No abstract

The control of crystal polymorphism is a long-standing issue in solid-state chemistry, which has many practical implications for a variety of commercial applications. At least four different crystalline forms of 1,3-bis(m-nitrophenyl) urea (MNPU), a classic molecular crystal system, are known to crystallize from solution in various concomitant combinations. Herein we demonstrate that the introduction of gold-thiol self-assembled monolayers (SAMs) of substituted 4'-X-mercaptobiphenyls (X = H, I, and Br) into the crystallization solution can serve as an effective means to selectively template the nucleation and growth of alpha-, beta-, and gamma-MNPU phases, respectively. Polymorph control in the presence of SAM surfaces persists under a variety of solution conditions and consistently results in crystalline materials with high phase purity. The observed selectivity is rationalized on the basis of long-range two-dimensional geometric lattice matching and local complementary chemical interactions at the SAM/crystal interfaces.

Crystalline clathrates formed from two-dimensional guanidinium sulfonate hydrogen-bonded networks connected by 4,4‘-biphenyldisulfonate “pillars” in the third dimension exhibit a “brick-like” molecular framework that is a predictable architectural isomer of a previously observed bilayer architecture based on the same pillars. The amount of void space in the brick framework is nominally twice that of the bilayer form, with the framework occupying only 30% of the total volume. The formation of the brick architecture can be attributed to steric templating by the included molecular guests and host−guest interactions that favor assembly of this framework over its bilayer counterpart. The brick framework conforms to the different steric demands and occupancies of various aromatic guests (1,4-dibromobenzene, 1-nitronaphthalene, nitrobenzene, and 1,4-divinylbenzene) by puckering of the flexible, yet resilient, hydrogen-bonded network and by rotation of the pillars about their long axes, the latter also governing the width of the pores in the framework. These observations demonstrate that cystal engineering, and the ability to direct architectural isomerism in porous molecular lattices by the appropriate choice of molecular guest, is simplified by the use of robust 2-D networks.

Atomic force microscopy (AFM) and chemical force microscopy (CFM) techniques have been used to characterize the chemical functionality of cholesterol monohydrate single-crystal surfaces in different solution environments. Both synthetic and natural crystals adopt a platelike habit in which the largest face is (001). Under aqueous and organic solution conditions, in situ contact mode topographical images reveal that the plate face is primarily terminated by bilayer molecular structures and therefore largely homogeneous. Adhesion force measurements obtained with chemically modified tips demonstrate that the functionality of the crystal surface can be altered by changes in the solution composition. The 3-hydroxyl end of cholesterol molecules is presented on the plate face in aqueous media, while alkyl tail groups terminate the surface in organic solutions. Contact angle measurements on (001) surfaces exposed to solvents of different hydrophilicity also show similar trends, providing additional support to these AFM assignments. These studies link molecular-level and macroscopic adhesion properties and demonstrate that solution environment can exert a strong influence on the surface properties of this important biomaterial.

Because tissues from all three germ layers contribute to the pharyngeal arches, it is not surprising that all major signaling pathways are involved in their development. We focus on the role of retinoic acid (RA) signaling because it has been recognized for quite some time that alterations in this pathway lead to craniofacial malformations. Several studies exist that describe phenotypes observed upon RA perturbations in pharyngeal arch development; however, these studies did not address whether RA plays multiple roles at distinct time points during development. Here, we report the resulting phenotypes in the hindbrain, the neural crest-derived tissues, and the pharyngeal endoderm when RA synthesis is disrupted during zebrafish gastrulation and pharyngeal arch morphogenesis. Our results demonstrate that RA is required for the postgastrulation morphogenesis and segmentation of endodermal pouches, and that loss of RA does not affect the length of the pharyngeal ectoderm or medial endoderm along the anterior-posterior axis. We also provide evidence that RA is not required for the specification of pharyngeal pouch endoderm and that the pharyngeal endoderm consists of at least two different cell populations, of which the pouch endoderm is sensitive to RA and the more medial pharyngeal endoderm is not. These results demonstrate that the developmental processes underlying pharyngeal arch defects differ depending on when RA signaling is disturbed during development.

The growth of anhydrous uric acid (UA) and uric acid dihydrate (UAD) crystals from supersaturated aqueous solutions containing methylene blue, a cationic organic dye, has been investigated. Low concentrations of dye molecules were found to be included in both types of crystal matrixes during the growth process. Incorporation of dye into UA crystals occurs with high specificity, affecting primarily [001] and [201] growth sectors, while UAD crystals grown from solutions of similar dye concentration show inclusion but little specificity. The orientation of the UA-trapped species was determined from polarization data obtained from visible light microspectrometry. To achieve charge neutrality, a second anionic species must also be included with the methylene blue into UA and UAD crystal matrices. Under high pH conditions, crystallization of 1:1 stoichiometric mixtures of methylene blue and urate yields methylene blue hexahydrate (MBU.6(H2O). The crystal structure of MBU.6(H2O) reveals continuous pi-pi stacks of planes of dye cations and urate anions mediated by water molecules. This structure provides an optimal geometry for methylene blue-urate pairs and additional support for the incorporation of these dimers in uric acid single-crystal matrices. The strikingly different inclusion patterns in UA and UAD demonstrate that subtle changes in the crystal surfaces and/or growth dynamics can greatly affect recognition events.

The secondary explosive RDX (1,3,5-trinitrohexahydro-1,3,5-triazine) has five known polymorphic forms, though only the α and β phases are observable at room temperature and pressure. Many literature reports suggest that the metastable β-RDX polymorph is extremely rare due to the limited number of solvents from which it can be grown and its facile conversion to the more stable α-RDX form. Herein we show that β-RDX can be consistently obtained using drop cast crystallization methods from a broad range of solvents [acetone, tetrahydrofuran (THF), nitromethane, dimethylsufoxide (DMSO)] and that crystals grown this way remain stable over extended periods of time up to a year. The distribution of α:β crystals as a function of solvent and drop concentration on various modified and unmodified glass surfaces was determined using Raman microscopy. β-RDX was the most prevalent phase deposited from dilute drops regardless of the solvent, though more concentrated DMSO drops tended to give either β or α exclusively. In cases where concomitant growth was observed, β crystals were typically smaller than α-RDX crystals. The morphological and thermal properties of β-RDX crystals were assessed, allowing for the most thorough characterization of the physical–chemical properties of this metastable phase to date. Mechanistic differences in the decomposition pathway of α-RDX and β-RDX single crystals upon exposure to electromagnetic radiation suggest some interesting intrinsic differences in their material properties.

A large percentage of molecular compounds can crystallize in different hydration states. Although hydrated and anhydrous crystal forms can exhibit different physical properties (e.g., solubility, stability, mechanical strength), establishing the contribution water makes to the properties can be elusive. Anhydrous (UA) and dihydrate (UAD) crystal forms of uric acid share a remarkably similar two-dimensional layer structure, though the presence or absence of water between the layers imparts these crystal forms with dramatically different mechanical properties. The quantitative and qualitative differences in how these two materials respond to uniaxial stress were investigated with nanoindentation and atomic force microscopy (AFM) imaging normal to the layer direction. Overall, UA was found to be both substantially harder and more brittle than UAD. Load–displacement curves and AFM images of the UA crystal surface postindent reveal slip planes in preferred crystallographic directions and oriented crack formation at higher load forces. UAD was markedly softer and also exhibited substantial creep in response to indentation. Time lapsed images of UAD indents suggest that some amount of “self-healing” on the surface may be possible at shallow indentation depths of ∼100 nm.