pH dependence of hydrochloric acid diffusion through gastric mucus: correlation with diffusion through a water layer using a membrane-mounted glass pH electrode

Solute diffusion coefficients (D) can indicate a dependence upon actual solute concentrations. Here a single compartment has been utilized, in which effective HCl diffusion to a membrane-mounted glass pH electrode can be measured across the pH spectrum. The study has investigated HCl diffusion through both mucus and water layers as a function of HCl concentration. The observed dynamic responses of a liquid-film and mucus-coated electrodes over a range of HCl concentrations suggest that the speed at which equilibrium is attained is pH dependent; equilibrium was reached rapidly under more acidic and alkaline conditions. Estimated values of DHCl also indicate a strong pH dependence for both liquid film and mucus. In both instances, a greater than 10-fold reduction in DHCl at pH 7.5 as compared with that at pH 3.5 has been demonstrated. Furthermore, estimated values of DHCl are approximately 4-fold smaller through the mucus gel, as compared with a water layer. The findings indicate that the most powerful influence on diffusional resistance is pH itself, whereby a marked drop in H+ diffusion is likely to occur towards neutral pH irrespective of the composition of the gel barrier. Possible implications of the findings are discussed in relation to mucosal protection from acid.     

Real versus artifactual symmetry-breaking effects in Hartree-Fock, density-functional, and coupled-cluster methods

We have examined the relative abilities of Hartree-Fock, density-functional theory (DFT), and coupled-cluster theory in describing second-order (pseudo) Jahn-Teller (SOJT) effects, perhaps the most commonly encountered form of symmetry breaking in polyatomic molecules. As test cases, we have considered two prototypical systems: the 2Sigmau+ states of D( infinity h) BNB and C3+ for which interaction with a low-lying 2Sigmag+ excited state leads to symmetry breaking of the nuclear framework. We find that the Hartree-Fock and B3LYP methods correctly reproduce the pole structure of quadratic force constants expected from exact SOJT theory, but that both methods appear to underestimate the strength of the coupling between the electronic states. Although the Tamm-Dancoff (CIS) approximation gives excitation energies with no relationship to the SOJT interaction, the random-phase-approximation (RPA) approach to Hartree-Fock and time-dependent DFT excitation energies predicts state crossings coinciding nearly perfectly with the positions of the force constant poles. On the other hand, the RPA excited-state energies exhibit unphysical curvature near their crossings with the ground (reference) state, a problem arising directly from the mathematical structure of the RPA equations. Coupled-cluster methods appear to accurately predict the strength of the SOJT interactions between the 2Sigmau+ and 2Sigmag+ states, assuming that the inclusion of full triple excitations provides a suitable approximation to the exact wave function, and are the only methods examined here which predict symmetry breaking in BNB. However, coupled-cluster methods are plagued by artifactual force constant poles arising from the response of the underlying reference molecular orbitals to the geometric perturbation. Furthermore, the structure of the "true" SOJT force constant poles predicted by coupled-cluster methods, although correctly positioned, has the wrong structure.     

Nanoengineered Silica-Polymer Composite Aerogels with No Need for Supercritical Fluid Drying

Owing to their low density, dielectric constant, thermal conductivity, high porosity and chemical inertness, monolithic aerogels could be useful in a variety of electronic, optical and chemical applications [1]. However, practical implementation has been slow, because aerogels are fragile, environmentally sensitive (hydrophilic) and most importantly, the final stage of their preparation involves supercritical fluid (SCF) extraction [1c]. It is reported herewith that for a nominal 3-fold increase in density, typical polymer crosslinked silica aerogels are not only stronger (> 300×) and less hydrophilic (< 10×) than the underlying silica backbone, but they can also withstand the capillary forces exerted upon their nanostructured framework by the residing meniscus of selected solvents, and thus they can be dried under ambient pressure without need for supercritical fluid (SCF) extraction. The best solvent identified for that purpose is pentane, and the resulting aerogels are both microscopically and macroscopically identical to their SCF-CO₂ dried counterparts. Being able to dry monolithic crosslinked aerogels without SCF extraction is expected to facilitate their commercial application.Employed by the Ohio Aerospace Institute.     

Porphyrins as photosensitizers to enhance night vision

Relative bleaching rates of bovine rhodopsin (rod outer segments) in the presence and absence of seven porphyrins and methylene blue were measured under exposure to lambdamax = 675 nm light, using UV-vis spectroscopy. Rate enhancements on the order of up to three times compared to the bleaching of rhodopsin alone where observed. Fluorescence measurements and other data suggests that the porphyrins act as photosensitizers and excite the visual pigment via electron or triplet state energy transfer. These mechanisms suggest that rhodopsin possesses a pocket, proximal to the Schiff base so that porphyrins act as photosensitizers.     

Electron densities from the Brueckner Doubles method

Summary The concept of the Brueckner orbital is examined, following a resurgence of interest in wavefunctions constructed from them. The distinction between Self Consistent Field, Natural and Brueckner orbitals are discussed. Total electron densities are calculated for several examples, and correlation densities are studied. It is found that the Brueckner orbitals are more localised than SCF orbitals. The total electron density constructed from the Brueckner reference determinant with Brueckner orbitals gives qualitatively similar pictures as other correlated methods. Brueckner orbitals are found to show dissociation well.     

Internally-referenced resonant mirror for chemical and biochemical sensing

The resonant mirror sensor is a planar optical sensor platform that uses frustrated total internal reflection to couple light into and out of a leaky waveguiding layer. The evanescent wave associated with the dielectric structure is very sensitive to changes in surface refractive index caused by the binding of macromolecules to immobilised proteins or other biorecognition species such as antibodies. However, such variations can also be generated by variations in the bulk analyte solution, via changes in the composition or temperature. In the device described here, an additional buried resonant mirror layer is incorporated into the sensor structure generating an internal reference resonant mirror. The efficacy of this internal reference system is demonstrated in both chemical and immunological systems--as a pH sensor monitoring the absorption of an encapsulated sulfonephthalein dye, and as a refractive index sensor measuring the adsorption of anti-protein A and binding of its corresponding antigen. In both cases the internally referenced resonant mirror provides a means by which errors due to fluctuations in light intensity, temperature and bulk composition may be accounted for.     

Fast second order electron transfer reactions coupled to redox protein electrochemistry: Experiment and digital simulation

Chemical modification of metal surfaces by chemisorption provides a versatile method for the production of electrode interfaces which can be selective for the direct electrochemistry of one redox protein over another. The electrochemistry of a mixture of horse heart cytochrome c and spinach plastocyanin has been investigated at gold surfaces made selective for first one and then the other protein. The resulting cyclic voltammetry is quite unusual, containing pre-shoulders to both reduction current and reoxidation current peaks. The results have been interpreted in terms of fast second order electron transfer reactions taking place between the two proteins in homogeneous solution. This rationalisation has been corroborated by an explicit digital simulation of the proposed reaction scheme, using second order RKI. There are three independently variable parameters to the simulation: forward kinetic parameter, reverse kinetic parameter, and concentration ratio of non-electrode-active species to electrode-active species. The simulation has been used to explore a number of interesting trends in these parameters. Five such sequences of simulated cyclic voltammograms are reported, together with peak current and potential data in most cases. Attention is drawn to the possibility for further interesting experimental mixed redox protein electrochemistry at selective surfaces.     

REMARKABLE INHIBITION OF OXYGEN QUENCHING OF PHOSPHORESCENCE BY COMPLEXATION WITH CYCLODEXTRINS

The phosphorescence of the 4-bromo-l-naphthoyl group is readily quenched by molecular oxygen in homogeneous solvents. However, when this lumophore is complexed with γ-cyclodextrin in aqueous solution at room temperature, its phosphorescence is observed even under 1 atm of oxygen! Phosphorescence decay data indicated that two types of probe/cyclodcxtrin complexes are formed with lifetimes of 600 u,s and 3.5 ms. Oxygen completely quenches the fast decay, but only partially quenches the slow decay.     

Ethylene-promoted intermolecular enyne metathesis

[reaction: see text] Intermolecular enyne metathesis between functional group-rich alkynes and vinyl ethers was promoted by ethylene cometathesis. The concentration of ethylene was optimized to suppress the competing formation of butadiene through background ethylene metathesis. The role of ethylene appears to be both protective and rate enhancing.