Variety Of Buffer Solutions Research Articles

Metal-organic framework polymethyl methacrylate composites for open-tubular capillary electrochromatography

Metal-organic frameworks (MOFs) are attractive as novel separation medium due to their distinguished properties including large surface area, accessible tunnels and diverse structures. Here, we report the incorporation of MOF CAU-1 (CAU=Christian-Albrechts-University) into polymethyl methacrylate (PMMA) to produce a new composite (CAU-1@PMMA), and the fabrication of CAU-1@PMMA coated capillary for open tubular capillary electrochromatography (CEC). CAU-1 contains unprecedented [Al8(OH)4(OCH3)8]12+ clusters connected by twelve aminoterephthalic acid linkers, and is highly porous and stable in a variety of buffer solutions. The incorporation of CAU-1 into PMMA not only increases surface area, but also electroosmotic flow (EOF). As a result, the CAU-1@PMMA coated capillary column gives higher column efficiency, larger column capacity, and shorter separation time for baseline separation of two groups of aromatic carboxylic acids than the PMMA coated capillary column. Besides, the incorporation of CAU-1 also improves the resolution for the CEC separation of basic sulfa drugs and structurally related peptides. The run-to-run, day-to-day and column-to-column precision for the EOF of CAU-1@PMMA coated capillary column is 0.3%, 0.4%, and 2.2% (relative standard deviation), respectively. The results show that MOFs composites are promising stationary phases for CEC applications.

Simple and Accurate Quantification of Quantum Dots via Single-Particle Counting

Quantification of quantum dots (QDs) is essential to the quality control of QD synthesis, development of QD-based LEDs and lasers, functionalizing of QDs with biomolecules, and engineering of QDs for biological applications. However, simple and accurate quantification of QD concentration in a variety of buffer solutions and in complex mixtures still remains a critical technological challenge. Here, we introduce a new methodology for quantification of QDs via single-particle counting, which is conceptually different from established UV-vis absorption and fluorescence spectrum techniques where large amounts of purified QDs are needed and specific absorption coefficient or quantum yield values are necessary for measurements. We demonstrate that single-particle counting allows us to nondiscriminately quantify different kinds of QDs by their distinct fluorescence burst counts in a variety of buffer solutions regardless of their composition, structure, and surface modifications, and without the necessity of absorption coefficient and quantum yield values. This single-particle counting can also unambiguously quantify individual QDs in a complex mixture, which is practically impossible for both UV-vis absorption and fluorescence spectrum measurements. Importantly, the application of this single-particle counting is not just limited to QDs but also can be extended to fluorescent microspheres, quantum dot-based microbeads, and fluorescent nano rods, some of which currently lack efficient quantification methods.

The Effects of Buffers on the Thermodynamics and Kinetics of Binding between Positively-Charged Cyclodextrins and Phosphate Ester Guests

Cyclodextrin derivatives in which the primary hydroxyl groups at the 6-positions of the sugar units have been replaced with 2-hydroxethylamino groups are known to bind phosphate esters with significant affinity. Association between the host and guest is mediated by a combination of hydrophobic and electrostatic interactions. Association constants for two cyclodextrin hosts with three phosphate ester guests are measured in a variety of buffer solutions, and it is found that the buffer has a large impact on the electrostatic component of the binding interaction. Negatively charged buffers such as phosphate and ADA (N-(2-acetamido)iminodiacetic acid) compete with the phosphate ester guests for binding to the positively charged cyclodextrins. This competition lowers the effective association constant between host and guest. Positively charged buffers such as Tris-HCl do not strongly compete and are thus more conducive to binding. The kinetics of dissociation of the host−guest complexes were measured in several buffers using 31P EXSY experiments. These measurements demonstrate that negatively charged buffers decrease the equilibrium constant by lowering the rate of association of these complexes but in most cases do not effect the dissociation rates.

Chemical Reactivity of Penicillins and Cephalosporins. Intramolecular Involvement of the Acyl-Amido Side Chain

The rate of degradation of 6-epi-ampicillin in acidic, neutral, and alkaline aqueous solutions was followed at 35 °C and an ionic strength of 0.5 mol dm-3 (KCl) by high-performance liquid chromatography (HPLC) and spectrophotometric assays. Pseudo-first-order rate constants were determined in a variety of buffer solutions, and the overall pH−rate profile was obtained by extrapolation to zero buffer concentration. The hydrolysis of 6-epi-ampicillin is subject to acid and hydroxide-ion catalysis and, for a penicillin, an unusual pH-independent reaction. Intramolecular general base-catalyzed hydrolysis by the side chain amido group is proposed to explain the enhanced rate of neutral hydrolysis of 6-epi-ampicillin and cephalosporins. The β-lactam of 6-epi-ampicillin also undergoes intramolecular aminolysis by nucleophilic attack of the 6-α side chain amino group to give a stable piperazine-2,5-dione derivative. The low effective molarity for intramolecular aminolysis of only 40 M is partly attributed to the u...

Stability kinetics of piperacillin in aqueous solutions

The degradation process of piperacillin in acidic, neutral and alkaline solutions was followed by both high-pressure liquid chromatographic and spectrophotometric assays. Pseudo-first-order rate constants were determined in a variety of buffer solutions. The overall pH-rate profile was determined at 35°C and an ionic strength of 0.5. β-Lactam moiety degradation occurred in acidic media to produce the hydrolysis products. In alkaline solutions, the piperazinyl ring of piperacillin was hydrolyzed about 20 times faster than the β-lactam moiety.

The tryptophan fluorescence lifetime puzzle. A study of decay times in aqueous solution as a function of pH and buffer composition

Tryptophan fluorescence decay kinetics have been systematically investigated in aqueous solutions as a function of pH as well as in a variety of buffer solutions. Below pH 7.0, the decay appears to be double exponential with a subnanosecond component confirming the previous findings of Rayner and Szabo (3). In the low pH region, where the proton concentration becomes kinetically significant, tryptophan fluorescence is collisionally quenched by [H+] with diffusion controlled rate and no experimental evidence is found regarding the appearance at low pH of a new tryptophan molecular species, namely the cationic form. At pH ≥ 7.0, the decay becomes triple-exponential with the appearance of a long component whose contribution to the total emission intensity increases rapidly with increasing pH at the expense of the other two. Lifetimes and relative intensities of each decay component depend in a complex way on pH and on the buffer chemical composition.

Detailed electrophoretic analyses of sera of healthy young men.

The electrophoretic patterns of plasma and serum obtained from healthy individuals have been described by a number of investigators and the results of such analyses are summarized in a recent paper (1). Appreciable fluctuations in the concentrations and percentage distributions of the major protein components are noted. A critical examination of these data shows that workers in this field use a variety of buffer solutions with different ionic strength and usually average the results obtained for males and females in different age groups. Furthermore, Bernfeld, Donahue, and Homburger (2) have observed large variations in the general appearance of electrophoretic patterns and have emphasized the necessity of considering the finer structures of the contour in the analytical procedure. The data obtained from electrophoretic analyses depend on the method by which the area of the electrophoretic pattern is subdivided. The analysis of electrophoretic patterns of serum proteins is generally done by the procedure of Tiselius and Kabat (3) which involves drawing a perpendicular from the base line to the minimum between two peaks, or the components may be resolved by drawing a series of symmetrical curves as described by Svedberg and Pedersen (4). The method of Tiselius and Kabat has the advantage of simplicity and the results approach those given by the Pedersen procedure (5, 6). Wiedemann (7) subdivided the electrophoretic areas of sera with the aid of Gaussian curves and stated that nine