Investigation of the molecular basis for silicon biofunctionality

Abstract

Despite advances in our understanding of the beneficial role of silicon in the biosphere, surprisingly little is known of the molecular mechanism by which silicon is absorbed, transported, accumulated and deposited by organisms. We used silicon-29 NMR spectroscopy to investigate the interaction of the rare amino acid 2,3-trans-3,4-cw-3,4-dihydroxy-L-proline (DHP) with aqueous silicon. Spectral data revealed the structure of three organosilicate complexes that DHP spontaneously forms with aqueous silicon at pH = 7.9, making DHP the first ever known Si-binding amino acid. Such a discovery has potential significance in accounting for silicon’s biological role in hydroxyproline-rich structural glycoproteins found in both mammals and plants. The cell wall morphology of wheat, Triticum aestivum L., grown in either -Si or +Si media was examined by optical and scanning electron microscopy. The preliminary results indicate that Si-deficiency causes swelling of the parenchyma cell walls, supporting the hypothesis that silicon may enhance cell wall integrity by cross-linking pectic polysaccharide molecules via complexation at the apiofuranose binding sites. Silicon-29 and 13C NMR spectroscopy were used to test this theory at the molecular level. Blood and urine were collected periodically from a human subject following ingestion of 29Si-enriched silicic acid and analyzed by ICP-OES and NMR spectroscopy. In addition to studying the kinetics of Si uptake and excretion, we obtained the first reliable speciation of Si-containing molecules in human biofluids, and demonstrated that mono- and disilicic acid are the predominant species.