Dynamics of small molecules within the F127 PEO-PPO-PEO triblock copolymer gel and sol phases studied at the molecular scale.

Triplet excited states of guest molecules with different hydrophobicities were used to probe the association and dissociation dynamics of these guests with F127 micelles in the gel and sol phases. The dynamics probed was on a longer length scale than amenable with fluorescence techniques, but at a shorter length scale than probed in translational diffusion studies. The mobility of the guests at the molecular scale showed that subtle changes in the guest's structure affect the guest's release time from the micelles, where the structural features of the guest are more important than the phase, gel vs. sol, of the system.

Mechanism of a Disassembly-Driven Sensing System Studied by Stopped-Flow Kinetics.

We carried out steady-state and stopped-flow photophysical measurements to determine the kinetics of a discrete disassembly driven turn-on fluorescent system. On and off rates for both DimerDye1 assembly and nicotine binding were determined. Relative rates for these competing processes provide insight on how this system can be optimized for sensing applications. Kinetics studies in artificial saliva showed that moving to more complex media has minimal effects on the sensing ability of the system.

Nonlinear Dependence on Na+ Ions for the Binding Dynamics of Cucurbit[6]uril with the trans-1-Methyl-4-(4-hydroxystyryl)pyridinium Cation.

The binding dynamics of the trans-1-methyl-4-(4-hydroxystyryl)pyridinium cation (HSP+) to cucurbit[6]uril (CB[6]) in the presence of Na+ cations were studied to establish the effect of the relative concentrations of the system's components (HSP+, CB[6], and Na+) on these dynamics. The formation of the HSP+@CB[6] complex was temporally uncoupled from the photoisomerization of trans-HSP+, while a nonlinear effect of the Na+ cation concentration on the HSP+@CB[6] dynamics was observed. This nonlinearity is a consequence of Na+ having the opposite effect on the association and dissociation rate constants for the HSP+@CB[6] complex, creating a conceptual framework for using such nonlinearities to control multistep reactions in cucurbit[n]uril chemistry.

Noninnocent Role of Na+ Ions in the Binding of the N-Phenyl-2-naphthylammonium Cation as a Ditopic Guest with Cucurbit[7]uril.

Na+ ions influence the mechanism for the binding of the ditopic guest N-phenyl-2-naphthylammonium cation (Ph-AH+-Np) to cucurbit[7]uril (CB[7]) by facilitating, at increased Na+ concentrations, the formation of a higher-order complex. Binding of the larger naphthyl moiety of Ph-AH+-Np forms the Ph-AH+-Np@CB[7] 1:1 complex (where "@" represents an inclusion complex) at low Na+ ion concentrations (≤5 mM), whereas the inclusion of the smaller phenyl moiety in CB[7] (CB[7]@Ph-AH+-Np) is transient. Ph-AH+-Np@CB[7] is formed by reactions with free CB[7] and CB[7]·Na+ (where "·" represents an exclusion complex) with displacement of the Na+ cation. Because of the latter reaction, the dissociation of Ph-AH+-Np@CB[7] is faster at higher Na+ concentrations. At high Na+ concentrations (≥25 mM), the Na+ ion stabilizes the inclusion of the phenyl moiety in CB[7] by capping the portal of CB[7]. The dynamics of the capped Na+·CB[7]@Ph-AH+-Np 1:1 complex is slower than in the absence of Na+ capping. This stabilization of the phenyl moiety inclusion in CB[7] by Na+ leads to the formation of the Na+·CB[7]@Ph-AH+-Np@CB[7] 2:1 host-guest complex, where each moiety of the ditopic guest is included in a different CB[7]. The opposing roles of Na+ cations in the formation of the two 1:1 complexes are essential for the switch in mechanism with changes in Na+ concentration and provide an example of systems chemistry, where new properties arise in the form of an increased diversity of complexes and altered complexation dynamics that depend on the system's composition.

Steric Demand and Rate-determining Step for Photoenolization of Di-ortho-substituted Acetophenone Derivatives.

Laser flash photolysis of ketone 1 in argon-saturated methanol yields triplet biradical 1BR (τ = 63 ns) that intersystem crosses to form photoenols Z-1P (λmax = 350 nm, τ ~ 10 µs) and E-1P (λmax = 350 nm, τ > 6 ms). The activation barrier for Z-1P re-forming ketone 1 through a 1,5-H shift was determined as 7.7 ± 0.3 kcal mol-1 . In contrast, for ketone 2, which has a less sterically hindered carbonyl moiety, laser flash photolysis in argon-saturated methanol revealed the formation of biradical 2BR (λmax = 330 nm, τ ~ 303 ns) that intersystem crosses to form photoenol E-2P (λmax = 350 nm, τ > 42 µs), but photoenol Z-2P was not detected. However, in more viscous basic H-bond acceptor (BHA) solvent, such as hexamethylphosphoramide, triplet 2BR intersystem crosses to form both Z-2P (λmax = 370 nm, τ ~ 1.5 µs) and E-2P. Thus, laser flash photolysis of ketone 2 in methanol reveals that intersystem crossing from 2BR to form Z-2P is slower than the 1,5-H shift of Z-2P, whereas in viscous BHA solvents, the 1,5-H shift becomes slower than the intersystem crossing from 2BR to Z-2P. Density functional theory and coupled cluster calculations were performed to support the reaction mechanisms for photoenolization of ketones 1 and 2.

Triplet Excited States and Singlet Oxygen Production by Analogs of Red Wine Pyranoanthocyanins.

During the maturation of red wines, the anthocyanins of grapes are transformed into pyranoanthocyanins, which possess a pyranoflavylium cation as their basic chromophore. Photophysical properties of the singlet and triplet excited states of a series of synthetic pyranoflavylium cations were determined at room temperature in acetonitrile solution acidified with 0.10 mol dm-3 trifluoroacetic acid (TFA, to inhibit competitive excited state proton transfer) and at 77 K in a rigid TFA-acidified isopropanol glass. In solution, the triplet states of these pyranoflavylium cations are efficiently quenched by molecular oxygen, resulting in sensitized formation of singlet oxygen, as confirmed by direct detection of the triplet-state decay by laser flash photolysis and of singlet oxygen monomol emission in the near infrared. The strong visible light absorption, the relatively small singlet-triplet energy differences, the excited state redox potentials and the reasonably long lifetimes of pyranoflavylium triplet states in the absence of molecular oxygen suggest that they might be useful as triplet sensitizers and/or as cationic redox initiators in polar aprotic solvents like acetonitrile.

Tuning the Binding Dynamics of a Guest-Octaacid Capsule through Noncovalent Anchoring.

Hydrophobic or hydrophilic substituents have different effects on the binding dynamics of pyrene derivatives with a 2:1 capsule formed from two octaacid cavitands, showing a subtle interplay of different kinetic factors. Anchoring of the methyl group of 1-methylpyrene within one cavitand slowed the association and dissociation dynamics of the 1:1 complex by at least 1000 times when compared to the 1:1 complex for pyrene. This slow down for the transient formation of the 1:1 complex is responsible for the overall increase in stability of the 2:1 complex without affecting the overall capsule dissociation. For 1-pyrenemethanol, its residence time in the 2:1 capsule is shorter compared to that of pyrene despite both guests having similar equilibrium constants for the binding of the second cavitand, suggesting that the hydroxymethyl substituent close to the equatorial region of the capsule can interact with water during the partial opening of the capsule.

New Insights into Peptide-Silver Nanoparticle Interaction: Deciphering the Role of Cysteine and Lysine in the Peptide Sequence.

We studied the interaction of four new pentapeptides with spherical silver nanoparticles. Our findings indicate that the combination of the thiol in Cys and amines in Lys/Arg residues is critical to providing stable protection for the silver surface. Molecular simulation reveals the atomic scale interactions that underlie the observed stabilizing effect of these peptides, while yielding qualitative agreement with experiment for ranking the affinity of the four pentapeptides for the silver surface.

Determination of the kinetics underlying the pKa shift for the 2-aminoanthracenium cation binding with cucurbit[7]uril.

The binding dynamics of the 2-aminoanthracenium cation (AH(+)) and 2-aminoanthracene (A) with cucurbit[7]uril (CB[7]) was studied using stopped-flow experiments. The kinetics was followed by measuring the fluorescence changes over time for AH(+) and A, which emit at different wavelengths. The studies at various pH values showed different mechanisms for the formation of the AH(+)@CB[7] complex, with this complex formed either by the binding of AH(+) or by the initial binding of A followed by protonation. In the latter case, it was possible to determine the protonation ((1.5 ± 0.4) × 10(9) M(-1) s(-1)) and deprotonation (89 ± 7 s(-1)) rate constants for complexed A/AH(+), which showed that the pKa shift of +3.1 for A/AH(+) in the complex is mainly due to a lower deprotonation rate constant.

Phototautomerization in pyrrolylphenylpyridine terphenyl systems.

[4-(2-Pyrrolyl)phenyl]pyridines 2-4 were synthesized and their photophysical properties and reactivity in phototautomerization reactions investigated by fluorescence spectroscopy and laser flash photolysis (LFP). The pKa for the protonation of the pyridine nitrogen in 2-4 was determined by UV-vis and fluorescence titration (pKa = 5.5 for 4). On excitation in polar protic solvents, 2-4 populate charge-transfer states leading to an enhanced basicity of the pyridine (pKa* ≈ 12) and enhanced acidity of pyrrole (pKa* ≈ 8-9) enabling excited-state proton transfer (ESPT). ESPT gives rise to phototautomers and significantly quenches the fluorescence of 2-4. Phototautomers 2-T and 4-T were detected by LFP with strong transient absorption maxima at 390 nm. Phototautomers 2-T and 4-T decayed by competing uni- and bimolecular reactions. However, at pH 11 the decay of 4-T followed exponential kinetics with a rate constant of 4.2 × 10(6) s(-1). The pyridinium salt 4H(+) forms a stable complex with cucurbit[7]uril (CB[7]) with 1:1 stoichiometry (ß11 = (1.0 ± 0.2) × 10(5) M(-1), [Na(+)] = 39 mM). Complexation to CB[7] increased the pKa for 4H(+) (pKa = 6.9) and changed its photochemical reactivity. Homolytic cleavage of the pyrrole NH leads to the formation of an N-radical because of the decreased acidity of the pyrrole in the inclusion complex.