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Measurements are presented of the magnesium isotopic composition of chlorophyll-a, extracted from cyanobacteria, relative to the isotopic composition of the culture medium in which the cyanobacteria were grown. Yields of 50-93% chlorophyll-a were achieved from the pigment extracts of Synechococcus elongatus, a unicellular cyanobacteria. This material was then digested using concentrated nitric acid to extract magnesium. Separation was accomplished using columns of cation-exchange resin, which achieved a 103 +/- 10% yield of magnesium from chlorophyll-a. This procedure ensured accurate measurement of the magnesium-isotopic ratios without isobaric interferences using a multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). We find a slight depletion in the heavier isotopes of magnesium in chlorophyll-a relative to culture medium, early growth phase: Delta Mg-26 = -0.71(+/- 0.35)parts per thousand and Delta Mg-25 = -0.37(+/- 0.18)parts per thousand; late growth phase: Delta Mg-26 = -0.53(+/- 0.20)parts per thousand and Delta Mg-25 = -0.26(+/- 0.11)parts per thousand, due to an apparent mass-dependent fractionation. We suggest that the small fractionation results from chelation during intracellular processes. A likely candidate for this chelation step involves the magnesium-chelatase enzyme, which mediates the insertion of magnesium to the tetrapyrrole ring during chlorophyll-a biosynthesis. Proof of this hypothesis can be tested with biological controls whereby steps in the enzymatic pathways of chlorophyll synthesis are selectively suppressed. (c) 2006 Elsevier Inc. All rights reserved.

One-dimensional 27Al, 23Na Magic-Angle-Spinning (MAS) NMR and 27Al Multiple-Quantum Magic-Angle-Spinning NMR (MQMAS) measurements are reported for the δ-isomer of the Al13 Keggin structure at high spinning speed and 14.1 T field. Values for the CQ and η parameters are on the same scale as those seen in other isomers of the Al13 structure. Density functional theory (DFT) calculations are performed for comparison to the experimental fits using the B3PW91/6-31+G* and PBE0/6-31+G* levels of theory, with the Polarizable Continuum Model (PCM). [ABSTRACT FROM AUTHOR]

To understand the corrosion of bulk oxide materials at the molecular scale, oxygen-isotope exchanges were examined in two classes of nanometer-sized ions, one cation and one anion, and subsets that differ by targeted single-atom substitutions. In niobate anions, the different oxygen sites within each molecule differ by ~ 103-104 in overall rates of isotopic exchange, yet all structural oxygens exhibit similar pH dependencies that relate to the dissociation pathways. In aluminate cations of the ε-Keggin structure, single-atom substitutions cause a 107-1010 variation of rates of oxygen-isotopic exchange into two sets of μ2-OH. Molecular-dynamic simulations indicate that metastable forms of these structures exist as loose, long-lived intermediates. A series of common steps is observed to access the intermediate structures, and these are best resolved in the symmetric ε-Keggin aluminate ions. In the first step, solvation forces, or a nucleophile, cause a near-surface metal to partly detach from a deeper overbonded oxygen via concerted motions of many atoms. Isotopically distinct oxygens then add to the newly undercoordinated metal in the partly detached metastable state. Protons transfer to more basic oxygens and oxygens shuffle. Finally, the metastable structure collapses and dehydrates. The number of such metastable states depends on the symmetry and composition of the starting structure and access to the metastable state controls the overall rates. Surprisingly, polyoxometalate ions with only 40-100 atoms already seem to capture much of the macroscopic chemistry observed for dissolving minerals and glasses. [ABSTRACT FROM AUTHOR]

The chemical speciation of silicon in xylem exudate from wheat ( Triticum aestivum L.) was examined by 29Si NMR spectroscopy. Wheat plants were grown to maturity in silicon-free nutrient medium, and then transferred to a solution containing 0.02 m m29Si-enriched silicic acid. After 30 min the shoots were excised and xylem exudate was collected. Within 10 min the Si concentration of the xylem exudate reached values greatly in excess of that of the starting nutrient solution, eventually reaching levels as high as 8 m m. Silicon-29 nuclear magnetic resonance spectra indicated the existence of only two Si-containing species in the xylem exudate, mono and disilicic acid (H4SiO4o and (HO)3Si(µ-O)Si(OH)3o) in a ratio of approximately 7 : 1. Significantly, there was no evidence of organosilicate complexes. Nevertheless, the efficiency by which the plant concentrates aqueous silicon indicates active mechanisms of silicon transport across root cell membranes. [ABSTRACT FROM AUTHOR]

„In DEr Zukunft sehe ich mich, die Hochschulverwaltung vor Frust zum Weinen gebracht zu haben. Die größte HerausforDErung für Wissenschaftler ist unser erstickenDEs und reflexives elitäres Denken...“ Dies und mehr von und über William   H. Casey finDEn Sie auf Seite 15542. [ABSTRACT FROM AUTHOR]