Phase behavior of aqueous solutions containing dipolar proteins from second-order perturbation theory

Due to the interplay of Coulombic repulsion and attractive dipolar and van der Waals interactions, solutions of globular proteins display a rich variety of phase behavior featuring fluid-fluid and fluid-solid transitions that strongly depend on solution pH and salt concentration. Using a simple model for charge, dispersion and dipole-related contributions to the interprotein potential, we calculate phase diagrams for protein solutions within the framework of second-order perturbation theory. For each phase, we determine the Helmholtz energy as the sum of a hard-sphere reference term and a perturbation term that reflects both the electrostatic and dispersion interactions. Dipolar effects can induce fluid-fluid phase separation or crystallization even in the absence of any significant dispersion attraction. Because dissolved electrolytes screen the charge-charge repulsion more strongly than the dipolar attraction, the ionic strength dependence of the potential of mean force can feature a minimum at intermediate ionic strengths offering an explanation for the observed nonmonotonic dependence of the phase behavior on salt concentration. Inclusion of correlations between charge-dipole and dipole-dipole interactions is essential for a reliable calculation of phase diagrams for systems containing charged dipolar proteins and colloids.     

Initial Steps of Amyloidogenic Peptide Assembly Revealed by Cold‐Ion Spectroscopy

The early stages of fibril formation are difficult to capture in solution. We use cold‐ion spectroscopy to examine an 11‐residue peptide derived from the protein transthyretin and clusters of this fibre‐forming peptide containing up to five units in the gas phase. For each oligomer, the UV spectra exhibit distinct changes in the electronic environment of aromatic residues in this peptide compared to that of the monomer and in the bulk solution. The UV spectra of the tetra‐ and pentamer are superimposable but differ significantly from the spectra of the monomer and trimer. Such a spectral evolution suggests that a common structural motif is formed as early as the tetramer. The presence of this stable motif is further supported by the low conformational heterogeneity of the tetra‐ and pentamer, revealed from their IR spectra. From comparison of the IR‐spectra in the gas and condensed phases, we propose putative assignments for the dominant motif in the oligomers.     

Continuous production of lactic acid in a cell recycle reactor

The production of lactic acid from glucose has been demonstrated using a CSTR (continuous stirred-tank reactor) with cell recycle. Studies were conducted withLactobacillus delbrueckii at a fermentation temperature of 42°C and a pH of 6.25. A cell density of 140 g dry weight/L and a volumetric productivity of 150 g/L.h, with complete glucose consumption, were obtained. It was not possible to obtain a lactic acid concentration above 60 g/L because of product inhibition. A cell purge was not necessary to maintain high viability bacteria culture or to obtain a steady state. At steady state the net cell growth appeared to be negligible. The specific glucose consumption for cell maintenance was 0.33 g glucose/g cells-h.     

Optimization of dynamic headspace sampling for the analysis of trace volatile components of grape juice: Use of a PTV injector for intermediate trapping

The use of a programmed temperature vaporizing (PTV) injector has been evaluated for the on-line concentration and injection of trace organic compounds either sampled from the head-space above grape juices or purged from solution. The Simplex method was used to improve the sensitivity of the method by optimization of the experimental conditions.     

Recoveries of organochlorine compounds (polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans) in water using steam distillation-solvent extraction at normal pressure

A simultaneous steam distillation-solvent extraction (SDE) procedure was used for determining polychrlorinated biphenyls, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCBs, PCDDs and PCDFs) at sub-ppb levels in water samples. Recoveries of 39.8–138.7% and a standard deviation of lower than 10% were achieved for the individual coplanar PCB and the 2,3,7,8-substituted PCDD/F congeners. SDE is a fast, clean, inexpensive and reproducible procedure for the determination of individual PCBs, PCDDs and PCDFs in waters at very low concentrations.     

Continuous production of lactic acid from glucose and lactose in a cell-recycle reactor

Growth and lactic acid production ofLactobacillus delbreuckii were compared using glucose and lactose as carbon sources. A continuous-flow stirred-tank fermenter was coupled with a cross-flow filtration unit to permit operation at high-cell concentrations. At steady state, yeast extract requirements for lactic-acid production were lower when glucose was used as a substrate than with lactose fermentation. Once steady state was obtained, with glucose feed, it was possible to lower the yeast extract concentration without affecting biomass concentration and lactic acid production. The lacticacid concentration that inhibited cell growth and lactic acid production was found to depend on the choice of a carbon substrate.     

Quantification of casein phosphorylation with conformational interpretation using Raman spectroscopy

Raman spectroscopy is emerging as a powerful method for obtaining both quantitative and qualitative information from biological samples. One very interesting area of research, for which the technique has rarely been used, is the detection, quantification and structural analysis of post-translational modifications (PTMs) on proteins. Since Raman spectra can be used to address both of these questions simultaneously, we have developed near infrared Raman spectroscopy with appropriate chemometric approaches (partial least squares regression) to quantify low concentration (4 microM) mixtures of phosphorylated and dephosphorylated bovine alpha(s)-casein. In addition, we have used these data in conjunction with Raman optical activity (ROA) spectra and NMR to assess the structural changes that occur upon phosphorylation.     

Enantiomeric purity of (+/-) -methyl jasmonate in fresh leaf samples and commercial fragrances

The enantiomeric purity of (+/-) -methyl jasmonate in fresh leaf material of Jasminum from different species and Rosmarinus officinalis was examined by solid-phase microextraction-GC-MS (SPME-GC-MS). For comparison with these natural products, commercial jasmine and rosemary fragrances were also studied. The extraction conditions were selected as a result of testing different values of temperature (40, 50, and 60 degrees C) and time (2, 15, 30, and 40 min). The results obtained in this work revealed a range of enantiomeric excesses for (+/-) -methyl jasmonate varying from 13 to 95% depending on the Jasminum specie considered. In contrast, (-) -methyl jasmonate always occurred as a pure enantiomer in all R. officinalis samples studied. This implies those Jasminum species in which the enantiomeric purity of (-) -methyl jasmonate is high enough and any R. officinalis sample might be used as natural sources of pure (-) -methyl jasmonate. Concerning the commercial fragrances, those of jasmine showed enantiomeric composition of (-) -methyl jasmonate ranging from 1 to 15% whereas those of rosemary exhibited practically the pure (-) -methyl jasmonate. This fact suggests the addition and nonaddition of the racemic mixture of methyl jasmonate to the commercial jasmine and rosemary samples, respectively.     

Controlling factors in the synthesis of cucurbituril and its homologues.

The acid-catalyzed synthesis of cucurbit[n]urils from formaldehyde and glycoluril is poorly understood. In this paper, we examine a wide range of reaction conditions that include the effects of acid type, acid concentration, reactant concentrations, and temperature to both probe the mechanism and optimize the yields of isolated cucurbit[n]urils, where n = 5-10. A mechanism for the formation of these cucurbit[n]urils is presented. Individual cucurbit[n]urils were unambiguously identified in reaction mixtures using ESMS and (13)C NMR.