Free energy surfaces for the alpha(1 --> 4)-glycosidic linkage: implications for polysaccharide solution structure and dynamics

We present a potential of mean force surface for rotation about phi and psi dihedral angles of the alpha(1 --> 4)-glycosidic linkage in the maltose disaccharide (4-O-alpha-d-glucopyranosyl-d-glucopyranose) in aqueous solution. Comparison of the vacuum and solution free energy surfaces for maltose shows the principal effects of water to be an increase in the rotational freedom of the alpha(1 --> 4) linkage brought about by lowering the energy barrier for syn to anti conformational changes as well as expansion of the range of low-energy phi,psi conformations. This free energy analysis thus provides a thermodynamic and conformational rationale for the effects of water on alpha(1 --> 4)-linked polysaccharides and carbohydrate glasses.     

Solvent effects on the aggregation state of lithium dialkylaminoborohydrides

DFT calculations were performed to determine the effects of ethereal solvents on the aggregation state of lithium dialkylaminoborohydrides (LABs). The calculations included dimerization energies in the gas phase, with continuum solvation only, microsolvation with coordinating ethereal ligands, and a combination of the microsolvation and continuum models. The continuum model alone overestimates the stability of the dimers, apparently due to the lack of steric effects from the coordinating ethereal ligands. The use of the combined microsolvation and continuum solvation models predicts lithium dimethylaminoborohydride to be a mixture of monomer and dimer in THF, and more sterically hindered lithium aminoborohydrides to exist primarily as monomers. The kinetics of amination of 1-chlorodecane by lithium dimethylaminoborohydride showed no detectable change in reaction rate with time, suggesting that the LAB reagent may exist primarily as a monomer in THF.     

Recombinant HDL-like nanoparticles: a specific contrast agent for MRI of atherosclerotic plaques

A new contrast agent for MRI based on recombinant HDL-like nanoparticles has been prepared. It shows a great potential as a contrast agent for atherosclerotic plaques in a relative short time (24 h post-injection) as it is selective for the plaques and is an endogenous molecule. It also can distinguish between different types of plaques as the enhancement obtained is different, depending on plaque composition.     

Solution structure and chelation properties of 2-thienyllithium reagents

[reaction: see text] The solution and chelation properties of 2-thienyllithium reagents with potential amine and ether chelating groups in the 3-position and related model systems have been investigated using low temperature 6Li, 7Li, 13C, and 31P NMR spectroscopy, 15N-labeling, and the effect of solvent additives. In THF-ether mixtures at low temperature 3-(N,N-dimethylaminomethyl)-2-thienyllithium (4) is ca. 99% dimer (which is chelated) and 1% monomer (unchelated), whereas 3-(methoxymethyl)-2-thienyllithium (5) is <10% dimer. Compound 5 crystallizes as a THF-solvated dimer, but there is no indication that the ether side chain is chelated in solution. Both 4 and 5 form PMDTA-complexed monomers almost stoichiometrically, similar to the model compound 2, in sharp contrast to phenyl analogues, which show very different behavior. The barriers to dimer interconversion are ca. 2 kcal/mol lower and chelation is significantly weaker in the 2-thienyllithium reagents than in their phenyl analogues.     

Comparison of different fluorimetric HPLC methods for analysis of acidic polyether toxins in marine phytoplankton

The human toxic syndrome, diarrhetic shellfish poisoning (DSP), is caused by polyether toxins that are present in bivalve molluscs but originate from some species of marine phytoplankton. During the last few years different HPLC methods with fluorescence detection (FLD) have been proposed for analysis of marine toxins, including polyether toxins, in shellfish and phytoplankton. Several derivatization reagents have been proposed in the literature, with the aim of converting the acidic DSP toxins into their corresponding fluorescent derivatives. In this work we report results obtained from HPLC–FLD analysis of extracts from phytoplankton, including Dinophysis spp., harvested off the south-west coast of Ireland. Three different reagents were used for fluorescent derivatization: 3-bromomethyl-6,7-dimethoxy-1-methyl-2(1H)-quinoxalinone (BrDMEQ), 9-chloromethylanthracene (CA), and in situ 9-anthracenyldiazomethane (ADAM). Derivatization was performed under conditions previously optimised. The DSP derivatives were cleaned using different SPE procedures then analysed by HPLC–FLD. In this study, the use of BrDMEQ, CA, and in situ ADAM was compared in terms of sensitivity and selectivity. Evaluation of HPLC methods for analysis of DSP toxin derivatives was also conducted; the presence of okadaic acid (OA), dinophysistoxin-2 (DTX-2), and pectenotoxin-2 seco acids (PTX1SAs) was detected in the sample extracts studied.     

Models for the biological production of glycerol and biosurfactants from potato-processing industry residuals

Applied Biochemistry and Biotechnology -     

Meta-conjugation and excited-state coupling in phenylacetylene dendrimers

Traditional pictures of optical properties in phenylacetylene dendrimers view the molecule as a collection of independent chromophores, linked by meta-substitution at the central phenyl ring. While this picture is reasonable for explaining the observed absorption trends, it breaks down in describing the emission behavior. We utilize a combination of ab initio theory and experiment to demonstrate that differences in the absorbing and emitting states can be described using an exciton model with very weak chromophore coupling for the absorption geometry and strong coupling for the emission geometry. This result may have significant implications for the design of energy-funneling dendrimeric molecules.     

Aqueous-phase, palladium-catalyzed cross-coupling of aryl bromides under mild conditions, using water-soluble, sterically demanding alkylphosphines

Sterically demanding, water-soluble alkylphosphines have been used in combination with various palladium salts in Suzuki, Sonogashira, and Heck couplings of aryl bromides under mild conditions in aqueous solvents. The tert-butyl-substituted ligands 2-(di-tert-butylphosphino)ethyltrimethylammonium chloride (t-Bu-Amphos) and 4-(di-tert-butylphosphino)-N,N-dimethylpiperidinium chloride (t-Bu-Pip-phos) in combination with palladium(II) salts were found to give catalysts that were significantly more active than catalysts derived from tri(3-sulfonatophenyl)phosphine trisodium (TPPTS). Suzuki couplings of unactivated aryl bromides occurred efficiently at room temperature in water/acetonitrile and water/toluene biphasic mixtures or in neat water. Notably, Suzuki couplings of hydrophilic aryl bromides gave high yields without using organic solvents for the reaction or purification. This methodology has been applied to a highly efficient synthesis of diflunisal. The catalyst derived from t-Bu-Amphos was recycled three times in Suzuki couplings in water/toluene before catalyst activity began to significantly drop. The average yield of four cycles was >80% per cycle. Heck and Sonogashira couplings were carried out under mild conditions (50 and 80 degrees C, respectively) with unactivated aryl bromides to give coupled products in high yield.     

A preliminary evaluation of the functional significance of alpha-1-acid glycoprotein glycosylation on wound healing

The laying down of collagen and fibrous tissue is a key process in wound healing, however excessive collagen (and glycoprotein) deposition causes hypertrophic and keloid scars, eg after burns. Collagen synthesis is increased in these scars compared with normal healing, as is collagenase activity, which controls the degradation pathway of collagen. The processes of wound healing are inextricably linked to those of the acute-phase response (APR): alpha-1-acid glycoprotein (AGP), a plasma glycoprotein that undergoes both an increase in concentration and an alteration in its glycosylation pattern during the APR. This study determined that AGP isolated from the plasma of burns patients was of an increased concentration and altered glycosylation pattern compared with normal plasma and was capable of directly interacting with type I collagen. It also had a profound effect on both collagen fibril formation and collagenase activity, to a degree dependent upon the percentage body surface area burned. Additionally, the results obtained provided the basis for predicting the formation of hypertrophic scars.