X-ray absorption spectroscopy of selenium-containing amino acids

Received: 2 April 1999 / Accepted: 17 September 1999     

Rainfall and parasitic wasp (Hymenoptera: Ichneumonoidea) activity in successional forest stages at Barro Colorado Nature Monument, Panama, and La Selva Biological Station, Costa Rica

1 In 1997, we ran two Malaise insect traps in each of four stands of wet forest in Costa Rica (two old‐growth and two 20‐year‐old stands) and four stands of moist forest in Panama (old‐growth, 20, 40 and 120‐year‐old stands). 2 Wet forest traps caught 2.32 times as many ichneumonoids as moist forest traps. The average catch per old‐growth trap was 1.89 times greater than the average catch per second‐growth trap. 3 Parasitoids of lepidopteran larvae were caught in higher proportions in the wet forest, while pupal parasitoids were relatively more active in the moist forest. 4 We hypothesize that moisture availability is of key importance in determining parasitoid activity, community composition and trophic interactions.     

Factors influencing the potential mobility and bioavailability of exchangeable ammonium ion in dried lake sediments

Abstract

The effect of drying temperature and oxidation on the level of exchangeable ammonium ion found in sediments has been examined using samples collected from along a polluted creek and from shallow lake bays. The sediments were dried at temperatures between 20°C and 100°C (either in air or under a nitrogen atmosphere), and the ammonium ion content was extracted into 0.1 M KCl prior to analysis using an ion selective electrode. Exposure to air during the drying stage usually resulted in lower ammonium values, while increasing the drying temperature altered the amount of displaceable (i.e. available) ammonium ion extracted, generally in an upward direction. The amount detected (5–25 μ g^?1) varied between sites, and surface sediment values differed from the 10–50 cm core material results. The pH of the extracts varied with the drying temperature used, indicating that the heating process promoted some chemical changes in the test samples. The study has demonstrated that in nutrient level surveys, the analytical data produced can depend greatly on the sample preparation procedure selected. It also indicated the type of changes which could occur when dredged sediments are land dumped.     

Modified speciation scheme for inorganic phosphate in lake sediments

Abstract

Speciation schemes for phosphate in soils or sediments tend to yield results which are ‘conditional’ and subject to error. In the proposed modified scheme, secondary phosphate-cation interactions have been minimised by including a cation exchange resin in the extraction system; re-sorption errors have been reduced by using low (1:400) solid to liquid extractant ratios; and slow dissolution rates have been balanced by using longer extraction times (up to 62 h). Variables examined during scheme development included the effect of varying sample weights, extraction time, reagent type, exchanger type, exchanger to sample weight ratios and pH on the amount of phosphate released from a polluted lake sediment. It was found that the amount of inorganic phosphate released by chemical reagents (or by cation exchanger) varied markedly with system pH, being greatest in the pH 2 to 3 region, decreasing to near zero in the pH 4 to 6 zone, and then rising to an intermediate value at pH > 8. Monitoring of the Ca, Mg, Fe and Al released into the various solutions (or transferred into the exchanger phase) indicated that one cannot clearly distinguish between phosphate initially present in association with alkaline earth compounds or iron-rich matrix components. The advantages and limitations of this ‘differential solubility’ scheme have been considered and alternative approaches briefly reviewed.     

SYNTHESIS OF NOVEL BICYCLIC NUCLEOSIDES RELATED TO NATURAL GRISEOLIC ACIDS

Abstract

Synthesis of novel, bicyclic nucleosides related to natural griseolic acids is described. The synthetic approach involves nucleobase construction at the C-2′ position of 1,4:3,6-diandydro-D-mannitol. The carbohydrate precursor used in the synthesis, 1,4:3,6-dianhydro-D-glucitol, can be prepared easily from D-glucitol. Bicyclic analogues of five naturally occurring nucleosides have been prepared. The single crystal X-ray structure of a representative example is presented and discussed.