Conformational stabilization at the active site of molluskan (Rapana thomasiana) hemocyanin by a cysteine-histidine thioether bridge A study by mass spectrometry and molecular modeling.

In some type-3 copper proteins (molluskan hemocyanin, catechol oxidase and fungal tyrosinase) one of the histidine residues, liganding the Cu(A) atom of the dinuclear copper active site, is covalently linked to a cysteine residue by a thioether bridge. The purpose of this study was to disclose the function of this bridge. Mass spectral analysis of a peptide, isolated from Rapana thomasiana (gastropodan mollusk) hemocyanin, indicated a stabilization of the peptide structure in the region of the bridge. Molecular modeling of three thioether containing type-3 copper proteins using the dead-end elimination method showed that the concerned histidine would be very flexible if not linked to the cysteine. Also, the side chain orientation of the histidine is rather exceptional, as evidenced by statistical data from the protein databank. It is suggested that the role of the bridge is to fix the histidine in an orientation that is optimal for coordination of the Cu(A) atom.

trans-Chlorido[6-chloro-4-(4-methoxy-benz-yl)-3-oxo-3,4-dihydro-pyrazin-2-yl]-bis-(triphenyl-phosphine)palladium(II).

The title compound, [Pd(C(12)H(10)ClN(2)O(2))Cl(C(18)H(15)P)(2)], is the inter-mediate of the reduction of a 3,5-dichloro-pyrazinone [Loosen, Tutonda, Khorasani, Compernolle & Hoornaert (1991 ▶). Tetra-hedron, 47, 9259-9268]. This species is formed by oxidative addition of coordinatively unsaturated Pd(0) to the reactive 3-position of the heterocycle. The coordination around the Pd atom is square planar, with two trans PPh(3) ligands. π-π inter-actions are observed between the centroid of the pyrazinone ring and planes of two adjacent phenyl rings, one from each PPh(3) group (3.25 and 3.078 Å), stabilizing the inter-mediate structure. This could explain the reduced reactivity towards substitution of the Cl atom by the formate anion, resulting in poor yield of the reduced compound. 3-Substituted pyrazinones are important precursors in the synthesis of 5-amino-piperidinone-2-carboxyl-ate (APC) systems.

Identification of steviol glucuronide in human urine.

Stevioside (250 mg capsules) was given three times daily to 10 healthy subjects. Steviol glucuronide (steviol 19-O-beta-D-glucopyranosiduronic acid; MM, 494.58; melting point, 198-199 degrees C) was characterized in the 24 h urine as the only excretion product of oral stevioside by MS, NMR, IR, and UV spectroscopy. This is the first report on the unambiguous identification of steviol glucuronide in human urine.

Design, synthesis, and SAR of a novel pyrazinone series with non-nucleoside HIV-1 reverse transcriptase inhibitory activity.

A series of novel pyrazinones designed as non-nucleoside reverse transcriptase inhibitors (NNRTIs) was synthesized and their anti-HIV structure-activity relationship (SAR) was studied.

Glycosylation sites of hemocyanins of Helix pomatia and Sepia officinalis.

Glycopeptides were isolated from functional units of two molluscan hemocyanins (Hcs). They were analyzed and localized in the sequences. A comparison with potential N-glycosylation sites of two other molluscan Hcs was made. An immunological cross-reactivity was observed between the beta-Hc and the alpha-macroglobulin of Helix pomotia. ELISA experiments with glycopeptide fractions indicated a competition.

Steviol quantification at the picomole level by high-performance liquid chromatography.

A simple and highly sensitive reversed-phase high-performance liquid chromatographic method (RP-HPLC) has been developed for the determination of steviol (SV) using dihydroisosteviol (DHISV) as an internal standard (IS). SV and DHISV were derivatized by reaction of the acids with 4-(bromomethyl)-7-methoxycoumarin in an aprotic solvent (DMF or acetone). The resulting ester derivatives were separated on an ODS column (250 x 4.6 mm i.d., 5 microm particle size) using fluorescence detection with excitation at 321 nm and emission at 391 nm. The mobile phase consisted of acetonitrile/water (80:20 v/v) with a flow rate of 1 mL min(-)(1). A linear relationship was observed for concentrations between 0.5 and 50 microg/mL of SV, and the detection limit was 100 pg. For application of this method to samples of beer fortified with stevioside, a simple procedure for extraction of the beer with diethyl ether and derivatization in DMF was applied. Whereas beer samples spiked with SV gave a linear response over the range 0.1-15 microg/mL beer, no SV could be detected in beer samples enriched in stevioside that had been stored for over 3 years. The application of the method to plant samples involved preparation of an acid fraction containing the SV analyte, derivatization, and sample cleanup using small silica columns and thin-layer chromatography. A sensitive determination of 594 ng of steviol present in 100 mg of dry plant material was performed with high precision and accuracy.

Metabolism of stevioside by chickens.

In intubation experiments (643-1168 mg per animal), most of the stevioside administered to chickens was recovered unchanged in the excreta, and only about 2% was converted into steviol. Neither stevioside nor steviol could be found in the blood. In chronic studies (667 mg of stevioside/kg of feed) with laying hens and meat-type chickens, no significant differences were found in feed uptake, weight gain, and feed conversion as the result of stevioside administration. The egg production and egg composition of laying hens were not influenced. Most of the stevioside taken up was found untransformed in the excreta, and about 21.5% or 7.3% was converted to steviol by meat-type chickens or laying hens, respectively. No stevioside or steviol could be detected in the blood or in the eggs of the different groups of animals. In anaerobic incubation experiments with chicken excreta, only a 20% conversion of stevioside into steviol was found. No harmful effects were observed in the chronic stevioside supplementation experiments nor in the intubation experiments in which very high stevioside doses were given.

Diterpene glycosides from Stevia phlebophylla A. Gray.

The rare Mexican species Stevia phlebophylla A. Gray was long considered to be the only known Stevia species, beside the well-known S. rebaudiana, containing the highly sweet diterpenoid steviol glycosides. We report a re-evaluation of this claim after phytochemically screening leaves obtained from two herbarium specimens of S. phlebophylla for the presence of steviol glycosides. Despite extensive MS analyses, no steviol glycosides could be unambiguously verified. Instead, the main chromatographic peak eluting at retention times similar to those of steviol glycosides was identified as a new compound, namely 16ß-hydroxy-17-acetoxy-ent-kauran-19-oic acid-(6-O-ß-D-xylopyranosyl-ß-D-glucopyranosyl) ester (1) on the basis of extensive NMR and MS data as well as the characterization of its acid hydrolysate. Seven more compounds were detected by ESIMS which are possibly structurally related to 1. It can therefore be concluded that S. phlebophylla is unlikely to contain significant amounts of steviol glycosides, if any.

New Flavonoids from Boldoa purpurascens Cav.

Phytochemical analysis of the leaves of BOLDOA PURPURASCENS Cav. led to isolation of four flavone glycosides, three of which are new compounds. Their structures have been determined by mass spectrometry and by 1 D and 2 D NMR analysis, i. e., 4',5-dihydroxy-6,7-methylenedioxyflavonol 3- O-alpha- L-rhamnopyranosyl-(1-->2)-beta- D-xylopyranoside ( 1), 4',5-dihydroxy-6,7-methylenedioxyflavonol 3- O-beta- D-xylopyranoside ( 2), and 4',5-dihydroxy-6,7-methylenedioxyflavonol 3- O-alpha- L-rhamnopyranosyl-(1-->2)-beta- D-glucopyranoside ( 3). The known compound was 4',5-dihydroxy-6,7-methylenedioxyflavonol 3- O-beta- D-glucopyranoside ( 4). The aglycone 4',5-dihydroxy-6,7-methylenedioxyflavonol is known as gomphrenol. Compounds 1 and 2 failed to show antifungal activity when tested against three different strains of fungi, i. e., FUSARIUM CULMORUM, BOTRYTIS CINEREA, and ASPERGILLUS FLAVUS.

Synthesis of 2(1H)-pyrazinone phosphonates via an Arbuzov-type reaction.

A simple and catalyst-free method for the synthesis of phosphonated 2(1H)-pyrazinones is described starting from 3,5-dichloropyrazinones. The method also works for 3-bromo- and 3-iodopyrazinones. Classical heating conditions as well as microwave-enhanced reaction conditions were tested.

Involvement of glycan chains in the antigenicity of Rapana thomasiana hemocyanin.

Functional unit (FU) RtH2-e from Rapana thomasiana hemocyanin (Hc) was degraded into small fragments with chymotrypsin. The glycopeptides were separated from the non-glycosylated peptides by chromatography on Concanavalin-A-Sepharose and characterized by mass spectrometry. The glycan part of the glycopeptides (all with common peptide stretch of 14 amino acids) consists of the classical trimannosyl-N,N-diacetylchitobiose core for N-glycosylation, predominantly extended with a unique tetrasaccharide that is branched on fucose. In inhibition ELISA experiments, the glycopeptides interfered in the complex formation between FU RtH2-e and rabbit antibodies against Rapana Hc (about 30% of inhibition). The inhibition also was retained after treatment of the glycopeptides with pronase in order to completely destroy the peptide part. The inhibitory effect of the non-glycosylated peptides, on the other hand, was very low. This study thus demonstrates that the glycans attached to FU RtH2-e contribute to the antigenicity of Rapana Hc.

Simultaneous liquid chromatography determination of polyamines and arylalkyl monoamines.

The diamines putrescine (PUT) and diaminopropane (DAP), the polyamines spermidine (SPD) and spermine (SPM), and the arylalkyl amines phenethylamine (PEA), tyramine (TYR), dopamine (DA), and salsolinol (SAL) were dansylated and baseline separated by LC using a Waters ODS-2 column. The dansyl derivatives were detected by fluorescence (lambda(ex): 337 nm; lambda(em): 520 nm). Besides the amine function, the phenolic OH groups of TYR, DA, and SAL were also dansylated (LC-MS, formation of N,O-didansyl [TYR] and N,O,O'-tridansyl derivatives [DA and SAL]). Calibration curves revealed response factors being appreciably lower for (N,O-didansyl) aminophenol TYR and (N,O,O'-tridansyl) DA and SAL than for N-dansylamines. However, the method is suitable as a cheap alternative to LC-MS for the simultaneous determination of polyamines and arylalkyl amines of large quantities of samples.

Mass spectral evidence for N-glycans with branching on fucose in a molluscan hemocyanin.

Glycopeptides, isolated from a trypsinolysate of functional unit (FU) RtH2-e of Rapana thomasiana hemocyanin subunit 2, were analysed by electrospray ionization mass spectrometry and MS/MS. From the molecular mass observed after deglycosylation, it was inferred that all glycopeptides shared the same peptide stretch 92-143 of FU RtH2-e with a glycosylation site at Asn-127. Besides the core structure Man(3)GlcNAc(2) for N-glycosylation, structures with a supplementary GlcNAc linked to either the Man(alpha1-3) or the Man(alpha1-6) arm and/or an additional tetrasaccharide unit connected to the other Man arm were observed, indicating the existence of microheterogeneity at the glycan level. The tetrasaccharide unit contains a central fucose moiety substituted with 3-O-methylgalactose and N-acetylgalactosamine, and linked to GlcNAc at the reducing end. This structure represents a novel N-glycan motif and is likely to be immunogenic. A second potential site for N-glycosylation in FU RtH2-e at Asn-17 was shown to be not glycosylated.

Study of the carbonyl products of terpene/OH radical reactions: detection of the 2,4-DNPH derivatives by HPLC-MS.

The oxidation of the terpenes alpha- and beta-pinene, limonene and Delta(3)-carene by hydroxyl radicals has been investigated in a fast-flow reactor coupled to a liquid nitrogen trap for collecting the carbonyl compounds. Identification of the products was performed via 2,4-dinitrophenylhydrazone (DNPH) derivatization of the carbonyls to form the mono- and di-DNPH derivatives, which were analysed by high-performance liquid chromatographic (HPLC)-DAD (diode array detector) and HPLC-mass spectrometry (HPLC-MS). Both electrospray ionization [ESI(-)] and atmospheric pressure chemical ionization [APCI(-)] were suitable for the detection of the DNPH derivatives of formaldehyde, acetaldehyde, myrtanal, campholene aldehyde, perillaldehyde, acetone, nopinone, trans-4-hydroxynopinone and 4-acetyl-1-methylcyclohexene. Also the mono-DNPH derivatives of the dicarbonyl compounds pinonaldehyde, endolim and caronaldehyde could be identified. The MS(2) spectra generated in the ion trap of the mass spectrometer allowed us to distinguish between aldehydes and ketones on the basis of the characteristic fragment ion m/ z 163 for the aldehydes. For the quantitative analysis of the mono-DNPH derivatives, ESI(-) in combination with single ion monitoring (SIM) detection showed the lowest detection limits. For the quantification of the dicarbonyl compounds, the acid-sensitive di-DNPH derivatives had to be formed by keeping the acidity in the acid-catalysed derivatization reaction at about 1.7 mM H(2)SO(4). Detection of these dicarbonyl compounds can only be performed by APCI(-) with somewhat lesser sensitivity than by HPLC-DAD.