously reported tetra(ethylhexanoate) pyromellitide 1, indicating that the ester groups in the latter significantly impede its aggregation. Morphology studies (AFM and TEM) on the aggregates formed by tetrahexyl pyromellitamide 2 in cyclohexane revealed highly uniform aggregates with different dimensions at different starting concentrations, suggesting that this molecule aggregates in a hierarchical fashion from a one-dimensional supramolecular polymer through hollow tubes or compressed helices to a network structure and then to a gel. This hypothesis is further supported by viscosity measurements that indicate a crossover point where individual supramolecular fibers get entangled at concentrations above ca. 3 mM in cyclohexane. Addition of 1 equiv of tetraalkylammonium salts of chloride or bromide, however, caused the viscosities of these pyromellitamide solutions to drop by a factor of 2-3 orders of magnitude, demonstrating the sensitivity of these aggregates to the presence of small anions. The sensitivity to anions does depend on the solubility of the salts used as small anion salts with little solubility in cyclohexane did not show this effect. Time-dependent viscosity studies showed that the aggregation of pyromellitamide 2 follows an exponential rate law, possibly related to the columnar rearrangements that are associated with the observed 6 angstroms contraction in d spacing in the XRD pattern of these gels. These results, particularly on the importance of kinetics of aggregation of self-assembled pyromellitamide gels, will be useful for future development of related materials for a number of applications, including tissue engineering and drug delivery.

Pyromellitamide aggregates and their response to anion stimuli.

The N,N',N'',N'''-1,2,4,5-tetra(ethylhexanoate) pyromellitamide is found to be capable of both intermolecular aggregation and binding to small anions. It is synthesized by aminolysis of pyromellitic anhydride with ethanolamine, followed by a reaction with hexanoyl chloride. The single-crystal X-ray structure of the pyromellitamide shows that it forms one-dimensional columnar stacks through an intermolecular hydrogen-bonding network. It also forms self-assembled gels in nonpolar solvents, presumably by a hydrogen-bonding network similar to the solid-state structure as shown by IR and XRD studies. Aggregation by intermolecular hydrogen bonding of the pyromellitamide is also observed by NMR and IR in solution. Fitting of NMR dilution data for pyromellitamide in d6-acetone to a cooperative aggregation model gave KE=232 M-1 and positive cooperativity of aggregation (rho=0.22). The pyromellitamide binds to a range of small anions with the binding strength decreasing in the order chloride>acetate>bromide>nitrate approximately iodide. The data indicate that the pyromellitamide binds two anions and that it displays negative cooperativity. The intermolecular aggregation of the pyromellitamide can also be altered using small anion stimuli; anion addition to preformed self-assembled pyromellitamide gels causes their collapse. The kinetics of anion-induced gel collapse are qualitatively correlated to the binding affinities of the same anions in solution. The cooperative anion binding properties and the sensitivity of the self-assembled gels formed by pyromellitamide toward anions could be useful in the development of sensors and switching/releasing devices.