Minding Their Ps and Ns

N. S. Wigginton
2011 Science  
Synthetic asymmetric catalysts have traditionally comprised transition metals bearing chiral ligands, though over the past decade, free amines, phosphate diesters, and urea derivatives have emerged as increasingly effective alternatives. The phosphate class of organocatalysts tends to rely on ion pair formation between a protonated substrate and a residual phosphate anion bearing a chiral, often polycyclic aromatic carbon framework. Zheng et al. were exploring the application of this type of
more » ... alyst toward asymmetric chlorination of phenyloxindoles, and they found that a particular phosphate diester afforded high enantioselectivity just after purifi cation by silica gel chromatography, yet minimal selectivity after an acid wash. This result led them to screen phosphate salts with a variety of cations, culminating in the discovery that calcium-a biochemically critical ion that has found comparatively few applications in synthetic catalysis-proved the optimal counterion for conferring selectivity. Though the mechanism remains unclear, the authors posit possible activation of the chlorine source (N-chlorosuccinimide) by calcium coordination, concomitant with oxindole activation by the phosphate. Phenyloxindoles with a range of substitution patterns were chlorinated in nearly quantitative yields within half an hour at room temperature, with enantioselectivities generally exceeding 90%. -JSY J. Am. Chem. Soc. 133, 10.1021/ ja109824x (2011). CLIMATE SCIENCE Reservoir Doubts As the last ice age came to an end, the concentration of atmospheric CO 2 increased rapidly, and its 14 C content declined precipitously between 17.5 and 14.5 thousand years ago. The favored explanation for this drop in the proportion of 14 CO 2 is that a large amount of old carbon-that is, carbon whose inventory of radioactive 14 C had been depleted with agewas outgassed from a deep ocean reservoir where it had been isolated for many thousands of years. That scenario makes good intuitive sense, but conclusive evidence for a reservoir of aged carbon has not yet been found. Hain et al. use an 18-box ocean model to simulate the history of deglacial CO 2 and evaluate the likelihood that an old deep-ocean CO 2 reservoir was in fact the source of the observed 14 C anomaly. They conclude that such a transfer of carbon was unlikely, based on inconsistencies ratio indeed remains relatively constant across many environments and time scales, including deep oceans and coastal waters, but questions remain about whether innate biochemical or environmental factors are responsible. Loladze and Elser compiled literature values of nutrient ratios in prokaryotic and eukaryotic microorganisms, which, combined with a theoretical model, suggest that the N:P ratio is determined by a balance of maximum macromolecule biosynthesis rates-specifi cally for nitrogen-rich proteins and phosphorus-rich ribosomal RNA. Although the analysis considered cases in which growth rates were optimal, an N:P ratio of ~16 isn't necessarily always desirable for effi cient with other records such as those of the 14 C history of other depths and regions of the ocean, ocean carbonate and oxygen changes over the interval, and constraints on mixing imposed by ocean circulation. -HJS Geophys. Res. Lett. 38, L04604 (2011). BIOCHEMISTRY Minding Their Ps and Ns In the mid-20th century, Alfred Redfi eld posited that the bulk ratio of nitrogen to phosphorus atoms (N:P) in marine microorganisms should maintain a relatively constant value of ~16. Work since then has shown that the Although a variety of factors contribute to global wildfi re activity, climate is thought to be a major determinant. Ohlson et al. now fi nd that tree species composition has had at least as strong an infl uence as climate on wildfi re activity over time in late Holocene boreal forests. By analyzing humus and peat records in northern Europe, the authors show that the invasion of Norway spruce, Picea abies, southwesterly across the region starting 4500 years ago was accompanied by a reduction in the frequency and severity of fi re. The presence of Norway spruce would tend to make the forest more dense and shaded in the drier summer months, which would increase the local humidity of the understory and inhibit the spread of fi re. Although the spruce invasion corresponded with a period of cooler and wetter climate, the tight spatial correlation of the arrival of spruce and the reduction in fi re in specifi c locations argues for a key role of species composition in governing the fi re regime. In turn, the reduction in fi re would have led to other ecological consequences, notably increased sequestration of carbon by the vegetation and the establishment of other species requiring longer-term continuity. Thus, the results show how changes to the dominant forest species can exert a cascading effect on the regional ecology for centuries or millennia to come. -AMS J. Ecol. 99, 395 (2011).
doi:10.1126/science.331.6021.1114-c fatcat:7ofydlazhffnnc7fpzn6zk3qfq