Silicification and biosilicification - Part 6. Poly- L-histidine mediated synthesis of silica at neutral pH

Biosilicification results in the formation of ornate silica structures, which are seen in diatoms, sponges, grasses and other biological systems. Proteins have been isolated from diatoms, sponges and grasses that are proposed to be responsible for biosilicification and have been sequenced and/or some of the key amino acids identified. Studies of the amino acids primary sequence in these proteins suggest that lysine, histidine, arginine, cysteine, proline and serine probably play a role in biosilicification. Homopolymers of lysine, arginine and proline have been shown to form silica structures in vitro at (or close to) neutral pH. Here we report, for the first time, the ability of poly-L-histidine (PLHis) to form silica structures from a silica precursor at neutral pH. It was observed by scanning electron microscopy that PLHis facilitates the formation of silica spheres in the size range 150-200 nm and interconnected structures make up relatively smaller particles.