A Miniature Electrolytic Conductivity Probe with a Wide Linear Range

A novel miniature electrolytic conductivity probe and its successful operation with modified bipolar pulse conductometry are presented. The probe based on a concentric ring‐disk electrode configuration of less than 1 mm in diameter featured extremely small detection volume, and conductivity was measured with a 1 μL solution. Rapid response, easy and quick washing, and virtually no consumption of samples by measurements were additional advantages of the suggested probe design. Poor linearity was observed with conventional AC conductometry due to a large cell constant and a small double layer capacitance of the probe. Measurement circuits for modified bipolar pulse conductometry were designed, and optimization of the various pulse parameters led to a wide linear dynamic range of 0.05–10 mS cm^?1, thus achieving high accuracy with a single‐point calibration. The suggested conductometric device could be conveniently applied to biological reagents and samples that are usually too little to be measured with conventional conductometers.     

Studies of hydrophobic bonding in aqueous alcohols: Enthalpy measurements and model calculations

The coefficients which measure the contribution of a pair of solute molecules to the excess enthalpy have been measured in water at 25°C for all pairs of alcohols which can be formed from the series methyl to n-butyl plus t-butyl as well as for ethanol with some of the higher alcohols and with the n-alkyl sulfonates through octyl. The methylene-group contribution to these coefficients is readily identifiable in suitable cases. These data and the corresponding free-energy and volume coefficients, where they are known, are analyzed in terms of amodel which specifies the core repulsion and solvation-layer overlap terms in the potential for the interaction of two solute molecules. The latter term has an adjustable parameter, the so-called Gurney free-energy parameter which is adjusted for each solute pair to fit the free-energy data. Its temperature and pressure derivatives are adjusted to fit the enthalpy and volume data, respectively. These parameters are compared with the corresponding thermodynamic coefficients of solvation as far as possible.This paper was presented at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.A portion of the talk presented at the H. S. Frank Symposium has been published else-where. The discussion section at the end of this paper pertains mainly to that portion.     

APPARATUS FOR THE MEASUREMENT OF QUANTUM YIELDS AND RATES OF PHOTOCHEMICAL REACTIONS

Abstract— A versatile irradiation apparatus for the quantitative study of photochemical reactions is described, which incorporates a commercial monochromator and utilizes a calibrated thermopile for the continuous measurement of light absorption during irradiation. The operating characteristics are discussed, and illustrative applications are cited. Using this apparatus, the quantum yield of the potassium ferrioxalate actinometer at 366 nm and 25C was determined as 1.27 ± 0.01 for a 6.0 times 10^-3 M solution, in excellent agreement with reported values of 1.26.     

Near-ultraviolet modification of Escherichia coli B ubiquinone in vivo and in vitro

Abstract— Ubiquinone Q-8 in Escherichia coli B was labeled by growing the bacteria in a synthetic growth medium containing p-hydroxybenzoic acid- ¹⁴C(U). The cells were then irradiated with light of wavelengths 313, 334, 366, and 405 nm, and Q-8- ¹⁴C was extracted and assayed. Q-8- ¹⁴C was also isolated from unirradiated bacteria, partially purified, and then irradiated similarly in hexane and assayed. Ubiquinone in the cells was four to eight times as resistant to radiation as ubiquinone in hexane. From 313 to 366 nm, the in vivo and in vitro action spectra were similar to each other and to the absorption spectra of synthetic Q-8 dissolved in hexane or ethanol, but at 405 nm the action spectra deviated from the absorption spectra. Quantum yields for the modification (loss) of ubiquinone in hexane decreased from 0·021. at 313 nm to 0·005 at 405 nm. The chromenol of Q-8 was not detected in labeled cells irradiated at 334 nm, but several more-polar photoproducts were separated by thin-layer chromatography.     

Single-Ion Contributions to Activity Coefficient Derivatives, Second Moment Coefficients, and the Liquid Junction Potential

We discuss several interrelated single-ion thermodynamic properties required to calculate the liquid junction potential between two solutions of the same binary electrolyte. According to a previously reported molecular theory of nonuniform electrolyte solutions in nonequilibrium, is determined by the transport numbers of the ions, and by the second moment coefficients H ⁽²⁾ of the charge densities around the ions. The latter may be viewed as the single-ion contributors to the second moment condition of Stillinger and Lovett. For a solution of a single binary electrolyte, we relate the H ⁽²⁾ (R) to the derivatives of the single-ion activity coefficients with respect to the ionic strength. In the light of these results, we examine, in some detail, the role played by the specific short-range interionic interactions in determining . We investigate this matter by means of integral equation calculations for realistic models of LiCl and NaCl aqueous solutions in the 0–1 mol-dm^–3 range. In addition to the hypernetted-chain (HNC) relation, we perform calculations under a new integral equation closure that is a hybrid between the HNC and Percus–Yevick closures. Like the HNC approximation, the new closure satisfies the Stillinger and Lovett condition. However, for the models considered in this study, the two closures predict different dependence of the H ⁽²⁾ and of on the specific part of the interionic interactions.     

Development of a novel method to populate native disulfide-bonded intermediates for structural characterization of proteins: implications for the mechanism of oxidative folding of RNase A

RNase A, a model protein for oxidative folding studies, has four native disulfide bonds. The roles of des [40-95] and des [65-72], the two native-like structured three-disulfide-bonded intermediates populated between 8 and 25 degrees C during the oxidative folding of RNase A, are well characterized. Recent work focuses on both the formation of these structured disulfide intermediates from their unstructured precursors and on the subsequent oxidation of the structured species to form the native protein. The major obstacles in this work are the very low concentration of the precursor species and the difficulty of isolating some of the structured intermediates. Here, we demonstrate a novel method that enables the native disulfide-bonded intermediates to be populated and studied regardless of whether they have stable structure and/or are present at low concentrations during the oxidative folding or reductive unfolding process. The application of this method enabled us to populate and, in turn, study the key intermediates with two native disulfide bonds on the oxidative folding pathway of RNase A; it also facilitated the isolation of des [58-110] and des [26-84], the other two native-like structured des species whose isolation had thus far not been possible.     

Simultaneous characterization of the reductive unfolding pathways of RNase B isoforms by top-down mass spectrometry

A novel method for characterization of the simultaneous reductive unfolding pathways of five isoforms of bovine pancreatic ribonuclease B (RNase B) is demonstrated. The results indicate that each isoform unfolds reductively through two three-disulfide-containing structured intermediates before proceeding to the fully reduced form, as in the reductive unfolding pathways of the A variant lacking the carbohydrate chain. The rates of reduction of bovine pancreatic ribonuclease A (RNase A) and RNase B and the formation and consumption of their reductive intermediates are identical, indicating that the unfolding events necessary to expose disulfide bonds for reduction are not affected by the oligosaccharide. The method utilizes top-down mass spectrometry and a naturally occurring tag on the protein, viz. the carbohydrate moiety, to obtain unfolding information of an ensemble of protein isoforms and is a generally applicable methodological advance for conducting folding studies on mixtures of different proteins.     

Acoustic micromachining of three-dimensional surfaces for biological applications

We present the use of an accessible micromachining technique (acoustic micromachining) for manufacturing micron-feature surfaces with non-discretely varying depth. Acoustic micromachining allows for non-photolithographic production of metal templates with programmable spatial patterns and involves the use of standard acoustic, cutting and electroplating equipment for mass production of vinyl records. Simple 3D patterns were transferred from an acoustic signal into working nickel templates, from which elastic polymer molds were obtained, featuring deep surface grooves and non-discrete (smooth) variations in the z-dimension. Versatility and applicability of the method is demonstrated in obtaining microfluidics structures, manufacturing high-surface area wavy polymer fibers, assembly of cell networks on scaffolds with 3D topography, and microcontact printing of proteins and cells.     

Pattern recognition in the prediction of protein structure. II. Chain conformation from a probability-directed search procedure

A procedure that finds the most probable conformational states of a protein chain is described. Single-residue conformations are represented in terms of four conformational states, α, ?, α*, and ?*. The conformation of the entire chain is represented by a sequence of single-residue conformational states; the distinct conformations in this representation are called “chain-states.” The first article in this series described a procedure that computes tripeptide conformational probabilities from the amino acid sequence using pattern recognition techniques. The procedure described in this article uses the tripeptide probabilities to estimate the probabilities of the chain-states. The chain-state probability estimator is a product of conditional and marginal probabilities (obtained from the tripeptide probabilities), with a penalty factor to eliminate conformations containing α-helices and ?-strands of excessive length. The probability estimator considers short-range conformational information, medium-range sequence information and some simple long-range information (through the restrictions on helix and strand lengths). Energy minimization calculations can be carried out in the region of conformational space corresponding to a particular chain-state. By selecting the most probable chain-states, the search can be focused on the most probable, or “important,” regions of the conformational space. These energy calculations are described in the third article of the series. The complete procedure described by the three articles is called PRISM, for pattern recognition-based importance sampling minimization.     

Attempts to prepare a pyramidal carbocation with an apical fluorine substituent

Gem-difluorides $\text{7}$ and $\text{8}$, different only in the location of a methyl substituent and each a potential precursor of the same pyramidal carbocation $\text{9}$, ionize instead to give allylic carbocation $\text{13}$ and homotropylium ion $\text{20}$ respectively.