To investigate the mode of action of cellulose-binding domains (CBDs), the Type II CBD from Pseudomonas fluorescens subsp. cellulosa xylanase A (XYLACBD) and cellulase E (CELECBD) were expressed as individual entities or fused to the catalytic domain of a Clostridium thermocellum endoglucanase (EGE). The two CBDs exhibited similar Ka values for bacterial microcrystalline cellulose (CELECBD, 1.62x10(6) M-1; XYLACBD, 1.83x10(6) M-1) and acid-swollen cellulose (CELECBD, 1.66x10(6) M-1; XYLACBD, 1.73x10(6) M-1). NMR spectra of XYLACBD titrated with cello-oligosaccharides showed that the environment of three tryptophan residues was affected when the CBD bound cellohexaose, cellopentaose or cellotetraose. The Ka values of the XYLACBD for C6, C5 and C4 cello-oligosaccharides were estimated to be 3.3x10(2), 1.4x10(2) and 4.0x10(1) M-1 respectively, suggesting that the CBD can accommodate at least six glucose molecules and has a much higher affinity for insoluble cellulose than soluble oligosaccharides. Fusion of either the CELECBD or XYLACBD to the catalytic domain of EGE potentiated the activity of the enzyme against insoluble forms of cellulose but not against carboxymethylcellulose. The increase in cellulase activity was not observed when the CBDs were incubated with the catalytic domain of either EGE or XYLA, with insoluble cellulose and a cellulose/hemicellulose complex respectively as the substrates. Pseudomonas CBDs did not induce the extension of isolated plant cell walls nor weaken cellulose paper strips in the same way as a class of plant cell wall proteins called expansins. The XYLACBD and CELECBD did not release small particles from the surface of cotton. The significance of these results in relation to the mode of action of Type II CBDs is discussed.

We apply perturbative effective mass theory as a broadly applicable theoretical model for quantum confinement (QC) in all Si and Ge nanostructures including quantum wells (QWs), wires (Q-wires) and dots (QDs). Within the limits of strong, medium, and weak QC, valence and conduction band edge energy levels (VBM and CBM) were calculated as a function of QD diameters, QW thicknesses and Q-wire diameters. Crystalline and amorphous quantum systems were considered separately. Calculated band edge levels with strong, medium and weak QC models were compared with experimental VBM and CBM reported from X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS) or photoluminescence (PL). Experimentally, the dimensions of the nanostructures were determined directly, by transmission electron microscopy (TEM), or indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline materials are best described by a medium confinement model, while amorphous materials exhibit strong confinement regardless of the dimensionality of the system. Our results indicate that spatial delocalization of the hole in amorphous versus crystalline nanostructures is the important parameter determining the magnitude of the band gap expansion, or the strength of the quantum confinement. In addition, the effective masses of the electron and hole are discussed as a function of crystallinity and spatial confinement.

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO 2 , and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si 3 N 4 and Al 2 O 3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters. V

The full-length glucoamylase from Aspergillus niger, G1, consists of an N-terminal catalytic domain followed by a semi-rigid linker (which together constitute the G2 form) and a C-terminal starch-binding domain (SBD). G1 and G2 both liberate glucose from insoluble corn starch, although G2 has a rate 80 times slower than G1. Following pre-incubation of the starch with SBD, the activity of G1 is uniformly reduced with increasing concentrations of SBD because of competition for binding sites. However, increasing concentrations of SBD produce an initial increase in the catalytic rate of G2, followed by a decrease at higher SBD concentrations. The results show that SBD has two functions: it binds to the starch, but it also disrupts the surface, thereby enhancing the amylolytic rate.z 1999 Federation of European Biochemical Societies.

Zinc oxide (ZnO) is a wide-band gap semiconductor that has attracted tremendous interest for optical, electronic, and mechanical applications. First-principles calculations by [C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 (2000)] have predicted that hydrogen impurities in ZnO are shallow donors. In order to determine the microscopic structure of hydrogen donors, we have used IR spectroscopy to measure local vibrational modes in ZnO annealed in hydrogen gas. An oxygen–hydrogen stretch mode is observed at 3326.3 cm−1 at a temperature of 8 K, in good agreement with the theoretical predictions for hydrogen in an antibonding configuration. The results of this study suggest that hydrogen annealing may be a practical method for controlled n-type doping of ZnO.