SCIENCE & TECHNOLOGY ESSENTIAL ELEMENT Scanning electron microscopy image shows silicon (blue) in young equisetum leaf. er plants for strengthening cell walls and in higher animals for bone and cartilage. Diatoms, radiolaria, some sponges, the Equisetaceae—commonly known as horsetail or scouring rushes—and other plants all require silicon to complete their life cycle. Agriculturally important plants such as rice, beans, wheat, soy, maize, and cucumbers depend on silicon for healthy growth and are adversely affected by sili con depletion. "Generally a lack of silicon reduces crop yields; causes the plants to lean or fall over; and reduces their resistance to pests, dis ease, drought, frost, and other types of stress," Knight says. IN MAMMALS, silicon deficiency has been linked to bone and heart disease, cancer, and neurodegenerative disorders. "One of the big questions is: How do plants isolate, transport, and deposit sili con?" Knights tells C&EN. "Since they readily do so at room temperatures and at mospheric pressures, the hope is that an understanding of the molecular-level mechanism involved will provide low-en ergy, and thus low-cost, synthetic routes to novel materials, as well as allowing us the MICHAEL FREEMANTLE, C&EN LOND Ν ability to investigate the creation of nanostructural molecular architecture. Ulti NTIL RECENTLY, NO ORGANOSILsilicon concentrations. And last month mately, this may lead to the realization of icon compounds had ever been they provided "the first direct evidence of such sci-fi concepts as biologically engi detected in living systems or an organosilicon complex formed during neering microcomputers and sensors, for even under biologically relevant the life cycle of an organism" \Jf. Chem. Soc, example." conditions in the laboratory. Da/ton Trans., 2002,307}. 'This complete absence of evidence that Casey points out that the dominant form "These results are startling," comments silicon forms compounds with the carbonof silicon in natural waters is the aqueous William H. Casey, professor of aqueous based materials found in living systems has monomer silicic acid (H4S1O4), in which sil geochemistry at the University of Cali led some workers to conclude that the field icon is tetrahedrally coordinated to four oxy fornia, Davis. "They suggest that Mother of silicon biochemistry is, essentially, a con gen atoms, as it is in most of Nature may employ organo tradiction in terms," says Christopher T. Earth's minerals. "The aque silicon complexes to modify G. Knight, a research scientist at the Uni ous chemistry of silicon(IV), the chemistry of silicon in versity of Illinois, Urbana-Champaign, who however, is enragingly simple solution." has been studying the aqueous chemistry yet difficult to modify" he says. Silicon is known to be pres of silicon for more than 20 years. The ele O n l y in the very highpressure ent in all living organisms. The ment's biological importance, he points mineral stishovite does silicon element occurs in the form of out, is attributed by these workers simply change its coordination num hydrated amorphous silica, to its role in producing solid oxide support ber to oxygens from four to six. referred to as opal, and is re structures along with an ability to alter tox "Most chemists know quired for the production of ic metal activity much more about the chem structural materials in singleistry of square-planar plat Over the past few years, Knight and a celled organisms through to inum complexes than they do group at Lakehead University, Ontario, led higher plants and animals, ex about the chemistry of their by chemistry professor Stephen D. Kinplains biosilicification expert Knight own planet, Earth," Casey ob rade, have been studying aqueous silicate Carole C. Perry who is a readserves wryly, pointing out that Earth's crust chemistry in an attempt to unravel silicon's er in bioinorganic chemistry at Notting is made of a rich array of aluminosilicate role in living systems. Last year, for exam ham Trent University, England. For many compounds. ple, they showed that stable silicon-car life forms, she continues, silicon can even bohydrate complexes can form in aqueous be considered to be an essential element. "There is evidence for extraordinarily solutions at biologically relevant p H and It is required, for example, in certain highrich silicon biological chemistries," he says.
HOW DOES NATURE PROCESS SILICON?
There is increasing evidence that organosilicon complexes play a pivotal role in living systems
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SCIENCE & TECHNOLOGY celled aquatic plants found by the billions in lakes and oceans," Knight explains. "Each cell possesses a beautiful, delicate, and precisely engineered shell of pure silica. One of nature's enduring mysteries is how diatoms and other plants actually build these unique silicate structures." Diatoms must isolate silicon from water, transport it across the cell membrane, and then deposit it as a solid," he continues. "To do this in the laboratory requires high temperatures, high pressures, or extreme p H levels. But diatoms somehow manage under normal physiological conditions." T h e group forced colonies of the diatom into a single stage of their life cycle by silicon starvation, fed them with ^ S i - e n riched silicate solution, and then recorded the 29Si N M R spectra of the diatoms in USING SILICON-29 nuclear magnetic resvivo. onance (NMR) spectroscopy as their primary analytical tool, Knight, Kinrade, and 'Although it is impossible to determine their coworkers have, over the past few its molecular structure, the complex does years, characterized a number of organosilnot contain the tetravalent silicon comicon complexes resulting from the intermonly found in aqueous systems, but inaction of dissolved silicon with organostead contains a penta- or hexavalent silications. In 1999, for example, the group con center," Knight explains. "Nitrogen-15 reported that addition of isotopic enrichment experaliphatic polyhydroxy comiments also suggest, alpounds —such as threitol, xylithough the evidence is cirtol, mannitol, and sorbitol—to cumstantial, that nitrogen aqueous silicate solutions unoccupies one of the coordider strongly alkaline condination sites." tions yielded high concenHe points out that the trations of stable polyoate signal-to-noise ratios of the complexes containing five- or spectra are poor. 'The point six-coordinated silicate anions is that there is a signal there, [Science, 285,1542 (1999)}. The although there is not very formation of these hypervamuch of the compound, lent anions depends on the loand it isn't there for very cation of the hydroxy groups long," he says. in the polyalcohols. Perry comments that claims for the detection of a "The remarkable ease by which these simple sugarlike AQUEOUS SILICATES Lakehead chemists attempt to explain transient organosilicon complex containing hexavalent molecules react to form hy- how plants manipulate silicon. Shown are Eric Deguns (left), silicon coordinated to at pervalent silicon complexes in Gillson, Kinrade, and Robin J. Hamilton. least one nitrogen in an oraqueous solution supports a longstanding supposition that such species ganism, if true, are groundbreaking and Transactions paper last month, the two play a significant role in the biological upcould provide the basis for the development chemists revealed evidence of a transient take and transport of silicon and in mineral of the biochemistry of the element. hexavalent silicon complex in the freshdiagenesis," the authors note. water diatom Naviadapelliculosa. They carKnight believes that his work with Kinried out the work with Lakehead underAccording to Casey, the discovery that rade and colleagues shows that the field of graduate chemistry student Ashley-M. E. simple polyalcohols cause Si(I V) to switch silicon biochemistry is no longer a contraGillson. to Si(V) and Si(VI) in aqueous solution is diction in terms. "It is finally beginning to remarkable. "A form of algae, diatoms are singletake shape," he concludes. •
"But we still haven't discovered how plants manipulate silicon. People have suspected for a long time that they do it by changing the coordination number of the element. "In organic-rich environments such as swamps, for example, the concentrations of silicon in solution are higher than you would predict from the equilibrium between solid silica and an aqueous solution containing only silicic acids," Casey continues. Hydrated silica and other biominerals usually exist in biological organisms as composite phases consisting of the mineral phase and organic components such as membranes or proteins and carbohydrates. Kinrade points out that silicates and carbohydrates are, respectively, the two most abundant classes of compounds in the inorganic and organic worlds. "That silicates and carbohydrates interact to form soluble organosilicon complexes has often been assumed when addressing issues of mineral weathering, biosilicification, and the apparent biological activity of silicon in higher plants and animals," he says. "\et proof was always lacking."
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"Prior to this work, catechol (1,2-dihydroxybenzene) was the only common natural ligand that could change the coordination number of aqueous silicon," he says. "Catechol is a common moiety in plant and microbe exudates and causes silicon to become six-coordinated in aqueous solutions." More recently, Kinrade, Knight, and coworkers reported 29Si N M R evidence that silicon-sugar acid complexes are so stable that they can be formed under the dilute neutral conditions of most groundwaters and biofluids [Chem. Commun., 2001,1564}. "The silicon concentrations in such solutions are at the very limits of detection by 29 Si N M R spectroscopy, even using materials isotopically enriched in 29Si and more than a week of N M R acquisition time," Kinrade points out. "Nonetheless, we detected at least two different pentaoxosilicon-carbohydrate complexes." Kinrade and Knight are now using 29Si N M R spectroscopy to examine living systems to see if they can detect any organosilicon compounds in vivo. In their Dalton
An understanding of the molecularlevel mechanism of silicon "may lead to the realization of such sci-fi concepts as biologically engineering microcomputers and sensors."
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