Biological Formation of CdS Semiconductor Quantum Crystallites.
12 June 1989
We have discovered and characterized semiconductor particles made by a biological process. Discrete cadmium:sulfide crystallites form in the yeasts Candida glabrata and Schizosaccharomyces pombe cultured in the presence of cadmium salts. Sequestration of Cd ions in these particles is a cellular detoxification mechanism. Monodisperse 20angstrom polycrystalline particles of cubic structure have been observed in the cytosol of the two yeasts. Additionally, zinc-blende polycrystalline particles of approximately 29angstrom diameter occur in the extracellular culture medium. The particles are too small to exhibit bulk CdS electronic properties (1). Rather, the optical spectra vary with size in the same fashion as colloidal CdS crystallites exhibiting the three-dimensional quantum size effect (2). The particles are stabilized against flocculation by biological sheaths which appear to terminate crystallite growth at discrete sizes. Cytoplasmic particles are coated with gamma glutamyl peptides of general structure (gamma-Glu-Cys) sub n -Gly in which n varies from 2 to 4.