On the Mass Fractal Character of Si-Based Structural Networks in Amorphous Polymer Derived Ceramics
The intermediate-range packing of SiNxC4−x (0 ≤ x ≤ 4) tetrahedra in polysilycarbodiimide and polysilazane-derived amorphous SiCN ceramics is investigated using 29Si spin-lattice relaxation nuclear magnetic resonance (SLR NMR) spectroscopy. The SiCN network in the polysilylcarbodiimide-derived ceramic consists predominantly of SiN4 tetrahedra that are characterized by a 3-dimensional spatial distribution signifying compact packing of such units to form amorphous Si3N4 clusters. On the other hand, the SiCN network of the polysilazane-derived ceramic is characterized by mixed bonded SiNxC4−x tetrahedra that are inefficiently packed with a mass fractal dimension of Df ~2.5 that is significantly lower than the embedding Euclidean dimension (D = 3). This result unequivocally confirms the hypothesis that the presence of dissimilar atoms, namely, 4-coordinated C and 3-coordinated N, in the nearest neighbor environment of Si along with some exclusion in connectivity between SiCxN4−x tetrahedra with widely different N:C ratios and the absence of bonding between C and N result in steric hindrance to an efficient packing of these structural units. It is noted that similar inefficiencies in packing are observed in polymer-derived amorphous SiOC ceramics as well as in proteins and binary hard sphere systems.
NMR determination of the fractal dimension in silica aerogels.
$^{29}\mathrm{Si}$ nuclear relaxation experiments were performed in different forms of silica (alcogel, aerogel, amorphous, and crystallized) doped with paramagnetic impurities. Magic-angle spinning is used to quench the nuclear-spin diffusion. Under this condition, the saturation recovery of the $^{29}\mathrm{Si}$ magnetization follows a power law in a very large time range (up to 5 orders of magnitude). This nonexponential relaxation m(t)\ensuremath{\sim}${\mathit{t}}^{\mathrm{\ensuremath{\alpha}}}$ reflects the mass distribution in the sample: M(r)\ensuremath{\sim}${\mathit{r}}^{\mathit{D}}$ with \ensuremath{\alpha}=D/6. In aerogels, the measured fractal dimension D\ensuremath{\approxeq}2.2 is in agreement with that determined by small-angle x-ray scattering. In densified materials, a dimension D\ensuremath{\approxeq}3 is actually observed.
Modulation of neuronal serotonin uptake by a putative endogenous ligand of imipramine recognition sites.
Imipramine inhibits the serotonin uptake by binding with high affinity to regulatory sites of this uptake located on axons that release serotonin. The number of imipramine recognition sites located on crude synaptic membrane preparations is reduced by two daily injections of imipramine or desmethylimipramine for 3 weeks. When the binding sites for [3H]imipramine are down-regulated the Vmax of the neuronal uptake of serotonin is increased. Moreover, in minces prepared from the brain hippocampus of rats receiving imipramine in a dose regimen that reduces the number of [3H]imipramine recognition sites, the efficiency of imipramine as a blocker of the serotonin uptake is diminished. Hence the high-affinity binding sites for [3H]imipramine may have a physiological role in modulation of serotonin reuptake. Probably this is mediated by an endogenous effector of these regulatory sites. A nonpeptidic constituent of rat brain capable of displacing [3H]imipramine from its high-affinity binding site and of inhibiting the serotonin uptake in a dose-related manner has been extracted and its partial purification is described.