Experimental and model investigation of the time-dependent 2-dimensional distribution of binding in a herringbone microchannel

A microfluidic device known to mix bulk solutions, the herringbone microchannel, was incorporated into a surface-binding assay to determine if the recirculation of solution altered the binding of a model protein (streptavidin) to the surface. Streptavidin solutions were pumped over surfaces functionalized with its ligand, biotin, and the binding of streptavidin to those surfaces was monitored using surface plasmon resonance imaging. Surface binding was compared between a straight microchannel and herringbone microchannels in which the chevrons were oriented with and against the flow direction. A 3-dimensional finite-element model of the surface binding reaction was developed for each of the geometries and showed strong qualitative agreement with the experimental results. Experimental and model results indicated that the forward and reverse herringbone microchannels substantially altered the distribution of protein binding (2-dimensional binding profile) as a function of time when compared to a straight microchannel. Over short distances (less than 1.5 mm) down the length of the microchannel, the model predicted no additional protein binding in the herringbone microchannel compared to the straight microchannel, consistent with previous findings in the literature.     

Enantioselective synthesis of the dimeric pyranonaphthoquinone core of the cardinalins using a late-stage homocoupling strategy

The enantioselective synthesis of a dimeric pyranonaphthoquinone closely related to the cardinalins is described. Whilst attempts to effect a double Hauser-Kraus annulation of enone 5 were unsuccessful using both bis-phthalide 4 and bis-sulfone 21, a single annulation of cyanophthalide 28 with enone 5 furnished functionalised naphthalene 31. Suzuki-Miyaura homocoupling of the aryl triflate 29 derived from 31 effected a late-stage construction of the biaryl bond and facilitated access to the biaryl 3. Double stereoselective lactol reduction installed the 1,3-cis stereochemistry of the pyran rings and a final double oxidative demethylation step furnished model dimer 1, completing the enantioselective synthesis of a dimeric pyranonaphthoquinone bearing the core structure of cardinalin 3.     

The Sharpless asymmetric aminohydroxylation reaction: optimising ligand/substrate control of regioselectivity for the synthesis of 3- and 4-aminosugars

An investigation of the factors responsible for the sense and magnitude of regioselectivity in the Sharpless asymmetric aminohydroxylation (AA) has been conducted. Theoretical investigations of ligand-osmium binding geometry and experimental investigations of the Sharpless AA reaction on a series of functionalized pent-2-enoic acid ester substrates demonstrate that the opposite regioselectivity afforded using PHAL and AQN ligands results from a change in substrate orientation with respect to the catalyst. Two distinct ligand binding domains within the catalyst have been proposed that undergo attractive interactions with the substrates. Selective access to each of the four potential regio- and stereo-isomeric AA products could be achieved through the appropriate choice of ligand and substrate. These results have been applied toward the efficient stereoselective synthesis of naturally occurring and regioisomeric 3- and 4-aminosugar derivatives.     

Structure and dynamics of surfactant interfaces in organically modified clays

Organic modification of clays with surfactants is required for the preparation of polymer-clay nanocomposites for a variety of applications. We have studied the structure and dynamics of interfaces in synthetic clays modified with phosphonium surfactants. The chemical shifts, line widths, and relaxation times measured by 31P, 13C, and 1H NMR and the relaxation times measured by impedance spectroscopy allow us to monitor the dynamics over a wide range of time scales. The results show that the phosphonium headgroup is most restricted and that the mobility increases with increasing separation from the clay surface. The carbon chemical shifts show that the 16-carbon and 12-carbon surfactant tails of hexadecyltributyl phosphonium and dodecytriphenyl phosphonium are disordered at the interface and experience mobility over a range of time scales. The dynamics depend most strongly on the structure of the surfactant headgroup, and tributylphosphoniums are more mobile than the triphenylphosphoniums. Two dimensional chemical shift anisotropy spin exchange experiments show that the phosphorus atoms in the triphenylphosphonium surfactant are immobile on the clay surface on a 1 s time scale. The dynamics measured by impedance spectroscopy show a similar dependence on headgroup structure, even though the processes occur on very different time scales and length scales. The relationship between the structure and dynamics of the interface and the properties of composites are considered.     

Ultrafast energy transfer dynamics of a bioinspired dyad molecule

A caroteno-purpurin dyad molecule was studied by steady-state and pump-probe spectroscopies to resolve the excited-state deactivation dynamics of the different energy levels as well as the connecting energy flow pathways and corresponding rate constants. The data were analyzed with a two-step multi-parameter global fitting procedure that makes use of an evolutionary algorithm. We found that following ultrafast excitation of the donor (carotenoid) chromophore to its S2 state, the energy flows via two channels: energy transfer (70%) and internal conversion (30%) with time constants of 54 and 110 fs, respectively. Additionally, some of the initial excitation is found to populate the hot ground state, revealing another limitation to the functional efficiency. At later times, a back transfer occurs from the purpurin to the carotenoid triplet state in nanosecond timescales. Details of the energy flow within the dyad as well as species associated spectra are disentangled for all excited-state and ground-state species for the first time. We also observe oscillations with the most pronounced peak on the Fourier transform spectrum having a frequency of 530 cm(-1). The dyad mimics the dynamics of the natural light-harvesting complex LH2 from Rhodopseudomonas acidophila and is hence a good model system to be used in studies aimed to further explain previous work in which the branching ratio between the competing pathways of energy loss and energy transfer could be manipulated by adaptive femtosecond pulse shaping.     

Rearrangement of a cyclohexyl radical to a cyclopentylmethyl radical on the avermectin skeleton

The rearrangement of a substituted cyclohexyl radical to a cyclopentylmethyl radical on the skeleton of avermectin B1 was observed experimentally and explored computationally. The Stork-Nishiyama methodology was applied to the macrocycle of interest followed by a Tamao oxidation. The expected 5-6 fused ring product was observed in minor amounts. The major product was a 5-5 fused ring resulting from apparent conversion of the initially formed cyclohexyl radical to a cyclopentylmethyl radical. Preliminary computational results indicate that substituents in the macrocycle induce the rearrangement.     

Continuum of mechanisms for nucleophilic substitutions of cyclic acetals

The effect of nucleophile strength on diastereoselectivity in the nucleophilic substitution of cyclic acetals was explored. Stereoselectivity remained constant and high as nucleophilicity increased until a threshold value was reached. Beyond this point, however, selection of Lewis acid determined whether stereochemical inversion or erosion was observed.     

Single sample extraction protocol for the quantification of NAD and NADH redox states in Saccharomyces cerevisiae

A robust redox extraction protocol for quantitative and reproducible metabolite isolation and recovery has been developed for simultaneous measurement of nicotinamide adenine dinucleotide (NAD) and its reduced form, NADH, from Saccharomyces cerevisiae. Following culture in liquid media, yeast cells were harvested by centrifugation and then lysed under nonoxidizing conditions by bead blasting in ice-cold, nitrogen-saturated 50 mM ammonium acetate. To enable protein denaturation, ice cold nitrogen-saturated CH(3)CN/50 mM ammonium acetate (3:1 v/v) was added to the cell lysates. Chloroform extractions were performed on supernatants to remove organic solvent. Samples were lyophilized and resuspended in 50 mM ammonium acetate. NAD and NADH were separated by HPLC and quantified using UV-Vis absorbance detection. NAD and NADH levels were evaluated in yeast grown under normal (2% glucose) and calorie restricted (0.5% glucose) conditions. Results demonstrate that it is possible to perform a single preparation to reliably and robustly quantitate both NAD and NADH contents in the same sample. Robustness of the protocol suggests it will be (i) applicable to quantification of these metabolites in other cell cultures; and (ii) amenable to isotope labeling strategies to determine the relative contribution of specific metabolic pathways to total NAD and NADH levels in cell cultures.     

Copper-Phosphido Catalysis: Enantioselective Addition of Phosphines to Cyclopropenes**

We describe a copper catalyst that promotes the addition of phosphines to cyclopropenes at ambient temperature. A range of cyclopropylphosphines bearing different steric and electronic properties can now be accessed in high yields and enantioselectivities. Enrichment of phosphorus stereocenters is also demonstrated via a Dynamic Kinetic Asymmetric Transformation (DyKAT) process. A combined experimental and theoretical mechanistic study supports an elementary step featuring insertion of a Cu^I-phosphido into a carbon-carbon double bond. Density functional theory calculations reveal migratory insertion as the rate- and stereo-determining step, followed by a syn-protodemetalation.     

Combining Nitridoborates,Nitrides and Hydrides—Synthesis and Characterization of the Multianionic Sr6N[BN2]2H3

Multianionic metal hydrides, which exhibit a wide variety of physical properties and complex structures, have recently attracted growing interest. Here we present Sr₆N[BN₂]₂H₃, prepared in a solid-state ampoule reaction at 800 °C, as the first combination of nitridoborate, nitride and hydride anions within a single compound. The crystal structure was solved from single-crystal X-ray and neutron powder diffraction data in space group P2₁/c (no. 14), revealing a three-dimensional network of undulated layers of nitridoborate units, strontium atoms and hydride together with nitride anions. Magic angle spinning (MAS) NMR and vibrational spectroscopy in combination with quantum chemical calculations further confirm the structure model. Electrochemical measurements suggest the existence of hydride ion conductivity, allowing the hydrides to migrate along the layers.