Acid-base control for biocatalysis in organic media: new solid-state proton/cation buffers and an indicator

Although great care is generally taken to buffer aqueous enzyme reactions, active control of acid-base conditions for biocatalysis in low-water media is rarely considered. Here we describe a new class of solid-state acid-base buffers suitable for use in organic media. The buffers, composed of a zwitterion and its sodium salt, are able to set and maintain the ionisation state of an enzyme by the exchange of H+ and Na+ ions. Surprisingly, equilibrium is established between the different solid components quickly enough to provide a practical means of controlling acid-base conditions during biocatalysed reactions. We developed an organosoluble chromoionophore indicator to screen the behaviour of possible buffer pairs and quantify their relative H+/Na- exchange potential. The transesterification activity of an immobilised protease, subtilisin Carlsberg, was measured in toluene in the presence of a range of buffers. The large observed difference in rates showed good correlation with that expected from the measured exchange potentials. The maximum water activities accessible without formation of hydrates or solutions of the buffers are reported here. The indicator was also used to monitor, for the first time in situ, changes in the acid-base conditions of an enzyme-catalysed transesterification reaction in toluene. We found that even very minor amounts of an acidic by-product of hydrolysis were leading to protonation of the enzyme, resulting in rapid loss of activity. Addition of solid-state buffer was able to prevent this process, shortening reaction times and improving yields. Solid-state buffers offer a general and inexpensive way of precisely controlling acid-base conditions in organic solvents and thus also have potential applications outside of biocatalysis.     

Thermodynamic Modeling of Actinide Complexation with Acetate and Lactate at High Ionic Strength

The stability constants of NpO ₂ ⁺ , UO ₂ ²⁺ Am³⁺, and Th⁴⁺ with acetate and lactate anions has been measured in 0.3–5.0m NaCl media at 25°C by the solvent extraction technique. For the 1:1 complexation, the values of the stability constants increased in the order: NpO ₂ ⁺ < Am³⁺ < ₂ ²⁺ < Th⁴⁺, in accordance with the actinide charge density and reflecting the strongly ionic bonding of the complexes. The Pitzer ionic interaction parameters were calculated and used to estimate the thermodynamic stability constants at I = 0. Because our data were collected mainly in the high ionic strength region values of ⁽¹⁾ were estimated from values reported in the literature. For all stability constants the Pitzer model gives an excellent representation of the data using three interaction parameters ⁽⁰⁾, ⁽¹⁾, and COn leave from Institute of     

Effects of frequency and duration on psychometric functions for detection of increments and decrements in sinusoids in noise

Psychometric functions for detecting increments or decrements in level of sinusoidal pedestals were measured for increment and decrement durations of 5, 10, 20, 50, 100, and 200 ms and for frequencies of 250, 1000, and 4000 Hz. The sinusoids were presented in background noise intended to mask spectral splatter. A three-interval, three-alternative procedure was used. The results indicated that, for increments, the detectability index d' was approximately proportional to delta I/I. For decrements, d' was approximately proportional to delta L. The slopes of the psychometric functions increased (indicating better performance) with increasing frequency for both increments and decrements. For increments, the slopes increased with increasing increment duration up to 200 ms at 250 and 1000 Hz, but at 4000 Hz they increased only up to 50 ms. For decrements, the slopes increased for durations up to 50 ms, and then remained roughly constant, for all frequencies. For a center frequency of 250 Hz, the slopes of the psychometric functions for increment detection increased with duration more rapidly than predicted by a "multiple-looks" hypothesis, i.e., more rapidly than the square root of duration, for durations up to 50 ms. For center frequencies of 1000 and 4000 Hz, the slopes increased less rapidly than predicted by a multiple-looks hypothesis, for durations greater than about 20 ms. The slopes of the psychometric functions for decrement detection increased with decrement duration at a rate slightly greater than the square root of duration, for durations up to 50 ms, at all three frequencies. For greater durations, the increase in slope was less than proportional to the square root of duration. The results were analyzed using a model incorporating a simulated auditory filter, a compressive nonlinearity, a sliding temporal integrator, and a decision device based on a template mechanism. The model took into account the effects of both the external noise and an assumed internal noise. The model was able to account for the major features of the data for both increment and decrement detection.     

Engineering quadrupole magnetic flow sorting for the isolation of pancreatic islets

Quadrupole magnetic flow sorting (QMS) is being adapted from the separation of suspensions of single cells (<15 μm) to the isolation of pancreatic islets (150–350 μm) for transplant. To achieve this goal, the critical QMS components have been modeled and engineered to optimize the separation process. A flow channel has been designed, manufactured, and tested. The quadrupole magnet assembly has been designed and verified by finite element analysis. Pumps have been selected and verified by test. Test data generated from the pumps and flow channel demonstrate that the fabricated channel and peristaltic pumps fulfill the requirements of successful QMS separation.     

High-order adaptive finite element-singly implicit Runge-Kutta methods for parabolic differential equations

We describe an adaptive mesh refinement finite element method-of-lines procedure for solving one-dimensional parabolic partial differential equations. Solutions are calculated using Galerkin's method with a piecewise hierarchical polynomial basis in space and singly implicit Runge-Kutta (SIRK) methods in time. A modified SIRK formulation eliminates a linear systems solution that is required by the traditional SIRK formulation and leads to a new reduced-order interpolation formula. Stability and temporal error estimation techniques allow acceptance of approximate solutions at intermediate stages, yielding increased efficiency when solving partial differential equations. A priori energy estimates of the local discretization error are obtained for a nonlinear scalar problem. A posteriori estimates of local spatial discretization errors, obtained by order variation, are used with the a priori error estimates to control the adaptive mesh refinement strategy. Computational results suggest convergence of the a posteriori error estimate to the exact discretization error and verify the utility of the adaptive technique.This research was partially supported by the U.S. Air Force Office of Scientific Research, Air Force Systems Command, USAF, under Grant Number AFOSR-90-0194; the U.S. Army Research Office under Contract Number DAAL 03-91-G-0215; by the National Science Foundation under Grant Number CDA-8805910; and by a grant from the Committee on Research, Tulane University.     

Spin-dependent reaction mechanism in Ti(d, p)Ti from proton-γ correlation

The γ-decays of the states in ⁴⁷Ti at 157 keV and 1547 keV have been studied following the ⁴⁶Ti(d, p)⁴⁷Ti reaction at 5.0 MeV incident energy. Protons were detected at 0° with a semiconductor detector. The angular correlations of the γ-rays with the reaction protons were measured. The E2/M1 mixing ratios of the γ-decays and the population ratios of the to the substates were deduced. From the non-zero population of the substate it was concluded that a spin-flip mechanism exists in the ⁴⁶Ti(d, p)⁴⁷Ti stripping reaction.     

Experiments with a symbolic programming system for complex analysis

The paper reports the results of work on the construction of LISP programs for various symbolic operations in complex analysis, including the evaluation of integrals around closed contours by the use of Cauchy's Theorem. It is concluded that the only difficulty in the way of the preparation of programs for all of the important and useful textbook calculations in complex analysis is that large amounts of fast storage in the computer are needed.     

The Rev Counter Decision

This case-based article arose out of a decision tree analysis carried out by the authors in an industrial firm. The company's identity and the actual product manufactured have both been altered for reasons of confidentiality. The various discussions outlined in the case are reported in the same order as they occurred within the firm whilst the problem was originally under study. The work reported was first carried out in 1971 and has been the subject of regular "follow-up" analyses since that date. A much simplified and restricted version of Parts I and II of this case was originally published in two articles appearing in the Financial Times during January 1972.