Silasesquioxanes are an interesting class of ligands for both main group and transition-metal elements. A variety of coordination environments can be supported by silasesquioxane ligands, and it is ...now possible to prepare metallasilsesquioxanes containing practically any stable element from Groups 1015. This article surveys the synthesis, characterization, and reactivity of silasesquioxane ligands. Structure, bonding and structural dynamics of both silasesquioxanes and metallasilasesquioxanes are also discussed.
The chemistry of silasesquioxanes and metallasilasesquioxanes can provide important insights into the chemistry of silica and silica-supported transition-metal catalysts. The uniqueness of silasesquioxanes as models for silica is discussed and several systems which provide molecular-level insights into the surface chemistry of silica and silica-supported catalysts are discussed.
Nitric-oxide synthase (NOS; EC 1.14.13.39) catalyzes the oxidation of L-arginine to nitric oxide (NO(.)) and L-citrulline via the intermediate N(omega)-hydroxy-L-arginine. Of the three distinct ...isoforms of NOS that have been characterized, the constitutive neuronal NOS (NOS I) generates NO(.) associated with long-term potentiation (LTP) and early brain development. All of the NOS isoforms contain an N-terminal oxidase and a C-terminal reductase domain connected by a Ca(2+)/calmodulin binding region. To activate NOS I, Ca(2+) has to bind to calmodulin, allowing electron transport through both domains. Calcium ions are tightly regulated in cells. However, a number of other metal ions that bind and activate calmodulin may also activate NOS I. One such metal ion may be Pb(2+), which is associated with neurobehavioral and psychological alterations, including the inhibition of LTP. The effect of various divalent cations on NOS I activity was tested, and the results presented herein demonstrate that Pb(2+) and Sr(2+) can activate NOS I to a level similar to that found for Ca(2+). Finally, there is a synergy between Pb(2+) and Ca(2+) resulting in maximal activation of NOS I using minimal concentrations of both metal ions.
Neuronal nitric oxide synthase (NOS I) is a Ca2+/calmodulin–binding enzyme that generates nitric oxide (NO•) and L-citrulline from the oxidation of L-arginine, and superoxide (O2•−) from the ...one-electron reduction of oxygen (O2). Nitric oxide in particular has been implicated in many physiological processes, including vasodilator tone, hypertension, and the development and properties of neuronal function. Unlike Ca2+, which is tightly regulated in the cell, many other divalent cations are unfettered and can compete for the four Ca2+ binding sites on calmodulin. The results presented in this article survey the effects of various divalent metal ions on NOS I–mediated catalysis. As in the case of Ca2+, we demonstrate that Ni2+, Ba2+, and Mn2+ can activate NOS I to metabolize L-arginine to L-citrulline and NO•, and afford O2•− in the absence of L-arginine. In contrast, Cd2+ did not activate NOS I to produce either NO• or O2•−, and the combination of Ca2+ and either Cd2+, Ni2+, or Mn2+ inhibited enzyme activity. These interactions may initiate cellular toxicity by negatively affecting NOS I activity through production of NO•, O2•− and products derived from these free radicals.
The reaction of (c-C6H11)7Si7O9(OH)3 (1a) with GaX3 (X = Cl or I) in the presence of Proton Sponge (C14H18N2) affords (c-C6H11)7Si7O12GaX- C14H18N2·H+ (2a, R = Cl; 2b, R = I). A single-crystal X-ray ...diffraction study of 2a reveals discrete ions with no significant interactions. Upon thermolysis both 2a and 2b liberate an equivalent of C14H18N2·H+X- to produce quantitative yields of (c-C6H11)7Si7O12Ga2 (3), a siloxy-bridged dimer which is isomorphous with previously characterized dimers containing Ti3+ and V3+. The reaction of 3 with Ph3PO affords (c-C6H11)7Si7O12Ga(OPPh3) (5), which was characterized by a single-crystal X-ray diffraction study. The reaction of trisilanol 1a with Ga(CH2SiMe3)3 affords 6, an interesting cluster derived from the reaction of two molecules of 1a with six molecules of Ga(CH2SiMe3)3 and six molecules of water.
Nitric oxide synthase (NOS) generates nitric oxide (NO·) by the oxidation of l-arginine. Spin trapping in combination with electron paramagnetic resonance (EPR) spectroscopy using ferro-chelates is ...considered one of the best methods to detect NO· in real time and at its site of generation. The spin trapping of NO· from isolated NOS I oxidation of l-arginine by ferro-N-dithiocarboxysarcosine (Fe(DTCS)2) and ferro-N-methyl-d-glucamide dithiocarbamate (Fe(MGD)2) in different buffers was investigated. We detected NO–Fe(DTCS)2, a nitrosyl complex, resulting from the reaction of NO· and Fe(DTCS)2, in phosphate buffer. However, Hepes and Tris buffers did not allow formation of NO–Fe(DTCS)2. Instead, both of these buffers reacted with Fe2+, generating sparingly soluble complexes in the absence of molecular oxygen. Fe(DTCS)2 and Fe(MGD)2 were found to inhibit, to a small degree, NOS I activity with a greater effect observed with Fe(MGD)2. In contrast, Fe(MGD)2 was more efficient at spin trapping NO· from the lipopolysaccharide-activated macrophage cell line RAW264.7 than was Fe(DTCS)2. Data suggested that Fe(DTCS)2 and Fe(MGD)2 are efficient at spin trapping NO· but their maximal efficiency may be affected by experimental conditions.
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