Bright colors have been observed when a metal island film is deposited on top of a silver mirror with a separating quartz layer. For spacer layer thicknesses that are varied from 0 to 140 nm, the visual appearance changes from blue/black to a series of brilliant spectrumlike colors. The sequence is repeated similarly for higher interlayer thicknesses. The phenomenon is analyzed in terms of a stratified medium theory by using TEM data and an electromagnetic model for the optical constants of the metal island film. For island films with a sufficiently high absorbance (> 0.35), the spectra are characterized by two sharp minima where the reflectivity drops to values below l0(-3). The observed thickness dependence is analyzed in terms of a complex combination of the phase shifts caused by the island film, the spacer, and the relevant interfaces.
A protein consisting of 60 kDa subunits (As-P60) was isolated from etiolated oat seedlings (Avena sativa L.) and characterized as avenacosidase, a beta-glucosidase that belongs to a preformed defence system of oat against fungal infection. The enzyme is highly aggregated; it consists of 300-350 kDa aggregates and multimers thereof. Dissociation by freezing/thawing leads to complete loss of enzyme activity. The specificity of the enzyme was investigated with para-nitrophenyl derivatives which serve as substrates, in decreasing order beta-fucoside, beta-glucoside, beta-galactoside, beta-xyloside. The corresponding orthonitrophenyl glycosides are less well accepted. No hydrolysis was found with alpha-glycosides and beta-thioglucoside. An anti-As-P60 antiserum was prepared and used for isolation of a cDNA clone coding for As-P60. A presequence of 55 amino acid residues was deduced from comparison of the cDNA sequence with the N-terminal sequence determined by Edman degradation of the mature protein. The presequence has the characteristics of a stroma-directing signal peptide; localization of As-P60 in plastids of oat seedlings was confirmed by western blotting. The amino acid sequence revealed significant homology (> 39% sequence identity) to beta-glucosidases that are constituents of a defence mechanism in dicotyledonous plants. 34% sequence identity was even found with mammalian and bacterial beta-glucosidases of the BGA family. Avenacosidase extends the occurrence of this family of beta-glucosidases to monocotyledonous plants.
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