Guadiamar Toxic Flood: Factors that Govern Heavy Metal Distribution in Soils

The remaining soil contamination after the removal of thesludge and the affected topsoil was studied in the northernpart of the Guadiamar river valley affected by the Aznalcóllartoxic spill. The easily soluble (NH₄NO₃-extraction)and the oxidisable fraction (H₂O₂/HNO₃-extraction)were analysed in samples of two calcareous and two non-calcareoussoils. Correlations between soil properties and heavy metal concentrations were tested with special respect to depth distribution. The spatial distributions of the easily soluble and the oxidisable fraction were highly skewed in both examineddepths (0–20 and 20–40 cm). Easily soluble heavymetal concentrations of a high percentage of samples exceededthresholds that have been given in the German soil protectionlaw particularly in the non-calcareous soils. Within the soillayers of the non-calcareous soils, the pH seems to control theeasily soluble concentrations. However no relation between thepH and depth distribution of heavy metals within the profilescould be found. Physical properties, which determine partlypenetration depth of the sludge and soil mixing caused by theremoval may be the more important factors. Accordingly, highheavy metal concentrations are to be expected even in thesubsoils of clayey sites. As heavy metal concentrations in theoxidisable fraction are still high, further oxidationaccompanied by pH lowering has to be expected. Thus, thestudied soils show a significant risk potential of availableheavy metals even after the removal of the sludge.     

De novo computational design of retro-aldol enzymes

The creation of enzymes capable of catalyzing any desired chemical reaction is a grand challenge for computational protein design. Using new algorithms that rely on hashing techniques to construct active sites for multistep reactions, we designed retro-aldolases that use four different catalytic motifs to catalyze the breaking of a carbon-carbon bond in a nonnatural substrate. Of the 72 designs that were experimentally characterized, 32, spanning a range of protein folds, had detectable retro-aldolase activity. Designs that used an explicit water molecule to mediate proton shuffling were significantly more successful, with rate accelerations of up to four orders of magnitude and multiple turnovers, than those involving charged side-chain networks. The atomic accuracy of the design process was confirmed by the x-ray crystal structure of active designs embedded in two protein scaffolds, both of which were nearly superimposable on the design model.