CHARACTERIZATION OF PEPTIDE-COATED CADMIUM-SULFIDE CRYSTALLITES

Glutathione-related isopeptides of general structure (γ-Glu-Cys)n-Gly, (γEC)nG), serve as a matrix for formation of nanometer-sized cadmium-sulfide crystallites in cultures of Schizosaccharomyces pombe and Candida glabrata and in vitro. The formation of these complexes constitute a cadmium detoxification response in these organisms. Crystallites formed in vivo and in vitro exhibit numerous properties analogous to quantum semiconductor clusters including luminescence and the ability to mediate the photoreduction of methyl viologen. The (γEC)nG peptides stabilize discrete sizes of the particles against accretion. Isopeptides from (γEC)2G to (γEC)4G preferentially stabilize CdS clusters having band gap transitions indicative of a lattice diameter near 20 A. Accretion can be facilitated in all clusters at elevated temperatures, acidic pH, or in the presence of free sulfide ions. However, particle coalescence is more facile in clusters containing isopeptides with only two dipeptide repeats (n = 2 peptides). The in vitro formation of CdS crystallites is also dependent on peptide length. Cd(II)-peptide complexes with a greater number of dipeptide repeats are an inferior matrix for crystallite formation. Cysteinyl peptides with α-peptide bonds bind Cd(II) ions and serve as effective matrices for CdS mineralization, but the resulting crystallites exhibit band gap transitions of higher energy than the transitions observed in 7-peptide coated particles. © 1990, American Chemical Society. All rights reserved.