Enzyme-assisted suicide: molecular basis for the antifungal activity of 5-hydroxy-4-oxonorvaline by potent inhibition of homoserine dehydrogenase

The structure of the antifungal drug 5-hydroxy-4-oxonorvaline (HON) in complex with its target homoserine dehydrogenase (HSD) has been determined by X-ray diffraction to 2.6 A resolution. HON shows potent in vitro and in vivo activity against various fungal pathogens despite its weak (2 mM) affinity for HSD in the steady state. The structure together with structure-activity relationship studies, mass spectrometry experiments, and spectroscopic data reveals that the molecular mechanism of antifungal action conferred by HON involves enzyme-dependent formation of a covalent adduct between C4 of the nicotinamide ring of NAD(+) and C5 of HON. Furthermore, novel interactions are involved in stabilizing the (HON*NAD)-adduct, which are not observed in the enzyme's ternary complex structure. These findings clarify the apparent paradox of the potent antifungal actions of HON given its weak steady-state inhibition characteristics.     

Stereospecific synthesis of 11S, 12S-oxido 5Z, 7E, 9E, 14Z-eicosatetraenoic acid

The first stereospecific synthesis of 11S,12S-oxido 5Z, 7E, 9E, 14Z-eicosatetraenoic acid has been achieved from 2-deoxy-D-ribose using either a Horner-Emmons or Wittig condensation to form the 9,10-trans or the 5,6-cis-double bond respectively.     

Study of pH-sensitive copolymer/phospholipid complexes using the langmuir balance technique: effect of anchoring sequence and copolymer molecular weight

The behavior of three copolymers of N-isopropylacrylamide (NIPAM), methacrylic acid (MAA), and hydrophobic moiety was studied at phospholipid monolayer/subphase interfaces. The hydrophobic moieties, N-terminal dioctadecylamine (DODA) and random octadecylacrylate (ODA), were used as anchoring groups. The interactions between a 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC) monolayer and the copolymers were studied using the Langmuir balance technique. The effect of subphase pH, distribution of anchors along the copolymer chain, and copolymer molecular weight on the nature of the interactions between the copolymer chains and the DSPC monolayer were investigated. A first-order kinetics model was used to analyze the copolymers adsorption at the DSPC monolayer/subphase interface and allowed the interaction area between the copolymer chains and the DSPC monolayer, A(x), to be determined. The interaction area appears to depend on the subphase pH and the copolymer molecular weight. On decreasing pH, the interaction area of high molecular weight copolymers increases significantly; this is consistent with the copolymer chain phase transition from an extended coil to a collapsed globule while pH is lowered. In the latter conformation, strong hydrophobic attractive interactions between the copolymer chains and the hydrophobic part of the DSPC monolayer favor the copolymer intercalation, which could eventually provoke the phospholipidic layer destabilization or rupture.     

Protein-induced bonding perturbation of the rhodopsin chromophore detected by double-quantum solid-state NMR

We have obtained carbon-carbon bond length data for the functional retinylidene chromophore of rhodopsin, with a spatial resolution of 3 pm. The very high resolution was obtained by performing double-quantum solid-state NMR on a set of noncrystalline isotopically labelled bovine rhodopsin samples. We detected localized perturbations of the carbon-carbon bond lengths of the retinylidene chromophore. The observations are consistent with a model in which the positive charge of the protonated Schiff base penetrates into the polyene chain and partially concentrates around the C13 position. This coincides with the proximity of a water molecule located between the glutamate-181 and serine-186 residues of the second extracellular loop, which is folded back into the transmembrane region. These measurements support the hypothesis that the polar residues of the second extracellular loop and the associated water molecule assist the rapid selective photoisomerization of the retinylidene chromophore by stabilizing a partial positive charge in the center of the polyene chain.     

Cycloaddition-elimination reactions of 4-methyl-5-(substituted)-imino-Δ2-1,2,3,4-thiatriazolines with isocyanates

4-Methyl-5-(substituted)imino-1,2,3,4-thiatriazolines 1 (R² ≠ Me) undergo cycloaddition-elimination reactions with isocyanates to yield 4-methyl-5-(substituted)imino-1,2,4-thiadiazolidine-3-ones 5 via the thermodynamically less stable isomers 4 . The latter have not been isolated, except for 4q which was shown to iso-merize rapidly into 5q with phenylsulfonyl isocyanate. The reactions of 1 are accelerated by using less bulky R² substituents and more electrophilic isocyanates, in accordance with the viewpoint that 1 reacts as a masked 1,3-dipole. The products 4i-n (= 5i-n ), derived from 1b , add isocyanates reversibly to give 2,3,4,5-tetrahydro-6aλ⁴-thia-1,3,4,6-tetraazapentalene-2,5-diones 9i-n , which have been isolated and characterized spectroscopically. Such compounds with a hypervalent sulfur atom thus occur as intermediates during the isomerization of 4 to 5 under the influence of isocyanates.     

An approach towards the synthesis of 4-phenyl-5-arylimino-δ2-1,2,3,4-thiatriazolines

4-Phenyl-5-arylimino-δ²-1,2,3,4-thiatriazolines are presumably formed by reacting 5-arylaminothiatriazoles with benzyne at 50°, but decompose in situ to benzothiazole derivatives by way of the two pathways (a) and (b) shown in Scheme 4.     

Synthesis of some 3-EthoxycarbonyI-5-hydroxy-4-oxo-2-pyrroline derivatives from 3(2H)furanones

A number of 3-ethoxycarbonyl-5-hydroxy-2-methyl-5-or-1,5-substituted 4-oxo-2-pyrrolines have been prepared, respectively, by the action of ammonium hydroxide or primary aliphatic amines on 2-arylidene or 2-N-acetyl-N-arylaminomethylene-4-ethoxycarbonyl-5-methyl-3(2H)furanones. The structures of these prepared compounds have been determined by spectroscopic data and chemical means.     

Cyclic imides. XI. Bisimide charge-transfer polymers

Condensation of two equivalents of an aromatic cyclic anhydride with an aromatic diamine, or of two equivalents of an aromatic amine with an aromatic bisanhydride, results in the formation of an aromatic bisimide. Thirty-one such bisimides were prepared. The bisimides in which the two imide groups arc connected by a fully coplanar system show extremely low solubilities and high melting and decomposition temperatures. Many are also strongly colored. It is suggested that these properties may result from intermolecular charge-transfer effects in the solid slate, giving charge-transfer polymers.     

Atmospheric pressure chemical ionization of alkanes,alkenes, and cycloalkanes

Normal and cyclic alkanes and alkenes form stable gas-phase ions in air at atmospheric pressure from 40 to 200°C when moisture is below 1 ppm. Ionization of alkanes in a ⁶³Ni source favored charge transfer over proton transfer through pathways involving [M?1]⁺ and [M?3]⁺ ions. Ion mobility spectra for alkanes showed sharp and symmetrical profiles while spectra for alkenes suggested fragmentation. Ion identifications were made by using mass spectrometry, and ionization pathways were supported by using deuterated analogs of alkanes and alkenes. Alkanes were ionized seemingly through a hydrogen abstraction pathway and did not proceed through an alkene intermediate. New methods for interpretation of mobility spectra utilizing ion mobility spectrometry, atmospheric pressure chemical ionization mass spectrometry, chemical ionization mass spectrometry, and ion mobility spectrometry-mass spectrometry data were demonstrated.     

Reactions of trichloroacetyl isothiocyanate with organic azides

Benzyl azide reacts with trichloroacetyl isothiocyanate to give 7a in chloroform solution, and 11 in acetone solution. These 1,2,4-oxathiazolidines were characterized by ¹³C nmr spectroscopy (Scheme 2), but could not be isolated since they deteriorated via the carbodiimide 8a into the 1,2,4-thiadiazolidine 9a . The oxathiazoline 6a is assumed as an intermediate and was trapped by isocyanates and dicyclohexylcarbodiimide to give the 1,2,4-thiadiazolidines 10a,b and 12 respectively. Isopropyl azide also reacts with trichloroacetyl isothiocyanate to give the labile oxathiazolidine 7b , which decomposes to the carbodiimide 8b and the 1,2,4-thiadiazolidine 9b . In the case of diphenylmethyl azide, however, no evidence was obtained for the presence of the oxathiazolidine 7c in the ¹H nmr spectra; only the carbodiimide 8c was observed. A mechanistic rationalization is presented in Scheme 1.