Efficient asymmetric synthesis of an azasugar in water

An extremely efficient asymmetric synthesis of a pyrrolidine azasugar was completed in only four steps in water, without the use of protecting groups and in 60% overall yield from a simple, achiral bis-electrophile.     

Synthesis of stable analogues of thiamine di- and triphosphate as tools for probing a new phosphorylation pathway

Thiamine (vitamin B1) is an essential nutritional factor metabolized inside the body in its mono-, di-, and triphosphate forms. Although the action of thiamine and thiamine diphosphate have been intensely investigated, many questions remain unanswered and the role of thiamine triphosphate is still especially unknown. To probe recent hypotheses on the implication of thiamine triphosphate in a new phosphorylation pathway involving synaptic proteins, we synthesized a series of thiamine di- and triphosphate analogues that are resistant to both enzymatic and chemical hydrolyses. The key step in the preparation of the title compounds is the coupling of thiamine propyl disulfide with adequately protected methylenebis-phosphonic acid, the corresponding triphosphate analogue, and difluoromethylenebisphosphonic acid.     

NMR relaxation study of the protonated form of 1,8-Bis(dimethylamino)naphthalene in isotropic solution: anisotropic motion outside of extreme narrowing and ultrafast proton transfer

The protonated form of 1,8-bis(dimethylamino)naphthalene (DMANH(+)) consists of a rigid, aromatic framework, substituted by two amino groups that are connected by a strong, symmetric (on the NMR time-scale) hydrogen bond bridge. The reorientational motion of the molecule in dimethylformamide-d7 solution was characterized by T(1) and NOE measurements for aromatic (13)C nuclei. Treating the reorientation of DMANH(+) as anisotropic rotational diffusion of a rigid body, the diffusion tensor was determined with good accuracy. Measurements and interpretation of (15)N T(1) and NOE indicate that the proton transfer between potential minima in the hydrogen bond bridge is faster than the molecular reorientation.     

Molecular gradients: an efficient approach for optimizing the surface properties of biomaterials and biochips

A variety of molecular gradients of alkanethiols with the structure HS-(CH(2))(m)-X (m = 15; X = COOH, CH(2)NH(2), or CH(3)) and oligo(ethylene glycol)-terminated alkanethiols with the structures HS-(CH(2))(15)-CO-NH-Eg(n) (n = 2, 4, or 6), HS-(CH(2))(15)-CO-NH-Eg(2)-(CH(2))(2)-NH-CO-(CH(2))(4)-biotin, and HS-(CH(2))(15)-CO-NH-Eg(6)-CH(2)-COOH were prepared on polycrystalline gold films. These gradients were designed to serve as model surfaces for fundamental studies of protein adsorption and immobilization phenomena. Ellipsometry, infrared spectroscopy, and X-ray photoelectron spectroscopy, operating in scanning mode, were used to monitor the layer composition, gradient profiles, tail group conformation, and overall structural quality of the gradient assemblies. The gradient profiles were found to be 4-10 mm wide, and they increased in width with increasing difference in molecular complexity between the thiols used to form the gradient. The oligo(ethylene glycol) thiols are particularly interesting because they can be used to prepare so-called conformational gradients, that is, gradients that display a variation in oligo(ethylene glycol) chain conformation from all trans on the extreme Eg(2,4) sides, via an amorphous-like phase in the mixing regimes, to helical at the extreme Eg(6) sides. We demonstrate herein a series of experiments where the above gradients are used to evaluate nonspecific binding of the plasma protein fibrinogen, and in agreement with previous studies, the highest amounts of nonspecifically bound fibrinogen were observed on all-trans monolayers, that is, on the extreme Eg(2,4) sides. Moreover, gradients between Eg(2) and a biotinylated analogue have been prepared to optimize the conditions for the immobilization of streptavidin. Ellipsometry and infrared spectroscopy reveal high levels of immobilization over a fairly broad range of compositions in the gradient regime, with a maximum between 50 and 60% of the biotinylated analogue in the monolayer. A pI gradient composed of (NH(3)(+)/COO(-))-terminated thiols was also prepared and evaluated with respect to its ability to separate differently charged proteins, pepsin, and lysozyme, on a solid surface.     

Determination of heptacaine hydrochloride in solution using nuclear analytical methods

The applicability of two nuclear analytical methods for the determination of hepatacaine hydrochloride in solution was studied, i. e. radionuclide X-ray fluorescence analysis and radiometric titrations based on precipitate formations. Methods used were evaluated on the basis of obtained sensitivity values.     

Stereoselectivity and competing reactions as studied by lipase-catalyzed esterifications in aqueous lecithin-based gelatin gels

 The enzyme catalyzed conversion of R/S-(±)-2-octanol with hexanoic acid to R/S-(±)-2-octyl hexanoate has been studied in different microenvironments and in the presence of the competing substrate ethanol. The reactions were performed in various gels made from aqueous gelatin solutions and liposome dispersions or isotropic liquid solutions, with or without oil and ethanol. The lipase Candida sp. (SP 525) was dissolved in the dispersions or solutions stabilized by the naturally occurring zwitterionic surfactant soybean lecithin. The sectioned porous gel was immersed in hexane containing 0.33 mol dm^-3 of racemic 2-octanol and hexanoic acid. Since ethanol acts both as a substrate and as a part of the gel it is of fundamental interest to know the phase behaviour of the used systems. Partial phase diagrams for the systems ethanol–water–soybean lecithin and ethanol/water (7:3)–oil–soybean lecithin were determined at 298.2 K. The oil was either castor oil or hexadecane. The conversion of R-2-octyl hexanoate was about 0.45 when no or small amounts of ethanol was present, but decreased considerably with high amounts of ethanol present and ethyl hexanoate became the main product. Hydrolysis of R-2-octyl hexanoate was favoured in the latter systems and hexanoic acid formed was immediately esterified to ethyl hexanoate. The conversion of R-2-octyl hexanoate and ethyl hexanoate depends only on the ethanol content present in the systems and is thus independent of the environment of the enzyme. However, the chiral esters synthesized from racemic 2-octanol and hexanoic acid showed high optical purities regardless of the ethanol content. Received: 1 July 1996 Accepted: 30 August 1996     

A kinetic study of methane conversion by a dinitrogen microwave plasma

Conversion of CH₄ with a N₂ microwave plasma (2.45 GHz) is studied. The experiments cover the absorbed microwave power range 300–700 W with 17–62% of methane in the gas mixture, with pressures of 10–40 mbar and flow rates of 140–650 ml· min^–1. The yields of C₂ hydrocarbons and dihydrogen are analyzed by gas chromatography. The distance of methane addition downstream of the plasma plays an important role on the composition and the concentration of the products obtained. This distance mainly determines the energy concentrated in the active species of the plasma when they react with methane. Different behaviors for acetylene formation, on the one hand, and for ethane and ethene formation, on the other hand, have been observed, and this finding allows us to propose a kinetic mechanism for the decay of methane and for the formation of C₂ hydrocarbons.     

Synthesis of 11C-amides using [11C]carbon monoxide and in situ activated amines by palladium-mediated carboxaminations

[11C]Carbon monoxide at low concentrations, aryl halides and amines were used in the palladium-mediated synthesis of twenty 11C-amides. In the study several approaches to improve the radiochemical yield were explored. Eight of the selected amides were prepared by in situ activation of the amines using lithium bis(trimethylsilyl)amide and the radiochemical yields of these reactions were improved compared to utilising a previous reported method. In the synthesis of 1-[carbonyl-11C]benzoyl-3-methyl-1H-indole (11) from 3-methyl-1H-indole (25), the corresponding organotin-amine was prepared prior to the acylation reaction. In a typical experiment, N-(4-hydroxyphenyl)[carbonyl-11C]acetamide (5) was prepared in 15% radiochemical yield using 4-aminophenol (20) but the yield increased to 63% when the amine was activated by lithium bis(trimethylsilyl)amide.     

Differential pulse polarographic determination of salicylaldehyde as its Girard-P derivative

A method for the determination of salicylaldehyde (2 × 10^–6–10^–4 M) by differential-pulse polarography, based on the in situ formation of its Girard-P derivative in aqueous solution at pH 2.5, is proposed. The relative standard deviation was 1.5% (ten determinations of 4 × 10^–5 M level). The applicability of this method was checked in synthetic samples containing salicyl alcohol,o-aminophenol, 2-methylphenol, salicylic acid and 4-aminobenzoic acid.     

Photoinduced electron transfer in platinum(II) terpyridinyl acetylide complexes connected to a porphyrin unit

A series of six new dyads consisting of a zinc or magnesium porphyrin appended to a platinum terpyridine acetylide complex via a para-phenylene bisacetylene spacer are described. Different substituents on the 4' position of the terpyridinyl ligand were explored (OC7H15, PO3Et2, and H). The ground-state electronic properties of the dyads are studied by electronic absorption spectroscopy and electrochemistry, and they indicate some electronic interactions between the porphyrin subunit and the platinum complex. The photophysical properties of these dyads were investigated by steady-state, time-resolved, and femtosecond transient absorption spectroscopy in N,N-dimethylformamide solution. Excitation of the porphyrin unit leads to a very rapid electron transfer (2-20 ps) to the nearby platinum complex followed by an ultrafast charge recombination, thus preventing any observation of the charge separated state. The variation in the rate of the photoinduced electron transfer in the series of dyads is consistent with Marcus theory. The results underscore the potential of the para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance.