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T A van Beek

Publications and source records attributed to T A van Beek.

16 recordsLinked to original sources
PubMed2003

Analysis of pharmaceutically relevant taxoids in wild yew trees from Sardinia.

Needles from a series of wild yews (Taxus baccata L.) from Sardinia were investigated for their contents of 10-deacetyl baccatin III (DAB-III, 1), paclitaxel (Taxol) (2) and taxine (3). Despite a common geographical origin, ample variation of the taxoid profile was discovered, and several samples were surprisingly devoid of all terpenoid markers above. This finding is unprecedented within the European yew, while the general lack of taxine might rationalize the observation that most plants investigated are actively and impunently browsed by goats.

Alkaloids↗
PubMed2001

Preparative isolation and dual column high-performance liquid chromatography of ginkgolic acids from Ginkgo biloba.

A chromatographic procedure for the preparative isolation of six different 6-alkylsalicylic acids (syn. ginkgolic acids) with as alkyl substituents C13:0, C15:0, C15:1, C17:1, C17:2 and, tentatively C17:3 from Ginkgo biloba leaves was developed. The procedure consisted of a combination of normal-phase, reversed-phase and argentation chromatography. The compounds were characterised by means of UV, 1H-NMR and 13C-NMR spectroscopy, and mass spectrometry after silylation. A 15 cm C18 RP-HPLC column connected in series with a 20 cm silver(I) loaded cation exchanger HPLC column in combination with the solvent methanol-water (93:7) acidified with 0.1% formic acid was capable of separating the ginkgolic acids C13:0, C15:1, C17:2, C15:0 and C17:1 within 21 min on an analytical scale. The separation is based on a combination of reversed-phase mechanisms and double bond complexation. Detection took place by UV at 311 nm. The separation is a good starting point for the development of a quantitative procedure for the five major ginkgolic acids in Ginkgo leaves and standardised extracts.

Chromatography, High Pressure Liquid↗
PubMed2001

Application of ABTS radical cation for selective on-line detection of radical scavengers in HPLC eluates.

The radical cation 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate), (ABTS*+) was utilized in an on-line HPLC method for the detection of radical scavengers in complex matrixes. The HPLC-separated analytes react postcolumn with the preformed ABTS*+, and the induced bleaching is detected as a negative peak by an absorbance detector at 734 nm. An optimized instrumental and experimental setup is presented. The method is suitable for both isocratic and gradient HPLC runs using mobile phases containing 100% organic solvent or its solution in water, weak acids, or buffers (pH 3-7.4). The method is sensitive, selective, relatively simple, applicable to compounds of different chemical natures; uses common instruments and inexpensive reagents; and has a time-saving, nonlaborious experimental protocol. It can also be used for quantitative analysis. The method was applied to several pure natural antioxidants and plant extracts. The minimum detectable concentration varied from 0.02 to 0.13 microg/mL, depending on the compound tested. The method can be applied to perform kinetic studies, which is illustrated by determination of Trolox equivalent antioxidant capacities (TEAC) of several known antioxidants in flow injection mode.

Benzothiazoles↗
PubMed2001

Evaluation and comparison of two improved techniques for the on-line detection of antioxidants in liquid chromatography eluates.

Two methods for the on-line detection in HPLC eluates of analytes possessing radical scavenging activity were improved and compared. The instrumental set-up of the method that is based on on-line inhibition of luminol chemiluminescence (CL) by antioxidants was improved using better quality syringe pumps, employing a diode array detector, and introducing a mixing/neutralisation coil and a pulse damper. Sensitivity of the HPLC-CL detection increased by a factor of 4. Post-column neutralisation of eluates improved compatibility of this detection method with acidified HPLC eluents. The second method, which is based on the post-column quenching of 2,2'-diphenyl-1-picrylhydrazyl radical (DPPH*), was improved by readjusting composition and flow-rate of the reagent, mounting an additional pulse damper and detecting unreacted DPPH* with a detector equipped with a tungsten lamp. Purging of the DPPH* solution with He gas prior to analysis was introduced. This led to 30-fold better detection limits. The improved methods were compared with respect to limits of detection, the radical scavenging mechanism involved, compatibility with common HPLC solvents and pH range, and some technical aspects. The techniques described have high potential for the rapid identification of radical scavengers in complex samples like plant extracts.

Antioxidants↗
PubMed2000

Elimination and detoxification of softwood extractives by white-rot fungi.

The ability of several white-rot fungal strains to remove and detoxify acetone extractives (pitch or resin) in Scots pine sapwood was investigated in stationary laboratory batch assays. Fungal pretreatment provided up to 62% total pitch reduction and significant decreases in pitch toxicity. The best strains were Bjerkandera sp. strain Stereum hirsutum and Trametes versicolor that eliminated over 93% of the problematic triglyceride fraction and 58-87% of other lipophilic extractive classes in only 2 weeks. Fungal removal of the wood extractives was accompanied by a 7.4-16.9-fold decrease in their inhibitory effect, as determined in the Microtox bioassay. Wood pretreatment by Bjerkandera sp. and T. versicolor caused limited losses of woody mass (less than 4% in 4 weeks); whereas S. hirsutum led to somewhat higher mass losses (7% in 4 weeks). These results indicate the potential of white rot fungi to control pitch deposition problems in pulping and to reduce the aquatic toxicity caused by naturally-occurring lipophilic extractives in forest industry effluents.

Acetone↗
PubMed2000

Screening of the needles of different yew species and cultivars for paclitaxel and related taxoids.

The needles of several yew species and cultivars were analysed by high-pressure liquid chromatography for paclitaxel, 10-deacetylpaclitaxel, cephalomannine, baccatin III, 10-deacetylbaccatin III and brevifoliol. About 750 samples were collected from five different locations in the Netherlands and the UK. The results of this screening show a large variation in taxane content between the different species and cultivars. The content of paclitaxel and 10-deacetylbaccatin III varied from 0 to 500 micrograms/g and 0 to 4800 micrograms/g dried needles, respectively. Brevifoliol was found in a very high concentration in Taxus brevifolia. 10-Deacetylpaclitaxel, cephalomannine and baccatin III were found in concentrations ranging from 0 to 500 micrograms/g dried needles.

Antineoplastic Agents, Phytogenic↗
PubMed2000

Rearrangement of O-cinnamoyltaxicin I to a novel C-13 spiro-taxane.

During the large-scale synthesis of an O-cinnamoyltaxicin I acetonide, an intermediate for the semisynthesis of 7-deoxypaclitaxel derivatives, side-product 3 was formed via a vinylogous retro-aldol reaction and a long-range hydride shift from O-cinnamoyltaxicin I (1) under alkaline reaction conditions. Compound 3 has two hemi-acetal bridges at C-1,C-9 and C-10,C-13. Compound 4 was formed from side-product 3 under acidic reaction conditions and is the first C-13 spiro-taxane described in the literature. This spiro-taxane has two acetal bridges between C-1, C-13 and C-10,C-13.

Alkaloids↗
PubMed1994

Structure-activity relationship of isolated avenanthramide alkaloids and synthesized related compounds as oviposition deterrents for Pieris brassicae.

The structure-activity relationship was investigated of compounds isolated from the eggs of Pieris brassicae, the large white cabbage butterfly, and eight synthesized related compounds as oviposition deterrents for this insect. The activity of all compounds was tested in a dual-choice bioassay. The two most active oviposition deterrents for P. brassicae were trans-2- [3-(4-hydroxyphenylpropenoyl)amino]-3,5-dihydroxybenzoic acid [8] and trans-2-[3-(3,4-dihydroxyphenylpropenoyl)amino]-3,5-dihydroxybenzo ic acid [2]. Among members of this compound class, alteration of the substituents of the cinnamic acid part of the molecule affected the oviposition deterrent activity more profoundly than other structural changes. Modification of the anthranilic acid part of the molecule resulted in lower activity.

Alkaloids↗
PubMed1992

Concentration of Ginkgolides and Bilobalide in Ginkgo biloba Leaves in Relation to the Time of Year.

The concentrations of ginkgolides A, B, and C, and bilobalide were determined in the leaves of 3 different Dutch GINKGO BILOBA trees from late spring until late autumn 1990. The concentration versus harvest time plots were roughly the same for all the compounds in each of the 3 trees. Concentration was lowest in spring and then gradually increased until a maximum in late summer or early autumn was reached. Thereafter the concentration declined until leaf fall. The difference in ginkgolide and bilobalide content between the 3 trees was very high. For ginkgolide B, the pharmacologically most potent compound, the maximum concentration was less than 3, 68 and 204 ppm respectively. Possible reasons for these differences and consequences for phytotherapy are discussed briefly.

Journal Article↗
PubMed1983

Screening of antimicrobial activity of some plants belonging to the Apocynaceae and Loganiaceae.

A review is given of the literature on the antimicrobial activity of various indole alkaloids. The data are briefly discussed. A screening was performed of a series of plants belonging to the indole alkaloid rich plant families Apocynaceae and Loganiaceae, 25 materials representing 14 Apocynaceae species and 54 materials representing 41 Strychnos species were investigated. No activity against the yeast or the fungus, used as test organisms was found. A number of extracts showed activity against the gram-positive test organisms, 4 species showed activity against gram-negative bacteria.

Alkaloids↗
PubMedDate not supplied

Phytochemical investigation of Tabernaemontana crassa.

From the stembark of Tabernaemontana crassa the alkaloid ibogaine was isolated as the major component. Ibogaine showed activity against the gram-positive Bacillus subtilis. Conopharyngine was identified as one of the minor compounds.

Alkaloids↗