The Roots of Remediation
Improving the bioremediation potential of soil bacteria is crucial to making them an economically viable player in strategies aimed at cleaning up contaminated sites. One of the key problems with developing and maintaining an efficient bacterial remediation system is keeping the bacteria alive and happy enough to feed on the toxic chemicals of interest. In many cases, it is necessary to provide food supplements (generally a carbon source) exogenously to the soil to keep the microflora growing and metabolically active enough to break down the pollutants. But one alternative to this approach is to take advantage of the fact that rhizosphere (root zone) bacteria have a natural physical protection and nutrition source to keep them alive as they go about their bioremediation task.

Researchers at the University of Oklahoma (Norman) recently reported on their study of the effect of root turnover as a source of nutrients for microbes involved in the remediation of difficult-to-digest contaminants (Environ. Sci. Tech. 2002, 36 (7), 1579–1583). They found that the seasonal turnover and breakdown of fine roots provided a natural in situ source of nutrient phenolics that spurred the growth of the bacterium Burkholderia sp. LB400—a strain that is capable of breaking down polychlorinated biphenyls (PCBs). Normally, these bacteria require the addition of exogenous biphenyl to trigger their growth and remediation efforts. But such supplements were not required with this rhizosphere method. With a minimum of 58% of the fine plant roots dying annually, the amount of phenolics made available by the turnover process provided a significant nutrient source.

Mary Beth Leigh and colleagues determined that the proper choice of plant species, in this case, Morus sp. (common mulberries), in conjunction with the appropriate bacterial strain, could create a stable system for remediating contaminating PCBs. This system could be established and maintained with minimal human intervention—requiring no added biphenyls—at least under controlled conditions in a rhizotron, a growth chamber specifically designed for the study of rooting and root behavior. The scientists showed that the most prominent fine-root flavones released by the dead roots provided a significant stimulatory effect on the bacteria; they believe that this research shows promise for the use of rhizosphere remediation in real contaminated sites.

Frederic V. Miluvec, a postdoctoral researcher at the UCSD laboratory, made the discovery while trying to create new sensors for computer disk drives by introducing magnetic materials into specially prepared silicon chips. The chips had been etched to create a dense series of microscopic channels, and had been immersed in a solution that left gadolinium nitrate, a magnetic compound, in the channels once the chips had dried. When Miluvec tried to cleave a magnetized silicon wafer with a diamond scribe, the chip exploded.

“It blew up in his face,” said Michael J. Sailor, a professor of chemistry and biochemistry at UCSD who heads the lab where Miluvec works, in a UCSD press release. “It was just a small explosion, like a cap going off in a cap gun. But it really surprised us, so we started looking more closely at it, because the gadolinium produced such a very clean burning flame.”

Researchers have long known that a silicon-based explosive would explode when mixed with potassium nitrate, also known as saltpeter. But the same result with gadolinium nitrate came as a complete surprise.

Sailor’s group is currently researching the applications that such a small, clean-burning explosion could provide. The absence of chemical impurities, they say, makes the gadolinium- and silicon-based explosive ideal for performing rapid chemical analysis of environmental contaminants and biohazards through flame emission spectrometry in the field. The explosive might be manufactured into a handheld device.

Other applications possible for the explosive, according to Sailor, include a propulsion source for microelectrical mechanical systems (MEMS) or computer chips that can explode from a remote command to prevent sensitive information from falling into the wrong hands.

Pradip Mascharak and colleagues from the University of California–Santa Cruz and the University of California–Davis have recently reported on new self-recognition chemistry for dimer formation of copper complexes, which, like many transition metal compounds, are important components of supramolecular research.

The scientists set out by reacting a copper (II) inorganic complex with a racemic mixture (R and S enantiomers) of the novel chiral ligand N-(1,2-bis(2-pyridyl)ethyl)pyridine-2-carboxamide (PEAH) to get the [Cu₂(PEA)₂](ClO₄)₂ dimer (Inorg. Chem. 2002, 41 (6), 1545–1549). Instead of obtaining a mixture of homochiral dimers (two R or two S PEA enantiomers in the same molecule) and heterochiral dimers (one R and one S in the same molecule), which would generally have been expected, the researchers observed (via X-ray crystallography) a 1:1 ratio of the homochiral complexes, with no heterochiral dimer present at all. The fact that the two different enantiomers would not appear in the same compound is an indication of chiral self-recognition occurring at some point in the complexation process.

Two of the same enantiomers of [Cu(PEAR(S))(Cl)(H2O)] can readily “clasp” each other to form a dimer complex, while the two opposite enantiomers, apparently, can not align for a successful interaction.

The researchers liken the occurrence to the mechanics of a handshake. As can be seen in the figure, the two like enantiomers (or “hands”) are able to align in such a way that they can wrap around each other and allow the free nitrogen ligands and the copper centers to readily interact. The opposite enantiomers can’t align in the same way—much as a handshake between left and right hands is awkward.

The group, says Mascharak, plans to build up from here, making ligands with increasing numbers of chiral carbons to gain a greater structural understanding of what can allow a metal to “choose” the chirality of its coordinating ligands.