Photoinduced electron transfer at liquid|liquid interfaces. Part VII. Correlation between self-organisation and structure of water-soluble photoactive species

The self-organisation of a variety of dyes at the water|1,2-dichloroethane interface was studied by admittance measurements, photocurrent–potential curves and light polarisation anisotropy of the photocurrent. The heterogeneous photo-oxidation of ferrocene was studied at interfaces sensitised by Sn(IV) meso-tetra-(4-carboxyphenyl) porphyrin dichloride (SnTPPC), chlorin e-6, protoporphyrin IX (protoIX) and Fe(III) protoporphyrin IX chloride (Fe-protoIX). Cyclic voltammograms and capacitance voltage curves exhibit different features associated with the self-assembly of the dye species at the liquid|liquid boundary. In the case of SnTPPC, the capacitance curves displayed the characteristic responses commonly associated with the specific adsorption of ionic species. On the other hand, chlorin e-6, protoIX and Fe-protoIX show rather complex behaviour suggesting not only changes in the excess charge but also in the dielectric permittivity of the interface. Differences in the photocurrent efficiency were also observed under the same experimental conditions. The relative magnitude of the photocurrent responses were rationalised in terms of the phenomenological electron transfer rate constant, the photon capture cross-sections and the lifetime of the triplet state as obtained from nanosecond flash photolysis. Finally, the average molecular orientation of the adsorbed photoactive species was estimated from the photocurrent dependence on the angle of light polarisation in total internal reflection. The results show a clear correlation between the orientation of the transition dipole and the distribution of the peripheral carboxyl groups responsible for the hydrophilic nature of the dyes.     

Fragmentation of N-linked glycans with a matrix-assisted laser desorption/ionization ion trap time-of-flight mass spectrometer

N-Linked glycans were ionized from several matrices with a Shimadzu-Biotech AXIMA-QIT matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight mass spectrometer. [M+Na]+ ions were produced from all matrices and were accompanied by varying amounts of in-source fragmentation products. The least fragmentation was produced by 2,5-dihydroxybenzoic acid and the most by alpha-cyano-4-hydroxycinnamic acid and 6-aza-2-thiothymine. Sialic acid loss was extensive but could be prevented by formation of methyl esters. Fragmentation produced typical low-energy-type spectra dominated by ions formed by glycosidic cleavages. MS(n) spectra (n = 3 and 4) were used to probe the pathways leading to the major diagnostic ions. Thus, for example, an ion that was formed by loss of the core GlcNAc residues and the 3-antenna was confirmed as being formed by a B/Y rather than a C/Z mechanism. The proposed structures of several cross-ring cleavage ions were confirmed and it was shown that MS3 spectra could be obtained from as little as 10 fmol of glycan.     

Lysine peroxycarbamates: free radical-promoted peptide cleavage

Strategies are reported that combine in one step a predictable chemical-based protein digestion with mass spectrometry. Lysine residue amino groups in peptides and proteins are modified by reaction with a peroxycarbonate derived from p-nitrophenol, and tert-butyl hydroperoxide. The peroxycarbonate reacts with lysine residues in peptides and proteins, and the resulting lysine peroxycarbamates undergo homolytic fragmentation under conditions of low-energy collision-induced dissociation (CID). Observed fragmentation of the peptides involves apparent free radical processes including Hofmann-L?ffler-type rearrangements that lead to peptide chain fragmentation. Strategies for directed cleavage of peptides by free radical promoted processes are feasible, and such strategies may well simplify schemes for protein analysis.     

Uniqueness of barbilian domains

W. Leissner has developed a plane geometry over any Z-ring R, in which a point is an element of R×R and a line is a set of the form {(x+ ra, y + rb):r R} where (x,y) R×R and (a,b) is from a Barbilian domain, i.e., a set of unimodular pairs from R×R satisfying certain axioms. In this note we generalize results of W. Benz guaranteeing the uniqueness of Barbilian domains over several classes of commutative rings. The author wishes to thank Gordon Keller and Douglas Costa for fruitful discussions, the referee for his improvements, and the University of Virginia for its hospitality while this work was done.     

Why are spherical stellar systems relaxed?

Spherical stellar systems, from globular star dusters to compact galaxy clusters, appear to be dynamically relaxed. In galaxies and galaxy clusters, collisional relaxation acts too slowly to produce the observed result; and a new argument suggests that the same may be true of globular star clusters. Violent relaxation requires special initial conditions and seems unable to produce sufficiently extended halos. It is here proposed that dynamical relaxation may result from tidal perturbations by external systems. If this explanation is correct, it has important implications for the early history of galaxies and galaxy clusters.This essay was awarded the fourth prize for 1976 by the Gravity Research Foundation.     

Optimizing two types of discrete functions,subject to linear restrictions

This paper is a collection of four topics; they are: a discussion of two knapsack problems (see, for example, Gilmore and Gomory, 1966), some interesting heuristic remarks on computational experience, some remarks on the underlying algebraic structures directly related to the knapsack functions, and a new proof technique for optimizing the functions mentioned. All functions in sight are discrete and defined on the integers. Also, the proofs of the theorems do not use convexity and monotonicity of the more abstract optimization techniques.     

Effect of pH on the electrochemical reduction of some heterocyclic quinones

The reduction of 1,10-phenanthroline-5,6-quinone(I), 5,8-quinolinequinone(II) and 6,7-dichloro-5,8-quinolinequinone(III) was investigated using cyclic voltammetry and coulometry at mercury electrodes and 50% dimethylsulfoxide+water solvent. Each compound is reduced to the corresponding hydroquinone in a diffusion-controlled, reversible two-electron process. The pH-dependence of the reversible potential indicated that the quinone forms were unprotonated, but the hydroquinones could be protonated at the heterocyclic nitrogen atom with pK_a = 5.3 for I and 3.5 for III. Careful analysis of the cyclic voltammetric peak shape revealed that the difference between the standard potentials for the introduction of successive electrons, E₂⁰ ? E₁⁰, was 70 ±20, >100 and 80 ± 20 mV for I–III. Investigation of the pH-dependence of E₁⁰ and E₂⁰ showed that the pK_a of the semiquinone of I was about 8.     

Iron(III)-catalyzed Prins-type cyclization using homopropargylic alcohol: a method for the synthesis of 2-alkyl-4-halo-5,6-dihydro-2H-pyrans

[reaction: see text] A new Prins-type cyclization between homopropargylic alcohol and aldehydes in the presence of FeX(3) to obtain 2-alkyl-4-halo-5,6-dihydro-2H-pyrans in good yield is described. Osmium-catalyzed cis dihydroxylation provided direct access to trans-2-alkyl-3-hydroxy-tetrahydro-pyran-4-ones. Anhydrous ferric halides are also shown to be excellent catalysts for the standard Prins cyclization using homoallylic alcohol. Isolation of an intermediate acetal provides substantiation of a proposed mechanism.     

Amino acid analysis by micellar electrokinetic chromatography with laser-induced fluorescence detection: application to nanolitre-volume biological samples from Arabidopsis thaliana and Myzus persicae

Amino acids were derivatised with 4-fluoro-7-nitrobenzo-2,1,3-oxadiazol (NBD-F), separated by micellar electrokinetic chromatography (MEKC), and detected by argon-ion (488 nm) laser-induced fluorescence. The optimised MEKC background electrolyte conditions were: 40 mM sodium cholate, 5 mM beta-cyclodextrin in 20 mM aqueous borate buffer, pH 9.1, with 7% v/v acetonitrile. Using these conditions, 19 amino acids were separated within 17 min. The limits of detection were in the range of 7.6-42.2 pmol/mL and limits of quantitation from 0.05-0.14 nmol/mL. The method was systematically validated for injection volume error, migration time variation, calibration linearity, accuracy, precision, and recovery. Nanolitre volume samples of phloem sap of individual sieve element cells from the plant Arabidopsis thaliana and honeydew from the aphid Myzus persicae were directly analysed with this method. Quantitative amino acid concentrations in these two biological matrices were profiled for the first time. This method is particularly important because it allows the complete profile of the amino acids obtained from individual phloem elements, allowing cell to cell and plant to plant variation to be quantified, which to date has not been possible with Arabidopsis thaliana.     

Highly stereoselective addition of stannylcuprates to alkynones

The addition of stannylcuprate reagents such as (Bu3Sn)(PhS)CuLi to alkynones has been found to proceed in high yield and with excellent stereoselectivity for the Z isomer of the product (>95%). The behavior of the stannylcuprates is thus very different from that of their "carbocuprate" counterparts such as Me2CuLi or Me2Cu(CN)Li2 which are nonstereoselective. Furthermore, in contrast to the reactions of (R3Sn)(PhS)CuLi with the corresponding alkynoates, the presence of a proton source in the reaction medium has no effect on the stereoselectivity of the reaction of alkynones.