How do sea-shells form?
Sea-shells are examples of biominerals. Biominerals are "nano-composite" materials where crystals are in the nanomater size-range (hence, nano) and are intimately associated with organic macromolecules (hence, composite). Determining how organisms such as diatoms and oysters produce their shells (made up of amorphous silica, aragonite and calcite) is related to the global cycling of important elements such as Si and C. Also, biominerals are have high toughness and fracture strength because they are nanocomposites, and are therefore of great interest to materials scientists.
Diatoms are the main sink for dissolved silicon in oceanwater. They precipitate amorphous silica tests in complex, intricate, and species-specific patterns. Experimentalists have suggested that proteins and polysaccharides found within the diatom silica is involved in the nucelation mechanism for silica precipitation. We are using molecular orbital theory to examine the interactions between dissolved Si(OH)4, amino-acids (portions of proteins) and polyalcohols (proxy compounds for polysaccharides) (Sahai and Tossell, 2000).
We have determined the relative stability and 29Si NMR shifts (Figs. c, d) of direct Si-O-C ester-like bonds versus hydrogen bonds between the monomeric silicic acid and the alcohol group on aliphatic organics such as serine and threitol , a four-carbon polyacohol. The identification of H-bonded versus Si-O-C covalently- bonded complexes may help identify the nucleation mechanism for the silica shells produced by diatoms.
Geometries, energies, and NMR shifts were calculated at the 6-31G* Hartree-Fock level. Solvation was accounted for by a spherical cavity in a self-consistent reaction field.
At neutral pHs, Si-O-C bonded aliphatic complexes of quadra-coordinated silicon (QSi) (Figs. c, d) are more stable than H-bonded complexes, but are difficult to distinguish from each other and from monomeric Si(OH)4 based on their 29Si isotropic shifts which are similar (-61 to 64 ppm, error is overprediction by 11ppm). Si-O-C bonded complexes of penta-coordinated silicon (PSi) (Figs. c, d) become stable only at very high pHs where Si(OH)4 is deprotonated. Unfortunately, computational error causes ambiguity in the interpretation of calculated isotropic 29Si shifts for PSi-serine complexes: the calculated shifts of 104 to 107 ppm, suggest that PSi-serine complexes could escape detection due to overlap with Q3 and Q4 inorganic polymerized silicon; on the other hand, consideration of the overprediction error of13 ppm for PSi makes the shifts 117 to 120 ppm, which could have been diagnostic of PSi O-C.
We have found, however, that anisotropic shifts of 29Si are diagnostic, with PSi having large anisotropies of ~80 to100 ppm whereas QSi has small anisotropies of ~ 0 to 40 ppm. Multinuclear NMR (13C, 17O) NMR experiments should also prove more useful than 29Si isotropic shifts alone. The 13C shift of the COH group in serine decreases by 1-7 ppm when serine is bonded to QSi or PSi through a Si-O-C bond, but increases by 6 ppm when serine is bonded to some other other organic such as a methyl group through a C-O-C bond. 17O NMR shift of the -COH group on serine increases by 10 to 30 ppm when bonded to Si, but decreases by 20 ppm when linked to methyl.