Separation of amino acids by ion mobility spectrometry

The mobilities of the 20 common amino acids were determined by electrospray ionization ion mobility spectrometry. It was found that each amino acid had a different drift time and hence a different reduced mobility constant K0. This difference in drift time was less than 0.1 ms in many cases. With the instrument used in this study it would not be possible to resolve mixtures of some of the amino acids. It would however be possible to determine any single amino acid. In addition, the detection limits were determined for the 20 amino acids. They ranged from 50 to 700 pg. This indicates that the detection limits were less than 3 pmol for all of the amino acids and that many amino acids had detection limits less than 1 pmol.     

Influence of ion pairing on the oxidation of iodide by MLCT excited states

The oxidation of iodide to diiodide, I(2)˙(-), by the metal-to-ligand charge-transfer (MLCT) excited state of [Ru(deeb)(3)](2+), where deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, was quantified in acetonitrile and dichloromethane solution at room temperature. The redox and excited state properties of [Ru(deeb)(3)](2+) were similar in the two solvents; however, the mechanisms for excited state quenching by iodide were found to differ significantly. In acetonitrile, reaction of [Ru(deeb)(3)](2+*) and iodide was dynamic (lifetime quenching) with kinetics that followed the Stern-Volmer model (K(D) = 1.0 ± 0.01 × 10(5) M(-1), k(q) = 4.8 × 10(10) M(-1) s(-1)). Excited state reactivity was observed to be the result of reductive quenching that yielded the reduced ruthenium compound, [Ru(deeb(-))(deeb)(2)](+), and the iodine atom, I˙. In dichloromethane, excited state quenching was primarily static (photoluminescence amplitude quenching) and [Ru(deeb(-))(deeb)(2)](+) formed within 10 ns, consistent with the formation of ion pairs in the ground state that react rapidly upon visible light absorption. In both solvents the appearance of I(2)˙(-) could be time resolved. In acetonitrile, the rate constant for I(2)˙(-) growth, 2.2 ± 0.2 × 10(10) M(-1) s(-1), was found to be about a factor of two slower than the formation of [Ru(deeb(-))(deeb)(2)](+), indicating it was a secondary photoproduct. The delayed appearance of I(2)˙(-) was attributed to the reaction of iodine atoms with iodide. In dichloromethane, the growth of I(2)˙(-), 1.3 ± 0.4 × 10(10) M(-1) s(-1), was similar to that in acetonitrile, yet resulted from iodine atoms formed within the laser pulse. These results are discussed within the context of solar energy conversion by dye-sensitized solar cells and storage via chemical bond formation.     

Studies on the autocatalyzed oxidation of amino acids by peroxomonosulfate

The kinetics of oxidation of six α‐amino acids (AA) by peroxomonosulfate (PMS) ion at pH 4.2 and 35°C are investigated once again. The rate of disappearance of peroxomonosulfate at constant [AA] and [H⁺] follows the equation The experimental results suggest that the hydroperoxide intermediate formed by the reaction between the hydrated form of aldehyde and PMS is more reactive and is responsible for the autocatalysis. The hydroperoxide reacts with the amino acid in the rate‐determining step. This observation is contrary to the earlier explanation that the autocatalysis is due to the Schiff's base from amino acid and aldehyde. The effect of H⁺, the structure of the aldehyde, etc. on the rate constants are discussed. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 475–483, 2003     

Solar-driven electrochemically assisted semiconductor-catalyzed iodide ion oxidation. Enhanced efficiency by oxide mixtures

Oxidation of iodide ion from an air-saturated solution under natural sunlight (900±50 W m⁻²) on the surfaces of TiO₂, ZnO, Fe₂O₃, MoO₃ and CeO₂ enhances by 6 to 12-fold on application of a cathodic bias of −0.2 to −0.3 V (vs NHE) to the semiconductors; light, the semiconductor and dissolved oxygen are essential for iodine generation. The semiconductors under an anodic bias of +0.2 to +0.3 V (vs NHE) fail to oxidize iodide ion from air-saturated solution under sunlight. Under cathodic bias, semiconductor mixtures like TiO₂-ZnO, TiO₂-Fe₂O₃ and ZnO-Fe₂O₃ show enhanced photocatalytic activity, indicating improved charge separation in oxide mixtures. The mechanism of photocatalysis under cathodic bias is discussed.      

Enantiomeric separation of unusual secondary aromatic amino acids

Summary High-performance liquid chromatographic and gas chromatographic methods were developed for the separation of unusual secondary aromatic amino acids. Amino acids containing 1,2,3,4-tetrahydroisoquinoline, 1,2,3,4-tetrahydronorharmane-1-carboxylic acid and 1,2,3,4-tetrahydro-3-carboxy-2-carboline moieties were synthetized in racemic or chiral forms. The high-performance liquid chromatography was carried out either on a teicoplanin-containing chiral stationary phase or on an achiral C₁₈ column. In the latter case the diastereomers of the amino acids formed by precolumn derivatization with the chiral reagents 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl isothiocyanate or 1-fluoro-2,4-dinitrophenyl-5-L-alanine amide were separated. The gas chromatographic analyses were based on separation on a Chirasil-L-Val column. Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997     

Haloketone dehalogenation by iodide ion

NaI in aqueous acid-THF is an efficient reagent for dehalogenation of α-haloketones.     

Isocratic ion chromatographic determination of underivatized amino acids by electrochemical detection

An isocratic chromatographic separation with amperometric detection of underivatized amino acids at a copper oxyhydroxide modified glassy carbon electrode is described. A simple and sensitive quantitation procedure of amino acids without the need of tedious and time-consuming derivatization step was achieved by coupling anion-exchange chromatography with electrochemical detection. The effects of hydroxide, nitrate and acetonitrile concentration in the mobile phase on the capacity factors and peak resolution was evaluated. Under the optimized isocratic chromatographic conditions (i.e. 60 mM NaOH) and under constant applied potentials (i.e. 0.55 V versus Ag/AgCl) the detection limit ranged between 4 and 24 pmol injected and the linear dynamic range spanned generally over three or four order of magnitude for all investigated amino acid compounds. Direct determination of several common free amino acids in beer and milk samples were performed.     

Synthesis of phosphinic and phosphonic analogs of aromatic amino acids

The reaction of aryl and aralkyl aldoximes with hypophosphorous acid resulted in aminophosphinic acids, which were oxidized into the corresponding aminophosphonic acids. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 137–140, January, 1997.     

Spectrophotometric determination of aromatic and heterocyclic amino acids in mixtures

A procedure for the determination of individual aromatic and heterocyclic amino acids in mixtures was developed with the use of the Vierordt method. Amino acids can be simultaneously determined by this procedure in the concentration range (0.4-3.0) x 10^-3 M. The determination errors for binary and ternary mixtures were no greater than 5 and 10%, respectively. The determination error increases with a decrease in the concentration of a particular component and with an increase in the number of components in the test mixture.     

Investigation of spectral diffusion in ribonuclease by photolabeling of intrinsic aromatic amino acids

Spectral diffusion dynamics in ribonuclease A was observed via the broadening of photochemical holes burned into the absorption spectrum of intrinsic tyrosine residues. Unlike previous results based on hole burning of chromophores in the pockets of heme proteins, where spectral diffusion develops according to a power law in time, the dynamics in ribonuclease follow a logarithmic law. The results suggest that the experiment preferentially labels the tyrosines located on the surface of the protein where the two-level system dynamics of the glass host matrix exert a strong influence.     

Kinetics of Ag/TiO2-photocatalyzed iodide ion oxidation

Abstract  

Ag-doped TiO₂ (anatase) samples (mass fraction w _Ag = 0.01 and w _Ag = 0.02) of 15.9 and 14.5 nm mean particle size and 11.46 and 10.14 m² g⁻¹ BET surface area were prepared by photodeposition. Doping results in surface plasmon resonance of the metallic silver nanoclusters at around 500 nm, but the absorption edge remains unaltered at 365 nm. Ag-doping remarkably enhances the photooxidation of iodide ion under UV light; iodine formation with Ag/TiO₂ with w _Ag = 0.01 is 16 times greater than with bare TiO₂. The reaction conforms to Langmuir–Hinshelwood kinetics with regard to both I⁻ and O₂. Increase of pH slows down iodine formation and sacrificial electron donors arrest the reaction. Pre-sonication of the catalyst slurry hinders the photocatalysis. Generation of iodine is much greater in acetonitrile than in water. Under the experimental conditions, Ag/TiO₂ with w _Ag = 0.01 is more efficient than Ag/TiO₂ with w _Ag = 0.02, and the enhanced photocatalysis is likely to be because of suppression of electron–hole pair recombination. Kinetic analysis reveals that increasing the Ag mass fraction from 0.01 to 0.02 enhances the surface pseudo-first-order rate constant but inhibits the adsorption of iodide ion and the oxygen molecule on the illuminated oxide surface.     

Metal ion affinity purification of proteins by genetically incorporating metal-chelating amino acids

Affinity tags are efficient tools for protein purification. They allow simple one-step purification of proteins to high purity. However, in some cases the tags cause structural and functional changes in a protein, and need to be removed. Therefore, affinity tags that are readily introduced into proteins with minimal perturbation and have specific affinity for purification are desired. Herein, two metal-chelating amino acids derived from 2,2′-bipyridine and 8-hydroxyquinoline were genetically incorporated into glutathione S-transferase (GST) and the mutant proteins were purified by using the metal ion affinity of the unnatural amino acids. The purification of the GST mutants containing 2-amino-3-(8-hydroxyquinolin-3-yl)propanoic acid (HQA) showed that the proteins could be efficiently enriched in Ni–NTA by the metal ion affinity of the unnatural amino acid and purified to excellent purity. This method should be very useful for general protein affinity purification, especially for proteins whose structure or function is affected by affinity tags fused to N- or C-terminals.