Light-Induced Oxidation of Unsaturated Lipids as Sensitized by Flavins

Triplet-excited riboflavin ((3)RF*) was found by laser flash photolysis to be quenched by polyunsaturated fatty acid methyl esters in tert-butanol/water (7:3, v/v) in a second-order reaction with k similar to 3.0 x 10(5) L mol(-1) s(-1) at 25 degrees C for methyl linoleate and 3.1 x 10(6) L mol(-1) s(-1), with Delta H double dagger = 22.6 kJ mol(-1) and Delta S double dagger = -62.3 J K(-1) mol(-1), for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <10(4) L mol(-1) s(-1). For comparison, beta-casein was found to have a rate constant k similar to 4.9 x 10(8) L mol(-1) s(-1). Singlet-excited flavin was not quenched by the esters as evidenced by insensitivity of steady-state fluorescence to their presence. Density functional theory (DFT) calculations showed that electron transfer from unsaturated fatty acid esters to triplet-excited flavins is endergonic, while a formal hydrogen atom transfer is exergonic (Delta G(HAT)degrees = -114.3, -151.2, and -151.2 kJ mol(-1) for oleate, linoleate, and linolenate, respectively, in acetonitrile). The reaction is driven by acidity of the lipid cation radical for which a pK(a) similar to -0.12 was estimated by DFT calculations. Absence of electrochemical activity in acetonitrile during cyclic voltammetry up to 2.0 V versus NHE confirmed that Delta G(ET)degrees > 0 for electron transfer. Interaction of methyl esters with (3)RF* is considered as initiation of the radical chain, which is subsequently propagated by combination reactions with residual oxygen. In this respect, carbon-centered and alkoxyl radicals were detected using the spin trapping technique in combination with electron paramagnetic resonance spectroscopy. Moreover, quenching of 3RF* yields, directly or indirectly, radical species which are capable of initiating oxidation in unsaturated fatty acid methyl esters. Still, deactivation of triplet-excited flavins by lipid derivatives was slower than by proteins (factor up to 10(4)), which react preferentially by electron transfer. Depending on the reaction environment in biological systems (including food), protein radicals are expected to interfere in the mechanism of light-induced lipid oxidation.

Organic additive-copper(II) complexes as plating precursors

Cu(II) ions previously coordinated with typical electroplating organic additives were investigated as an alternative source of metal for plating bath. The coordination complexes were isolated from reaction between CuSO(4) and organic additives as ligands (oxalate ion, ethylenediamine or imidazole). Deposits over 1010 steel were successfully obtained from electroplated baths using the complexes without any addition of free additives, at pH = 4.5 (H(2)SO(4)/Na(2)SO(4)). These deposits showed better morphologies than deposits obtained from CuSO(4) solution either in the absence or presence of oxalate ion as additive (40 mmol L(-1)), at pH = 4.5 (H(2)SO(4)/Na(2)SO(4))It is suggestive that the starting metal plating coordinated with additives influences the electrode position processes, providing deposits with corrosion potentials shifted over + 200 mV in 0.5 mol L(-1) NaCl (1 mV s(-1)). The resistance against corrosion is sensitive to the type of additive-complex used as precursor. The complex with ethylenediamine presented the best deposit results with the lowest pitting potential (-0.27 V vs 3.0 mol L(-1) CE). It was concluded that the addition of free additives to the electrodeposition baths is not necessary when working with previously coordinated additives. Thus, the complexes generated in ex-situ are good alternatives as plating precursors for electrodeposition bath. (C) 2009 Elsevier B.V. All rights reserved.

Interaction of bovine serum albumin (BSA) with ionic surfactants evaluated by electron paramagnetic resonance (EPR) spectroscopy

EPR spectra of 5- and 16-doxyl stearic acid nitroxide probes (5-DSA and 16-DSA, respectively) bound to bovine serum albumin (BSA) revealed that in the presence of ionic surfactants, at least, two label populations coexist in equilibrium. The rotational correlation times (tau) indicated that component I displays a more restricted mobility state, associated to the spin labels bound to the protein; the less immobilized component 2 is due to label localization in the surfactant aggregates. For both probes, the increase of surfactant concentration leads to higher motional levels of component 1 followed by a simultaneous decrease of this fraction of nitroxides and its conversion into component 2. For 10 mM cethyltrimethylammonium chloride (CTAC), the nitroxides are 100% bound to the protein, whereas at 10mM N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and sodium dodecyl sulfate (SDS) the fractions of bound nitroxides are reduced to 18% and 86%, respectively. No significant polarity changes were observed in the whole surfactant concentration range for component 1. Moreover, at higher surfactant concentration, component 2 exhibited a similar polarity as in the pure surfactant micelles. For 16-DSA the surfactant effect is different: at 10mM of HPS and CTAC the fractions of bound nitroxides are 76% and 49%, respectively, while at 10 mM SDS they are present exclusively in a micellar environment, consistent with 100% of component 2. Overall, both SDS and HPS are able to effectively displace the nitroxide probes from the protein binding sites. while CTAC seems to affect the nitroxide binding to a significantly smaller extent. (C) 2008 Elsevier B.V. All rights reserved.

Synthesis and preliminary analytical evaluation of the chemiluminescence from (4-[4-(dichloromethylsilanyl)-butyl]-4`-methyl-2,2`-bipyridyl)bis(2,2`-bipyridyl)ruthenium(II) covalently bonded to silica particles

This paper describes, for the first time, a simple and effective synthetic route for covalently bonding the chemiluminescence reagent, (4-[4-(dichloromethylsilanyl)-butyl]-4’-methyl-2,2’-bipyridyl)bis(2,2’-bipyridyl)ruthenium(II) onto silica particles. The subsequent preparation of chemically regeneratable detection cells and their preliminary analytical evaluation with both sequential injection analysis and flow injection analysis are also reported. Unoptimised analytical figures of merit were established for standard solutions of codeine and sodium oxalate with detection limits calculated from three times the standard deviation of the blank signal, of 1 × 10^–8 M and 3 × 10^–7 M respectively. The chemically immobilised reagent exhibited some intriguing solvent and kinetic effects, which are also briefly discussed.

The rapid analysis of heroin drug seizures using micellar electrokinetic chromatography with short-end injection

A simple and rapid method for the analysis of heroin seizures by micellar electrokinetic chromatography with short-end injection is described. Separations were performed using an uncoated fused silica capillary, 50 cm×50 mm I.D.×360 mm O.D. with an effective separation length of 8 cm. The system was run at 25°C with an applied negative voltage of –25 kilovolts. Injection of each sample was for 2 s at –50 mbar. UV detection was employed with the wavelength set at 210 nm. The background electrolyte consisted of 85:15 (water:acetonitrile, v/v) containing final concentrations of 25 mM SDS and 15 mM sodium borate, pH 9.5. Samples and standards were prepared in 0.1% v/v acetic acid and diluted in the run buffer containing 1 mg/ml of N,N-dimethyl-5-methoxytryptamine as an internal standard. Under these conditions a text mixture containing caffeine, paracetamol, morphine, codeine, heroin, and acetylcodeine was resolved within 1.5 min. The method was used to determine the concentration of heroin in heroin seizure samples, and the results were in good agreement with those obtained by a validated gas chromatographic method.

Aqueous/micellar peroxyoxalate chemiluminescent detection for ion chromatography

This thesis covers the development of the traditionally fluorescent bis(8-quinolinol-5-sulfonic acid) magnesium (II) fluorophore as a chemiluminescent emitter. A brief description of luminescence spectroscopy and its application to analytical chemistry lays the foundation to the discussion of the results obtained herein. This includes the synthesis and identification of two so called ‘water soluble’ aryl oxamides 2,2’-oxalyl-bis(trifluoromethanesulfonyl) imino] ethylene-bis(N- methylpyridinium) trifluoromethane sulfonate (PETQ) and 2,2’-oxalyl-bis(trifluoromethanesulfonyl) imino]ethylene-bis(N-pyridinium) chloride (PETH), previously developed for the US navy as a possible emergency light source, yet the synthetic methodology were incomplete. The inconsistencies of the synthetic methods for PETQ and PETH were overcome with yields satisfactory for their preliminary analytical evaluation. The evaluation of these aryl oxamides, including 4,4’-oxalyI- bis[(trifluoromethanesulfonyl) imino]ethylene-bis(l-methyM-benzylpiperidinium) trifluoromethanesulfonate (BPTQ), 4,4’-oxalyl-bis [(trifluoromethylsulfonyl)imino] ethylene-bis(N-methylmorpholinium)trifluoromethanesulfonate (METQ) and the oxalate bis(2,4,6-trichlorophenyl) oxalate (TCPO) were performed with the peroxyoxalate chemiluminescent reaction using bis(8-quinolinol-5-sulfonic acid) magnesium (II) as the fluorophore. A univariate optimisation of this system resulted in 0,0082 mol 1-1 the detection limit of magnesium in the absence of cationic surfactants and 0.0041 mol 1-1 in their presence for the majority of these compounds. The oxamides were found to be insoluble in water with long ulrasonication periods required to dissolve the compound, with solvents such as acetonitrile preferred. The determination of other chemiluminescent metal-8HQS chelates to replace magnesium -8HQS in the peroxyoxalate were limited to Al (III), Cd (II), Ca (II), In (II) and Zn (II), unfortunately these metals all possessed poorer detection limits than those obtained using magnesium The base reaction conditions used for the flow injection system with chemiluminescent detection were transferred to an ion chromatographic configuration for the separation of magnesium from other cations on an exchange column. After a univariate and simplex optimisation of these conditions, the detection limit of magnesium was found to be 0.0411 mol 1-1 which was less than the limits that could be achieved with fluorescent detection, The further development of this reaction to incorporate the displacement of magnesium from Mg-EDTA by other metals that possessed a higher conditional stability constant than magnesium also proved to be problematic with interferences from not only EDTA but from the eluant (lactic acid) from the cation column. Using this system the detection limits of the displacing metals were found to be in the order of 10 mg 1-1 which was substantially less that what was observed when exactly the same configuration was used with fluorescent detection. The final component of the thesis entails the discussion of the background emission that results from the reaction of oxamides/oxalates with hydrogen peroxide. A detailed investigation into the reaction of TCPO and hydrogen peroxide in the presence of various additives, such as imidazole , heavy atoms and triethylamine illustrated the existence of a further intermediate in fee mechanism for this reaction. The species responsible for this emission was attributed to the degradation product 2,4,6-trichlorophenyi of TCPO, which was supported by the non-existent background present with the oxamides that do not contain this degradation product.

Chiral organotin hydrides as enantioselective reducing agents

This thesis reports on the feasibility of the utilization of organotin hydrides as enaantioselective free radical reducing agents. The chiral organotin hydrides prepared contain the bulky chiral (1R,2S,5R)-menthyl substituent and in some cases also contain a stereogenic tin centre. Reaction of (1R,2S,5R)-menthylmagnesium chloride (MenMgC1) with triphenyltin chloride in THF proceeds with epimerization of the C-1 carbon of the menthyl group and results in a mixture of (1R,2S,5R)-menthyltriphenyltin and (1S,2S.5R)-menthyltriphenyltin. Addition of Lewis bases such as triphenylphosphine to the THF solution of triphenyltin chloride prior to the addition of the Grignard reagent suppresses epimerization and enables isolation of pure (1R,2S,5R)-menthyltriphenyltin. (1R,2S,5R)-Menthyltriphenyltin is the precursor for the synthesis of (1R,2S,5R)-menthyldiphenyltin hydride as well as (1R,2S,5R)-menthyl-containing organotin halide derivatives. A crystal structure of (1R,2S,5R)-menthylphenyltin dibromide and (1R,2S,5R)-menthylphenyltin dichloride confirmed the configuration of the menthyl substituent in these compounds. Reaction of MenMgC1 with diphenyltin dichloride in THF proceeds with no epimerization of the C-1 carbon of the menthyl group and bis((1R,2S,5R)-menthyl)diphenyltin is formed. A crystal structure of (1R,2S,5R)-menthyltriphenyltin confirmed the configuration of the menthyl substituent. Bis((1R,2S,5R)-menthyl)diphenyltin is used to form bis((1R,2S,5R)-menthyl)phenyltin hydride as well as other bis(1R,2S,5R)-menthyl derivatives. A series of chiral non-racemic triorganotin halides and triorganotin hydrides containing one or two (1R,2S,5R)-menthyl substituents as well as various potentially intramolecular coordination substituents were synthesized and characterized. The intramolecular substituents include the 8-(dimethylamino)naphthyl, 2-[(1S)-1-dimethylaminoethyl]phenyl, 2-(4,4-dimethyl-2-oxazoline)-5-methylphenyl and the 2-(4-(S)isopropyl-2-oxazoline)-5-methylphenyl substituents. Each compound containing a stereogenic tin centre was synthesized as diastereomeric mixtures. AM1 calculations of these compounds provide good qualitative predictability of the molecular geometries observed in the solid state as well as the diastereomeric ratios observed in solution. X-ray analysis of some of the organotin halides containing intramolecular coordination substituents revealed a tendency towards penta-coordination at the tin centre as a result of N-Sn interactions. The chiral organotin hydrides synthesized were found to be poor enantioselective free radical reducing agents. However, the addition of one molar equivalent of achiral or chiral Lewis acids to the free radical reduction reactions involving these organotin hydrides results in remarkable increases in enantioselectivity. There are numerous examples in which enantioselectivities exceed 80% and three examples of enantioselectivites which are equal and above 90% with one outstanding enantioselective outcome of ≥99%. These results appear to be the highest enantioselectivites for organotin hydride radical reductions reported to date. There is strong evidence to suggest that the chiral menthyl group of the organotin hydride directs the stereochemical outcome in the reduced product. The results also suggest that an increase in the number of menthyl substituents attached to tin or the introduction of intramolecular coordination substituents does not necessarily results in a greater increase in enantioselectivity. Preliminary studies into the synthesis of organotin hydrides containing Lewis acid functionalities are also reported. A zirconium chloride functionality was found to be incompatible with organotin hydride. However, an organotin hydride containing a trialkylboron Lewis acid functionality attached via an alkyl chain was successfully synthesized. Although this reagent was only stable in the preparative THF solution, it was still found to be effective at reducing benzaldehyde to benzyl alcohol.

A 7Li and 19F NMR relaxation study of LiCF3SO3 in plasticised solid polyether electrolytes

⁷Li and ¹⁹F NMR relaxation time (T₁, T₂, T_1ρ) measurements have been used to probe the dynamics of LiCF₃SO₃ dissolved in an amorphous co-polymer poly(ethylene oxide-co-propylene oxide), and in particular the influence of the plasticising agents propylene carbonate and dimethyl formamide. The changes in relaxation behaviour of ¹⁹F and ⁷Li with increasing plasticiser concentration are very different, as is the effect of each plasticiser. These differences can be explained qualitatively in terms of the interaction between the plasticiser and the ions. Preliminary ⁷Li T_1ρ measurements reveal two components at low temperatures.

Ion association and molar conductivity in polyether electrolytes

The ion association behaviour observed in our earlier studies of a polyether electrolyte system at elevated temperatures, was reminiscent of the molar conductivity behaviour typical of low dielectric constant systems. Further investigation of this relationship has led to some suggestions about the types of ionic species present in the polymer electrolyte systems. FT-IR spectroscopy has been used in this work to contrast ion association in an amorphous polyether electrolyte with two liquid electrolytes, N,N-dimethyl-formamide and tetraethylenegylcol dimethylether, containing lithium trifluoromethan sulfonate.

FT-IR investigation of Ion association in plasticized solid polymer electrolytes

FT-IR spectroscopy has been utilized to monitor ion association in plasticized solid polymer electrolytes (SPEs). The SPEs were prepared from a random copolymer of ethylene oxide (EO) and propylene oxide (PO) and the salt lithium trifluoromethanesulfonate (lithium triflate, LiTf). Tetraethylene glycol dimethyl ether (tetraglyme) and N,N‘-dimethylformamide (DMF) were chosen as model plasticizers. Despite having a similar dielectric constant to that of the polymer host, ε ~ 5, the incorporation of tetraglyme into the SPEs resulted in increased ion association. The addition of a higher dielectric constant solvent , DMF, ε = 36.7, resulted in decreased ion association in the SPE. The effects of salt concentration (0.05−1.25 mol dm^-3) and temperature (25−100 °C) upon ion association in SPEs were also investigated. At low salt concentrations, ion association was found to increase with temperature, however, at 1.25 mol dm^-3 the temperature dependence of ion association was dominated by concentration effects. There appears to be a maximum in the fraction of “free” ions at a LiCF₃SO₃ concentration of 0.4 mol dm^-3, preceded by a minimum at approximately 0.2 mol dm^-3, consistent with the molar conductivity behavior previously observed in these electrolytes.

Triflate ion association in plasticized polymer electrolytes

Ion association in plasticised solid polymer electrolytes (SPEs) has been monitored using FT-IR spectroscopy. The SPEs were prepared from a random co-polymer of ethylene oxide (EO) and propylene oxide (PO) and the salt lithium trifluoromethane sulfonate (lithium triflate, LiTf). Tetraethylene glycol dimethylether (tetraglyme, ε˜5) and N,N'-dimethyl formamide (DMF, ε = 36.7) were chosen as model plasticisers. Decreased ion association resulted from plasticization with DMF, indicating that the addition of a higher dielectric constant solvent increases the fraction of dissociated ions in the SPE. The incorporation of tetraglyme into these SPEs results in increased ion association, despite the similar dielectric constants of the plasticiser and polymer host. The effects of salt concentration (0.05–1.25 mol dm^{− 3} solvent) upon ion association in SPEs was also investigated. There appears to be a minimum in the number of “free” ions at a LiTf concentration of 0.2 mol dm^{− 3} solvent followed by a maximum at approximately 0.4 mol dm^{− 3} solvent, consistent with the molar conductivity behaviour previously observed in these electrolytes.

Ionic conductivity studies of polymeric electrolytes containing lithium salt with plasticizer

New plasticized polymer electrolytes were synthesized based on poly ethylene oxide (PEO), Poly (N,N-dimethylamino-ethyl-methacrylate) (PDMAEMA), LiN(CF₃SO₂)₂ (LITFSI) as the salt and tetraethylene glycol dimethyl ether(tetraglyme) and EC + PC as plasticizers. The preparation and characterization of the polymer electrolytes were investigated as a function of temperature and various concentrations of LITFSI. Impedance spectroscopy and differential scanning calorimeter (DSC) were used to characterize the effects of various temperature, lithium salt concentration and two plasticizers on conductivity. The complex of PDMAEMA/PEO/LiTFSI/tetraglyme (S2) exhibits higher conductivity (4.74 × 10⁻⁴ S cm⁻¹at 25 °C) than PDMAEMA/PEO/LiTFSI/EC + PC (S1).

NMR determination of ionic structure in plasticized polyether-urethane polymer electrolytes

Solid polymer electrolytes based on amorphous polyether-urethane networks combined with lithium or sodium salts and a low molecular weight cosolvent (plasticizer) have been investigated in our laboratories for several years. Conductivity enhancements of up to two orders of magnitude can be obtained whilst still retaining solid elastomeric properties. In order to understand the effects of the plasticizers and their mechanism of conductivity enhancement, multinuclear NMR has been employed to investigate ionic structure in polymer electrolyte systems containing NaCF₃SO₃, LiCF₃SO₃ and LiClO₃ salts.

With increasing dimethyl formamide (DMF) and propylene carbonate (PC) concentration the increasing cation chemical shift with fixed salt concentration indicates a decreasing anion-cation association consistent with an increased number of charge carriers. ¹³C chemical shift data for the same systems suggests that whilst DMF also decreases cation-polymer interactions, PC does the opposite, presumably by shielding cation-anion interactions. Temperature dependent ⁷Li spin-lattice relaxation times indicate the expected increase in ionic mobility upon plasticization with a shift of the T₁ minimum to lower temperatures. The magnitude of T₁ at the minimum increases upon addition of DMF whereas there is a slight decrease when PC is added. This also supports the suggestion that the DMF preferentially solvates the cation whereas the action of PC is limited to coulomb screening, hence freeing the anion.

Conductivity and NMR properties of plasticized polyethers complexed with lithium salts

NMR provides a tool whereby the dynamic properties of specific nuclei can be investigated. In the present study, a poly(ethylene oxide-co-propylene oxide) network has been used as the polymer host to prepare solid polymer electrolytes (SPE) containing either LiClO₄ or LiCF₃SO₃. In addition, a low molecular weight plasticizer [propylene carbonate (PC), dimethyl formamide (DMF) or tetraglyme] has been added to several of the samples to enhance the mobility of the polymer and, thus, of the ionic species. The effects of plasticizer and salt concentration on the ionic structure and mobility in these SPEs, as measured by NMR relaxation times, and correlation to the conductivity behaviour in these systems are discussed. Temperature dependent triflate diffusion coefficients, as measured by Pulsed Field Gradient ¹⁹F-NMR, in plasticized SPEs are also reported.

An nmr investigation of ionic structure and mobility in plasticized solid polymer electrolytes

²³Na and ¹⁹F nuclear magnetic resonance spectroscopy is used to investigate the effect of plasticizer addition on ionic structure and mobility in a urethane crosslinked polyether solid polymer electrolyte. The incorporation of dimethyl formamide and propylene carbonate plasticizers in a sodium triflate/polyether system results in an upfield chemical shift for the ²³Na resonance consistent with decreased anion-cation association and increased cation-plasticizer interactions. The ¹⁹F resonances appears less susceptible to changes in chemical environment with only minor chemical shift changes recorded. Spin lattice relaxation measurements for the ¹⁹F nucleus are also reported. Two minima are observed in the relaxation measurements consistent with both an inter and intramolecular relaxation mechanism.