Arsenic calamity in the Indian subcontinent What lessons have been learned?

Groundwater arsenic (As) contamination in West Bengal (WB, India) was first reported in December 1983, when 63 people from three villages of two districts were identified by health officials as suffering from As toxicity. As of October 2001, the authors from the School of Environmental Studies (SOES) have analyzed >105 000 water samples, >25 000 urine/hair/nail/skin-scale samples, screened approximately 86 000 people in WB. The results show that more than 6 million people in 2700 villages from nine affected districts (total population approximately 42 million) of 18 total districts are drinking water containing >/=50 mug l(-1) As and >300 000 people may have visible arsenical skin lesions. The As content of the physiological samples indicates that many more may be sub-clinically affected. Children in As-affected villages may be in special danger. In 1995, we had found three villages in two districts of Bangladesh where groundwater contained >/=50 mug l(-1) As. The present situation is that in 2000 villages in 50 out of total 64 districts of Bangladesh, groundwater contains As above 50 mug l(-1) and more than 25 million people are drinking water above >/=50 mug l(-1) As. After years of research in WB and Bangladesh, additional affected villages are being identified on virtually every new survey. The present research may still reflect only the tip of iceberg in identifying the extent of As contamination. Although the WB As problem became public almost 20 years ago, there are still few concrete plans, much less achievements, to solve the problem. Villagers are probably in worse condition than 20 years ago. Even now, many who are drinking As-contaminated water are not even aware of that fact and its consequences. 20 years ago when the WB government was first informed, it was a casual matter, without the realization of the magnitude this problem was to assume. At least up to 1994, one committee after another was formed but no solution was forthcoming. None of the expert reports has suggested solutions that involve awareness campaigns, education of the villagers and participation of the people. Initially, international aid agencies working in the subcontinent simply did not consider that As could be present in groundwater. Even now, while As in drinking water is being highlighted, there have been almost no studies on how additional As is introduced through the food chain, as large amounts of As are present in the agricultural irrigation water. Past mistakes, notably the ceaseless exploitation of groundwater for irrigation, continue unabated today; at this time, more groundwater is being withdrawn than ever before. No efforts have been made to adopt effective watershed management to harness the extensive surface water and rainwater resources of this region. Proper watershed management and participation by villagers are needed for the proper utilization of water resources and to combat the As calamity. As in groundwater may just be nature's initial warning about more dangerous toxins yet to come. What lessons have we really learned?     

Response: Why had long-lived electron-transfer states of donor-substituted 10-methylacridinium ions been overlooked? Formation of the dimer radical cations detected in the near-IR region

Photoexcitation of an acetonitrile solution of the 9-(1-naphthyl)-10-methylacridinium ion results in formation of the electron-transfer state that forms the long-lived dimer radical cation with the acridinium ion, which was clearly detected as the transient absorption spectrum in the near-IR region.     

Study of self-association of water in supercritical CO2 by Monte Carlo simulation: Does water have a specific interaction with CO2?

The extent of the self-association of water in supercritical CO₂ has been investigated in a wide range of density and temperature by the test particle insertion technique. The results show that the association constant for water decreases with temperature and weakly depends on CO₂ density. This weak density dependence provides evidence for the lack of a strong specific CO₂–water interaction. Comparing calculated association constants with its gas-phase values shows that the association constant is at most ca. 38% lower than its gas-phase value in the high density–low temperature region. Inspection of the simulated radial distribution functions revealed that forming modest water–CO₂ complexes does not result in substantial interference in H-bonding of water molecules.     

Why substituting the asparagine at position 35 in Bacillus circulans xylanase with an aspartic acid remarkably improves the enzymatic catalytic activity? A quantum chemistry-based calculation study

Xylanases from Bacillus circulans (BCX) are known as configuration-retaining glycoside hydrolases, which hydrolyze xylans with two glutamic acid residues (Glu78 and Glu172) serving as catalytic active residues according to a double displacement mechanism. Existing experimental researches show that mutating the asparagines (Asn) to aspartic acid (Asp) at position 35 next to Glu172 can obviously improve the catalytic activity of BCX. To better understand the inherent mechanism for the experimental finding, we performed quantum chemistry calculations on two model systems to mimic the catalyses of wild-type and mutant BCXs. Geometrical structures and relative energies of intermediates and transition states involved in the hydrolysis reactions are given in detail. It is found that in the wild-type model system Asn35 interacts with Glu172 via a loose hydrogen bond, while in the mutant model system Asp35 forms a very tight hydrogen bond with Glu172. The glycosidic bond cleavage is proposed to be the rate-determining step for the hydrolysis reaction, whose barrier varies from 98 to 65 kJ mol⁻¹ when Asn35 is replaced by Asp35, showing the presence of Asp35 remarkably reduces the energy demand for the hydrolysis reaction. The present result provides a theoretical elucidation for why a single amino acid substitution can importantly influences catalytic activity of BCX.     

Why is Re-Re bond formation/cleavage in [Re(bpy)(CO)3]2 different from that in [Re(CO)5]2? Experimental and theoretical studies on the dimers and fragments

The Re(NN)(CO)3(THF) (NN=bpy=2,2'-bipyridine or dmb=4,4'-dimethyl-2,2'-bipyridine) radical, produced by homolysis of [Re(NN)(CO)3]2 in THF solution by visible irradiation, dimerizes with rate constants kd=20 +/- 3 and 11 +/- 4 M(-1) s(-1) for NN=dmb and bpy, respectively. The dimerization processes are strikingly slow compared to those of typical metal radicals including Re(CO)5 (kd approximately 10(9) M(-1) s(-1)). In order to explain such slow reactions, we have performed B3LYP hybrid DFT and fully ab initio RHF and MP2 calculations on several conformations of [Re(bpy)(CO)3]2 (cis, trans, skewed cis, skewed trans) and [Re(CO)5]2 (staggered) and on their constituent monomer radicals and anions. The calculations show that the most stable geometry of [Re(bpy)(CO)3]2 is skewed cis, and the experimental infrared spectrum and photochemical properties of the [Re(bpy)(CO)3]2 dimer are best described by the calculated properties of the skewed cis conformer in which there is no low-lying unoccupied orbital that is predominantly sigma(MM) in character. The Re(bpy)(CO)3(THF) ligand radical is more stable than the 5-coordinate "17-electron" metal radical, Re(bpy)(CO)3, suggesting that the extremely slow dimerization rate most likely arises from the solvent blocking the binding site (i.e., the estimated fraction of the five-coordinate monomer is 1.6 x 10(-2)). Theoretical results are consistent with our experimental results that the dimerization process proceeds via the Re centered radical, which is involved in a pre-equilibrium favoring the ligand-centered radical. Furthermore, time-dependent DFT calculations on [Re(bpy)(CO)3]2 and [Re(bpy)(CO)3]- identify the origin of UV-vis absorption in THF.