Quality Control of Krill Oil by Nuclear Magnetic Resonance (NMR) Spectroscopy: Composition and Detection of Foreign Species

Krill oil is currently among the most highly promoted products in the dietary supplement market, which, due to its high price, can be potentially adulterated with fish species and artificial oil. For a holistic control of krill oil quality, ¹H, ¹³C, and ³¹P nuclear magnetic resonance (NMR) spectroscopies were used. The fatty acid and phospholipid composition as well as secondary ingredients, such as homarine, amino acids, and chitin, were examined. The following phospholipid species were detected: phosphatidylcholine (75–85?mol %), phosphatidylethanolamine (4–7?mol%) and their lyso derivatives 1-lysophosphatidylcholine (1–2?mol%)–2-lysophosphatidylcholine (10–16?mol%) and lysophosphatidylethanolamine (1?mol%). In the -2 position of phospholipids, the content of eicosapentaenoic acid (mean 68.23%; relative standard deviation 2.23%) was twice as high as the content of docosahexaenoic acid (mean 31.77%; relative standard deviation 4.79%). ¹³C NMR spectroscopy was used to distinguish between krill and fish oil-based dietary supplements. The adulteration of krill oil can be detected by fatty acid distribution in the sn-2 triacylglycerol position. The sensitivity of the method is about 10% (w/w) of fish content in blends, which is enough to detect deliberate adulteration. The same methodology can be used to recognize synthetically modified krill oil. The method was successfully applied to 30 commercially available krill and fish oil supplements.     

Methylglyoxal modification hinders amyloid conversion of prion protein

1. Download high-res image (138KB)
2. Download full-size image

     

Ferricenium salts as true substrates of glucose oxidase

The steady-state kinetics of D-glucose oxidation by ferricenium dyes RFc⁺PF₆ ^- (R = H, Me, Et, n-Bu, MeCH₂CMe₂, and Cl) and 1,1′-Et₂Fc+PF₆-catalyzed by glucose oxidase from Aspergillus niger was investigated as a function of RFc⁺ and D-glucose concentrations at pH 6.7, 25°C in the presence of 2% (v/v) Triton X-100. The enzymatic bleaching is characterized by large steady-state portions on the kinetic curves for all ferricenium ions studied. The reaction follows the MichaelisMenten kinetics demonstrating a high affinity of RFc⁺ toward the active site of reduced glucose oxidase (GO). The reaction rate is weakly sensitive to the nature of RFc⁺, and the apparent V_m(app) values decrease only twofold on going from the most to the least reactive salt in the series (l,l′-Et₂Fc⁺ and CIFc⁺, respectively), although their observed redox potentials differ by 160 mV. Remarkably, the reactivity of RFc⁺ does not increase with increasing their oxidative power. The apparent Michaelis constants K_m(app) are also weakly sensitive to the nature of RFc⁺. The profiles for the steady-state rate vs [HFc⁺] and [D-glucose] were rationalized in terms of the “ping-pong” mechanism typical of the catalysis by GO. Ferrocenecarboxylic acid (FcCOOH) appeared to be a competitive inhibitor of GO with the inhibition constant of (3 ±1) x 10-³M. The pH profile for the ferricenium fading is bell-shaped with the optimum around 7.5. A simple routine for a rapid in situ preparation of the ferricenium dye, which is ready for spectrophotometric assaying of the GO activity, is presented. The apparent V_m(app) and K_m(app) values for this substrate are similar to those for HFc⁺PF₆.     

Synthesis,Molecular Structure and Coordination Chemistry of the First 1‐Aza‐3,7‐diphosphacyclooctanes

The first representatives of a novel type of cyclic bis‐phosphines, namely, 1‐aza‐3,7‐diphosphacyclooctanes ( 4 , 5 ), were synthesized by condensation of 1,3‐bis(arylphosphino)propanes ( 2 , 3 ; aryl = phenyl or mesityl), formaldehyde and 5‐aminoisophthalic acid. Only the meso isomers were obtained, in good to satisfactory yield. The cyclic bis‐phosphines readily form P,P chelate complexes ( 6 , 7 ) with [PtCl₂(cod)] (cod = 1,5‐cyclooctadiene). The bisphosphine 4 and the corresponding complex 6 are soluble in water in the presence of two equivalents of alkali metal hydroxide. The molecular structures of 1‐(meta‐dicarboxyphenyl)‐3,7‐dimesityl‐1‐aza‐3,7‐diphosphacyclooctanes ( 5 ) and cis‐{P,P‐1‐(meta‐dicarboxyphenyl)‐3,7‐diphenyl‐1‐aza‐3,7‐diphosphacyclooctane}dichloroplatinum(II) ( 6 ) are reported.     

Synthesis and properties of stereoregular cyclic polysilanols: cis-[PhSi(O)OH](4), cis-[PhSi(O)OH](6), and tris-cis-tris-trans-[PhSi(O)OH](12)

New stereoregular cyclic polysilanols of the general formula [PhSi(O)OH]n (n = 6 and 12) have been selectively obtained in high yields by the reaction of cagelike oligophenylmetallasiloxanes with dilute solutions of hydrochloric acid at low temperatures. An alternative method was used to prepare cis-[PhSi(O)OH](4) from sodium phenylsiloxanolate, cis-[(Na(+))(4)[PhSi(O)O(-)](4)].(1-butanol)(x). All compounds were fully characterized by NMR and IR spectroscopy and molecular weight determinations. The structure of cis-[PhSi(O)OH](6) was confirmed by single-crystal X-ray analysis. Furthermore, a series of stereoregular cyclosiloxanes containing triorganylsiloxy groups at each silicon atom was prepared by the reactions of the cyclic polysilanols with triorganylchlorosilanes Me(3)SiCl, Me(2)ViSiCl, and Me(2)(CH(2)Cl)SiCl.     

Adsorption and aggregation of cationic amphiphilic polyelectrolytes on silica

The adsorption of two cationic amphiphilic polyelectrolytes, which are copolymers of two charged monomers, triethyl(vinylbenzyl)ammonium chloride and dimethyldodecyl(vinylbenzyl)ammonium chloride (which is the amphiphilic one) with different contents of amphiphilic groups (40% (40DT) and 80% (80DT)), onto the hydrophilic silica-aqueous solution interface has been studied by in situ null ellipsometry and tapping mode atomic force microscopy (AFM). Adsorption isotherms for both polyelectrolytes were obtained at 25 degrees C and at different ionic strengths, and the adsorption kinetics was also investigated. At low ionic strength, thin adsorbed layers were observed for both polyelectrolytes. The adsorption increases with polymer concentration and reaches, in most cases, a plateau at a concentration below 50 ppm. For the 80DT polymer, at higher ionic strength, an association into aggregates occurs at concentrations at and above 50 ppm. The aggregates were observed directly by AFM at the surface, and by dynamic light scattering in the solution. The adsorption data for this case demonstrated multilayer formation, which correlates well with the increase in viscosity with the ionic strength observed for 80DT.     

DSC and adiabatic calorimetry study of the polymorphs of paracetamol

Monoclinic (I) and orthorhombic (II) polymorphs of paracetamol were studied by DSC and adiabatic calorimetry in the temperature range 5 - 450 K. At all the stages of the study, the samples (single crystals and powders) were characterized using X-ray diffraction. A single crystal → polycrystal II→ I transformation was observed on heating polymorph II, after which polymorph I melted at 442 K. The previously reported fact that the two polymorphs melt at different temperatures could not be confirmed. The temperature of the II→I transformation varied from crystal to crystal. On cooling the crystals of paracetamol II from ambient temperature to 5 K, a II→ I transformation was also observed, if the 'cooling-heating' cycles were repeated several times. Inclusions of solvent (water) into the starting crystals were shown to be important for this transformation. The values of the low-temperature heat-capacity of the I and II polymorphs of paracetamol were compared, and the thermodynamic functions calculated for the two polymorphs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Polarized X-ray absorption spectroscopy of single-crystal Mn(V) complexes relevant to the oxygen-evolving complex of photosystem II

High-valent Mn-oxo species have been suggested to have a catalytically important role in the water splitting reaction which occurs in the Photosystem II membrane protein. In this study, five- and six-coordinate mononuclear Mn(V) compounds were investigated by polarized X-ray absorption spectroscopy in order to understand the electronic structure and spectroscopic characteristics of high-valent Mn species. Single crystals of the Mn(V)-nitrido and Mn(V)-oxo compounds were aligned along selected molecular vectors with respect to the X-ray polarization vector using X-ray diffraction. The local electronic structure of the metal site was then studied by measuring the polarization dependence of X-ray absorption near-edge spectroscopy (XANES) pre-edge spectra (1s to 3d transition) and comparing with the results of density functional theory (DFT) calculations. The Mn(V)-nitrido compound, in which the manganese is coordinated in a tetragonally distorted octahedral environment, showed a single dominant pre-edge peak along the MnN axis that can be assigned to a strong 3d(z(2))-4p(z) mixing mechanism. In the square pyramidal Mn(V)-oxo system, on the other hand, an additional peak was observed at 1 eV below the main pre-edge peak. This component was interpreted as a 1s to 3d(xz,yz) transition with 4px,y mixing, due to the displacement of the Mn atom out of the equatorial plane. The XANES results have been correlated to DFT calculations, and the spectra have been simulated using a TD (time-dependent)-DFT approach. The relevance of these results to understanding the mechanism of the photosynthetic water oxidation is discussed.     

The first hexanuclear copper(I) carboxylate: X-ray crystal structure and reactivity in solution and gas-phase reactions

The carboxylate ligand-exchange reaction of copper(I) trifluoroacetate by 3,5-difluorobenzoate yielded a new product, [Cu(O2C(3,5-F)2C6H3)] (1). Single crystals of 1 suitable for X-ray structural characterization were obtained by sublimation-deposition procedures at 230 degrees C. An X-ray diffraction study revealed a remarkable planar hexanuclear copper(I) core supported by bridging carboxylates, the first such structural type among other known copper(I) carboxylates. The Cu...Cu distances within the core range from 2.7064(8) to 2.8259(8) A and fall into the category of cuprophillic interactions. The hexacopper unit remains intact upon gas-phase deposition with a planar polyarene, coronene (C24H12), to give [Cu6(O2C(3,5-F)2C6H3)6](C24H12) (2). Density functional theory calculations suggest the latter compound to be a cocrystallization product having electrostatic interactions between the hexacopper complex and coronene. However, cocrystallization affects the photophysical properties of 2. While copper(I) 3,5-difluorobenzoate (1) exhibits photoluminescence at ca. 554 nm (lambda(ex) = 350 nm) in the solid state, compound 2 is nonluminescent at room temperature in the visible region. Gas-phase and solution reactions of 1 with alkyne ligands, diphenylacetylene (C14H10) and 1,4-bis(p-tolylethynyl)benzene (C24H18), result in the rupture of the [Cu6] core to afford dinuclear organometallic copper(I) complexes. The latter have a dimetal core cis-bridged by two carboxylate groups with acetylene ligands eta(2)-coordinated to each copper(I) center.     

Genus one polyhedral surfaces,spaces of quadratic differentials on tori and determinants of Laplacians

We prove a formula for the determinant of the Laplacian on an arbitrary compact polyhedral surface of genus one. This formula generalizes the well-known Ray–Singer result for a flat torus. A special case of flat conical metrics given by the modulus of a meromorphic quadratic differential on an elliptic surface is also considered. We study the determinant of the Laplacian as a functional on the moduli space of meromorphic quadratic differentials with L simple poles and L simple zeros and derive formulas for variations of this functional with respect to natural coordinates on . We give also a new proof of Troyanov’s theorem stating the existence of a conformal flat conical metric on a compact Riemann surface of arbitrary genus with a prescribed divisor of conical points.