Crystal tuning: Organic molecules can be xenophobic, preferring to crystallize with their own kind. Though useful for purification, this precludes the tuning of crystal properties by doping or mixing. Nanoporous steroids provide an exception, as their channels can accept a variety of termini (hexagons and spheres). The steroids can be cocrystallized in any ratio to give a wide range of chiral, potentially porous crystalline materials.
Previous
work has shown that certain steroidal bis-(N-phenyl)ureas,
derived from cholic acid, form crystals in the P61 space group with unusually wide unidimensional
pores. A key feature of the nanoporous steroidal urea (NPSU) structure
is that groups at either end of the steroid are directed into the
channels and may in principle be altered without disturbing the crystal
packing. Herein we report an expanded study of this system, which
increases the structural variety of NPSUs and also examines their
inclusion properties. Nineteen new NPSU crystal structures are described,
to add to the six which were previously reported. The materials show
wide variations in channel size, shape, and chemical nature. Minimum
pore diameters vary from ∼0 up to 13.1 Å, while some of
the interior surfaces are markedly corrugated. Several variants possess
functional groups positioned in the channels with potential to interact
with guest molecules. Inclusion studies were performed using a relatively
accessible tris-(N-phenyl)urea. Solvent removal was
possible without crystal degradation, and gas adsorption could be
demonstrated. Organic molecules ranging from simple aromatics (e.g.,
aniline and chlorobenzene) to the much larger squalene (Mw = 411) could be adsorbed from the liquid state, while
several dyes were taken up from solutions in ether. Some dyes gave
dichroic complexes, implying alignment of the chromophores in the
NPSU channels. Notably, these complexes were formed by direct adsorption
rather than cocrystallization, emphasizing the unusually robust nature
of these organic molecular hosts.
Nanoparticulate hydroxyapatites (HAps) reproducing some of the subnanometre length scale intermolecular interactions characteristic of hard tissue have been prepared for nuclear magnetic resonance (NMR) structural elucidation. HAps were precipitated at physiological pH and temperature from dilute aqueous solutions, in the presence of acidic polysaccharides (chondroitin sulfate, dermatan sulfate, hyaluronic acid, dextran sulfate, polygalacturonic acid), and of hydrophilic poly aminoacids (poly-l-glutamate, poly-l-asparagine, poly-l-lysine). The HAp resembles that of bone with respect to its 31P NMR properties, broad reflections in X-ray powder diffraction, and the coexistence of an ordered crystalline core, surrounded by a less ordered surface containing water and hydrogen phosphate. 13C{31P} rotational echo double resonance (REDOR) NMR, which probes carbon−phosphorus proximities below ca. 1 nm, shows that all the HAps are molecular composites in which each biopolymer forms intimate intermolecular associations with mineral ions. REDOR effects between mineral phosphorus and ring carbons of the polysaccharides, and the side-chain terminal carboxylate and amide carbonyls of poly-l-glutamate and poly-l-asparagine, and the side chain carbons of poly-l-lysine, closely recapitulate those seen in native bone. Such model HAp−biopolymer composites will prove useful in studies of the role of biopolymers in biomineralization, and in high resolution biomineral structure elucidation.
Kristall‐Tuning: Organische Moleküle können bevorzugt mit Molekülen der eigenen Sorte kristallisieren, was eine Feinabstimmung der Kristalleigenschaften durch Dotierung oder Vermischung verhindert. Nanoporöse Steroide (siehe Bild) bilden eine Ausnahme, weil sie in ihren Kanälen verschiedene Endgruppen (angedeutet durch Sechsecke und Kugeln) beherbergen können. Sie können in beliebigem Verhältnis cokristallisiert werden und liefern eine breite Spanne von chiralen, potenziell porösen kristallinen Materialien.
Amide-linked side-chains can substitute for esters in crystalline nanoporous steroidal ureas (NSPUs). This efficient conjugation method increases the versatility of NPSUs, and should aid the inclusion of complex functional units in the crystal channels.
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