Hydrophobic dodecanethiol-capped silver nanocrystals ranging from 50 to 80 Å in diameter were synthesized using arrested growth methods. Size-selective precipitation was employed to isolate nanocrystals homogeneous in size and shape which exhibited close-packed structural order after drying on a carbon or mica substrate. Elemental analysis, H NMR and FTIR spectroscopies were used to characterize the compositional features of the adsorbed thiolate ligands on nanocrystals suspended in solution and condensed in nanocrystal films. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to probe the structure of both individual nanocrystals and superlattices, and to estimate the energetic interactions between these sterically stabilized particles. In terms of these measurements, the effects of the capping ligand coverage, particle faceting and shape, and interparticle attractions on superlattice formation are elucidated. In particular, the concept of nanocrystals as “soft spheres” which order with an effective hard sphere diameter is introduced. In addition, the ways in which nanocrystals deviate from such an ideal model are discussed.
Nitric oxide synthase (NOS) enzymes synthesize nitric oxide, a signal for vasodilatation and neurotransmission at low levels, and a defensive cytotoxin at higher levels. The high active-site conservation among all three NOS isozymes hinders the design of selective NOS inhibitors to treat inflammation, arthritis, stroke, septic shock, and cancer. Our structural and mutagenesis results identified an isozyme-specific induced-fit binding mode linking a cascade of conformational changes to a novel specificity pocket. Plasticity of an isozyme-specific triad of distant second- and third-shell residues modulates conformational changes of invariant first-shell residues to determine inhibitor selectivity. To design potent and selective NOS inhibitors, we developed the anchored plasticity approach: anchor an inhibitor core in a conserved binding pocket, then extend rigid bulky substituents towards remote specificity pockets, accessible upon conformational changes of flexible residues. This approach exemplifies general principles for the design of selective enzyme inhibitors that overcome strong active-site conservation.
324. [26] Standard deviation, s is calculated according to Equation 1 where D and N are the average diameter and the number of particles, respectively. The size distribution is evaluated by the ratio s /D. s = {å [N i (D i ±D) 2 ]/[N±1]} 1/2[27] Transmission electron microscopy (TEM) image and electron diffraction spectroscopy (EDS) patterns were obtained with a JEOL JEM 2000FX microscope.[28] At high temperature, above the blocking, the magnetization curve is simulated by the Langevin relationship shown in Equation 2, where m = M s pD3/6, M(D), M s , D, and H are the magnetization, the saturation magnetization, the particle diameter, and the applied field, respectively.[29] Diffractomer D22 in LURE ([30] The magnetic studies were performed using a commercial SQUID magnetometer Cryogenic S600. 521.[38] X-ray diffraction measurements (XRD) were carried out using a STOE Stadi P goniometer with a Siemens Kristalloflex X-ray generator with a cobalt anticathode (l = 1.7809 ) driven by a personal computer through a DACO±MP interface.The future growth of the electronics sector will depend on developing faster integrated circuits (ICs) and on reducing costs. In respect of both these objectives, the assembly in solution, or at a suitable substrate, of complex metal, semiconductor and insulator nanocrystal architectures is of particular interest. [1±14] In the medium term, it is expected such studies will lead to the assembly of addressable arrays of functional nanoscale heterostructures. In the longer term, it is possible such studies will lead to the cost-effective assembly (partial or complete) in solution or at a suitable substrate of ICs capable of processing information at unprecedented speeds. If, however, these expectations are to be realized it will be necessary to develop practical strategies for the assembly of complex nanocrystal architectures in solution or at a suitable substrate. One strategy is to adsorb molecules incorporating a binding site at the surface of each nanocrystal of a dispersion. The function of these molecules is to uniquely define the position of a nanocrystal from a given dispersion in the nanocrystal architecture to be assembled. Upon mixing a number of such dispersions, each nanocrystal will recognize and selectively bind a nanocrystal from another dispersion or a well-defined region on a patterned substrate. By this means, it will be possible to program the parallel assembly of identical multiple copies of the desired nanocrystal architecture in solution or at a suitable substrate. Toward this end, the gold nanocrystals of an aqueous dispersion have been modified by chemisorption of a modified biotin and their subsequent aggregation induced by addition of streptavidin, a biotin binding protein. The size and structure of the resulting aggregates have been studied by dynamic light scattering, small-angle X-ray scattering and transmission electron microscopy. Apart from being the first example of nanocrystal assembly based on protein binding, these findings further support the view that t...
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