Redox cycling amplified electrochemical detection of DNA hybridization: application to pathogen E. coli bacterial RNA

An electrochemical genosensor in which signal amplification is achieved using p-aminophenol (p-AP) redox cycling by nicotinamide adenine dinucleotide (NADH) is presented. An immobilized thiolated capture probe is combined with a sandwich-type hybridization assay, using biotin as a tracer in the detection probe, and streptavidin-alkaline phosphatase as reporter enzyme. The phosphatase liberates the electrochemical mediator p-AP from its electrically inactive phosphate derivative. This generated p-AP is electrooxidized at an Au electrode modified self-assembled monolayer to p-quinone imine (p-QI). In the presence of NADH, p-QI is reduced back to p-AP, which can be re-oxidized on the electrode and produce amplified signal. A detection limit of 1 pM DNA target is offered by this simple one-electrode, one-enzyme format redox cycling strategy. The redox cycling design is applied successfully to the monitoring of the 16S rRNA of E. coli pathogenic bacteria, and provides a detection limit of 250 CFU μL⁻¹.     

Inversion of particle-stabilized emulsions of partially miscible liquids by mild drying of modified silica particles

Using a system of modified silica particles and mixtures of water and 2,6-lutidine to form particle-stabilized emulsions, we show that subtle alterations to the hydration of the particle surface can cause major shifts in emulsion structure. We use fluorescence confocal microscopy, solid state nuclear magnetic resonance (NMR) and thermo-gravimetric analysis (TGA) to explore this sensitivity, along with other shifts caused by modifications to the silica surface chemistry. The silica particles are prepared by a variant of the St?ber procedure and are modified by the inclusion of 3-(aminopropyl)triethoxysilane and the dye fluorescein isothiocyanate. Treatment prior to emulsification consists of gently drying the particles under carefully controlled conditions. In mixtures of water and 2,6-lutidine of critical composition, the particles stabilize droplet emulsions and bijels. Decreasing particle hydration yields an inversion of the emulsions from lutidine-in-water (L/W) to water-in-lutidine (W/L), with bijels forming around inversion. So dependent is the emulsion behavior on particle hydration that microscopic differences in drying within a particle sample can cause differences in the wetting behavior of that sample, which helps to stabilize multiple emulsions. The formation of bijels at emulsion inversion is also crucially dependent on the surface modification of the silica.     

Gd(III)[15-metallacrown-5] recognition of chiral α-amino acid analogues

Chiral Ln(III)[15-metallacrown-5] complexes with phenyl side chains have been shown to encapsulate aromatic carboxylates reversibly in their hydrophobic cavities. Given the importance of selective guest binding for applications of supramolecular containers in synthesis, separations, and materials design, the affinity of Gd(III)[15-metallacrown(Cu(II), L-pheHA)-5] hosts for a series of chiral carboxylate guests with varying substitutions on the α-carbon (phenylalanine, N-acetyl-phenylalanine, phenyllactate, mandelate, methoxyphenylacetate) has been investigated. Differential binding of S- and R-phenylalanine was revealed by X-ray crystallography, as the S-enantiomer exclusively forms associative hydrogen bonds with oxygen atoms in the metallacrown ring. Selective guest binding in solution was assessed with isothermal titration calorimetry, which measures the sequential guest binding in the hydrophobic cavity first and the hydrophilic face of the host, and a cyclic voltammetry assay, which quantifies guest binding strength in the hydrophobic cavity of the host exclusively. In solution, the Gd(III)[15-metallacrown(Cu(II), L-pheHA)-5] hydrophobic cavity exhibits modest chiral selectivity for enantiomers of phenylalanine (K(S)/K(R) = 2.4) and mandelate (K(S)/K(R) = 1.22). Weak binding constants of ～100 M(-1) were measured for neutral and -1 charged carboxylates with hydrophilic functional groups (ammonium, N-acetyl, methyl ether). Weaker binding relative to the unsubstituted guests is attributed to unfavorable interactions between the hydrophilic functionalities of the guest and the hydrophobic cavity of the host. In contrast, binding constants greater than 2000 M(-1) were measured for α-hydroxy analogues phenyllactate and mandelate. The significantly increased affinity likely arises from the guests being bound as a -2 anion upon metal-assisted deprotonation in the Gd(III)[15-metallacrown(Cu(II), l-pheHA)-5] cavity. It is established that guest binding affinity in the hydrophobic cavity of the host follows the general trend of neutral zwitterion < monoanion < dianion, with hydrophilic functional groups decreasing the binding affinity. These results have broad implications for the development of metallacrowns as supramolecular catalysts or in chiral separations.     

Assessment of the electronic structure of 2,2'-pyridylpyrrolides as ligands

The ligand class 2,2'-pyridylpyrrolide is surveyed, both for its structural features and its electronic structure, when attached to monovalent K, Cu, Ag, Au, and Rh. The influence of pyrrolide ring substituents is studied, as well as the question of push/pull interaction between the pyridyl and pyrrolide halves. The π donor ability of the pyrrolide is found to be less than that of an analogous phenyl. However, in contrast to the phenyl analog, the HOMO is pyrrolide π in character for pyridylpyrrolide complexes of copper and rhodium, while it is conventionally metal localized for planar, d(8) rhodium pyridylphenyl. Monovalent three-coordinate copper complexes show great deviations from Y-shaped toward T-shaped structures, including cases where the pyridyl ligand bonds only weakly.     

On the construction of quasidiabatic state representations of bound adiabatic state potential energy surfaces coupled by accidental conical intersections: incorporation of higher order terms

The quadratic vibronic coupling model is an important computational tool for simulating photoelectron spectra involving strongly coupled electronic states in polyatomic molecules. However, recent work has indicated the need for higher order terms, with most of the initial studies focusing on molecules with symmetry-required degeneracies. In this study we report an extension of our approach for constructing fully quadratic representations of bound electronic states coupled by conical intersections, which allows for the inclusion of higher order terms, demonstrated here employing a quartic expansion. Procedures are developed that eliminate unphysical behavior for large displacements, a problem likely to be an endemic to anharmonic expansions. Following work on representing dissociative electronic states, Lagrange multipliers are used to constrain the constructed representation to reproduce exactly the energy, energy gradients, and∕or derivative couplings at specific points, or nodes, in nuclear coordinate space. The approach is illustrated and systematically studied using the four lowest electronic states of triazolyl, (CH)(2)N(3).     

Decoherence and surface hopping: when can averaging over initial conditions help capture the effects of wave packet separation?

Fewest-switches surface hopping (FSSH) is a popular nonadiabatic dynamics method which treats nuclei with classical mechanics and electrons with quantum mechanics. In order to simulate the motion of a wave packet as accurately as possible, standard FSSH requires a stochastic sampling of the trajectories over a distribution of initial conditions corresponding, e.g., to the Wigner distribution of the initial quantum wave packet. Although it is well-known that FSSH does not properly account for decoherence effects, there is some confusion in the literature about whether or not this averaging over a distribution of initial conditions can approximate some of the effects of decoherence. In this paper, we not only show that averaging over initial conditions does not generally account for decoherence, but also why it fails to do so. We also show how an apparent improvement in accuracy can be obtained for a fortuitous choice of model problems, even though this improvement is not possible, in general. For a basic set of one-dimensional and two-dimensional examples, we find significantly improved results using our recently introduced augmented FSSH algorithm.     

Communication: A new spectroscopic window on hydroxyl radicals using UV + VUV resonant ionization

A 1 + 1' multiphoton ionization (MPI) detection scheme for OH radicals is presented. The spectroscopic approach combines initial excitation on the well-characterized A(2)Σ(+)-X(2)Π band system with vacuum ultraviolet (VUV) ionization via autoionizing Rydberg states that converge on the OH(+) A(3)Π ion state. Jet-cooled MPI spectra on the (1,0) and (2,0) bands show anomalous rotational line intensities, while initial excitation on the (0,0) band does not lead to detectable OH(+) ions. The onset of ionization with the (1,0) band is attributed to an energetic threshold; the combined UV + VUV photon energies are above the first member of the autoionizing (A(3)Π)nd Rydberg series. Comparison of the OH 1 + 1' MPI signal with that from single photon VUV ionization of NO indicates that the cross section for photoionization from OH A(2)Σ(+), v' = 1 is on the order of 10(-17) cm(2).     

Linear and nonlinear optical properties of cationic bipyridyl iridium(III) complexes: tunable and photoswitchable?

A series of cationic Ir(III) substituted bipyridyl ()(N(∧)N (N(∧)N-bpy) complexes incorporating electron-donor and -acceptor substituents, [Ir(C(∧)N-ppy-R')(2)(N(∧)N-bpy-CH═CH-C(6)H(4)-R)][X] (X(-) = PF(6)(-) or C(12)H(25)SO(3)(-)), 2 (a, R = NEt(2) and R' = Me; b, R = O-Oct and R' = Me; c, R = NO(2) and R' = C(6)H(13); C(∧)N-ppy = cyclometalated 2-phenylpyridine, [Ir(C(∧)N-ppy-Me)(2)(N(∧)N-bpy-CH═CH-thienyl-Me)][PF(6)], 2d, and the dithienylethene (DTE)-containing complex 2e have been synthesized and characterized, and their absorption, luminescence, and quadratic nonlinear optical (NLO) properties are reported. Density functional theory (DFT) and time-dependent-DFT (TD-DFT) calculations on the complexes facilitate a detailed assignment of the excited states involved in the absorption and emission processes. All five complexes are luminescent in a rigid glass at 77 K, displaying vibronically structured spectra with long lifetimes (14-90 μs), attributed to triplet states localized on the styryl-appended bipyridines. The second-order NLO properties of 2a-d and related complexes 1a-d with 1,10-phenanthrolines have been investigated by both electric field induced second harmonic generation (EFISH) and harmonic light scattering (HLS) techniques. They are characterized by high negative EFISH μβ values which decrease when the ion pair strength between the cation and the counterion (PF(6)(-), C(12)H(25)SO(3)(-)) increases. The EFISH response is mainly controlled by metal-to-ligand charge-transfer/ligand-to-ligand charge-transfer (MLCT/L'LCT) processes. A combination of HLS and EFISH techniques is used to evaluate both the dipolar and octupolar contributions to the total quadratic hyperpolarizability, demonstrating that the major contribution is controlled by the octupolar part. The incorporation of a photochromic DTE unit into the N(∧)N-bpy ligand (complex 2e) allows the luminescence to be switched ON or OFF. The photocyclisation of the DTE unit can be triggered by using either UV (365 nm) or visible light (430 nm), leading to an efficient quenching of the ligand-based 77 K luminescence, which can be restored upon irradiation of the closed form at 715 nm. In contrast, no significant modification of the EFISH μβ value is observed upon photocyclization, suggesting that the quadratic NLO response is dominated by the MLCT/L'LCT processes, rather than by the intraligand excited states localized on the substituted bipyridine ligand.     

Hydrothermal descriptive chemistry and single crystal structure determination of cesium and rubidium thorium fluorides

Two new cesium thorium fluorides and three new rubidium thorium fluorides have been synthesized hydrothermally and structurally characterized. The structures of two polymorphs of CsTh(3)F(13) are described in space group P6/mmm with a = 8.2608(14) and c = 8.6519(17) and space group Pmc2(1) with a = 8.1830(16), b = 7.5780(15), and c = 8.6244(17). The analogous orthorhombic compound RbTh(3)F(13), with a = 8.1805(16), b = 7.4378(15), and c = 8.6594(17) in space group Pmc2(1), is also reported. Two other rubidium thorium fluorides are also described: RbTh(2)F(9) crystallizes in the space group Pnma where a = 8.9101(18), b = 11.829(2), and c = 7.4048(15), and Rb(7)Th(6)F(31) crystallizes in the space group R3 where a = 15.609(2) and c = 10.823(2). Comparison of these materials was made on the basis of their structures and synthesis conditions. The formation of these species in hydrothermal fluids appears to be dependent upon the concentration of the alkali fluoride mineralizer solution and, thus, the ratio of alkali ions to thorium in the system.     

Bidirectional "ping-pong" energy transfer and 3000-fold lifetime enhancement in a Re(I) charge transfer complex

The synthesis and photophysics of a new Re(I)-carbonyl diimine complex, Re(PNI-phen)(CO)(3)Cl, where the PNI-phen is N-(1,10-phenanthroline)-4-(1-piperidinyl)naphthalene-1,8-dicarboximide is reported. The metal-to-ligand charge transfer (MLCT) emission lifetime was increased approximately 3000-fold at room temperature with respect to that of the model complex [Re(phen)(CO)(3)Cl] as a result of thermal equilibrium between the emissive (3)MLCT state and a long-lived triplet ligand-centered ((3)LC) state on the PNI chromophore. This represents the longest excited state lifetime (τ = 651 μs) that has ever been observed for a Re(I)-based CT photoluminescence at room temperature. The energy transfer processes and the associated rate constants leading to the establishment of the excited state equilibrium were elucidated by a powerful combination of three techniques (transient visible and infrared (IR) absorption and photoluminescence), each applied from ultrafast to the micro/milliseconds time scale. The MLCT excited state was monitored by transient IR using CO vibrations through time intervals where the corresponding signals obtained in conventional visible transient absorption were completely obscured by overlap with strong transients originating from the pendant PNI chromophore. Following initial excitation of the (1)LC state on the PNI chromophore, energy is transferred to form the MLCT state with a time constant of 45 ps, a value confirmed in all three measurement domains within experimental error. Although transient spectroscopy confirms the production of the (3)MLCT state on ultrafast time scales, Fo?rster resonance energy transfer calculations using the spectral properties of the two chromophores support initial singlet transfer from (1)PNI* to produce the (1)MLCT state by the agreement with the experimentally observed energy transfer time constant and efficiency. Intersystem crossing from the (1)MLCT to the (3)MLCT excited state is believed to be extremely fast and was not resolved with the current experiments. Finally, triplet energy was transferred from the (3)MLCT to the PNI-centered (3)LC state in less than 15 ns, ultimately achieving equilibrium between the two excited states. Subsequent relaxation to the ground state occurred via emission resulting from thermal population of the (3)MLCT state with a resultant lifetime of 651 μs. The title chromophore represents an interesting example of "ping-pong" energy transfer wherein photon excitation first migrates away from the initially prepared (1)PNI* excited state and then ultimately returns to this moiety as a long-lived excited triplet which disposes of its energy by equilibrating with the photoluminescent Re(I) MLCT excited state.