Evidence for intramineral macromolecules containing protein from plant silicas

Macromolecular assemblages intimately associated with biogenic silica in plants are released on solubilization of the siliceous phase by treatment with buffered aqueous solutions of HF following treatment of plant material with concentrated oxidizing acids (perchloric, nitric and sulphuric) to remove cytoplasmic contents and the largely polysaccharidic cell wall. The non-dialysable material, which may form 0.015-0.030% dry weight of the silica, has an amino acid composition rich in Pro-Glu, Pro-Lys, or Ser-Asp-Gly depending on the extent of treatment with oxidizing acids. The materials are suggested to have an intrasilica location with materials of similar composition being extracted from the branches of Equisetum telmateia (a primitive plant) and from hairs from the lemma of the grass Phalaris canariensis. The role of such material in regulating nucleation, particle growth and aggregate structure development in silicas is discussed.