Antiparallel Chains Research Articles

Stress-strain curves and corresponding structural parameters in mulberry and non-mulberry silk fibers

Mulberry fiber (Bivoltine) and non-mulberry fiber (Tassar) were subjected to stress-strain studies and the corresponding samples were examined using wide angle X-ray scattering studies. Here we have two different characteristic stress-strain curves and this has been correlated with changes in crystallite shape ellipsoids in all the fibers. Exclusive crystal structure studies of Tassar fibers show interesting feature of transformation from antiparallel chains to parallel chains.

Cysteine shotgun–mass spectrometry (CS-MS) reveals dynamic sequence of protein structure changes within mutant and stressed cells

Questions of if and when protein structures change within cells pervade biology and include questions of how the cytoskeleton sustains stresses on cells--particularly in mutant versus normal cells. Cysteine shotgun labeling with fluorophores is analyzed here with mass spectrometry of the spectrin-actin membrane skeleton in sheared red blood cell ghosts from normal and diseased mice. Sheared samples are compared to static samples at 37 °C in terms of cell membrane intensity in fluorescence microscopy, separated protein fluorescence, and tryptic peptide modification in liquid chromatography-tandem mass spectrometry (LC-MS/MS). Spectrin labeling proves to be the most sensitive to shear, whereas binding partners ankyrin and actin exhibit shear thresholds in labeling and both the ankyrin-binding membrane protein band 3 and the spectrin-actin stabilizer 4.1R show minimal differential labeling. Cells from 4.1R-null mice differ significantly from normal in the shear-dependent labeling of spectrin, ankyrin, and band 3: Decreased labeling of spectrin reveals less stress on the mutant network as spectrin dissociates from actin. Mapping the stress-dependent labeling kinetics of α- and β-spectrin by LC-MS/MS identifies Cys in these antiparallel chains that are either force-enhanced or force-independent in labeling, with structural analyses indicating the force-enhanced sites are sequestered either in spectrin's triple-helical domains or in interactions with actin or ankyrin. Shear-sensitive sites identified comprehensively here in both spectrin and ankyrin appear consistent with stress relief through forced unfolding followed by cytoskeletal disruption.

The continuing debate over the structure of cellulose: historical perspective and outlook

The crystalline structure of native cellulose, with its unique microfibrillar morphology resulting from a complex biogenesis mechanism, has not only been the most studied subject in polymer science, but is still generating great interest along with lively debates and controversies. This issue of Cellulose features such a debate in the form of two articles dealing with the conformational analysis of the glucosidic bond in the cellulose chains. This debate is but the latest incident in a long string of events, which represent the foundation of our current understanding of this important material. This makes this an opportune moment to reflect on some of the major milestones in the history of cellulose structure evaluation, which also often involved debates and controversies. It was in 1913, i.e. close to one century ago, that Nishikawa and Ono showed the first diffraction patterns of two cellulose-based plant materials, namely bamboo and hemp, taking advantage of the discovery of the interference phenomena of X-rays reported 1 year earlier by Laue, Friedrich and Knipping. Following this early report, diffraction diagrams of a large number of cellulose materials were duly recorded and subjected to analysis, as it was felt that the knowledge of the crystalline structure of cellulose was of great importance for the development and improvement of cellulose-based products. While recording diffraction patterns of these specimens, it was soon realized that even the X-ray data sets of the most crystalline samples remained limited and it was therefore hard to deduce from them alone the required structural details. It is only when the diffraction data could be complemented by the occurrence of new techniques that substantial advances toward the structural description of cellulose could be made. The sequential emergence of such techniques explains the succession of steps leading to the present understanding. In the progress toward the molecular description of the crystal structure of cellulose from X-ray diffraction data, a first landmark was the incorporation of the stereochemistry of the glucosyl moieties in the molecular models built by, among others, Meyer and Mark (1928), later revised by Meyer and Misch (1937), who presented a model where the crystal of cellulose consisted of two antiparallel chains. This last model, which remained the established standard for at least two decades, was confronted with a series of conflicting data, resulting from new techniques. In 1958, the recording of electron diffraction diagrams of frozen Valonia cell wall fragments allowed Honjo and Watanabe to obtain diffraction data sets far better H. Chanzy—affiliated with the Universite Joseph Fourier and member of the Institut de Chimie Moleculaire de Grenoble.

Supramolecular patterns in benzyladeniniump-toluenesulfonate

In the title compound (systematic name: 6-benzylamino-7H-purin-3-ium p-toluenesulfonate), C(12)H(12)N(5)(+)·C(7)H(7)O(3)S(-), the adenine moiety exists as the N(3)-protonated N(7)-H tautomer. The dihedral angle between the adenine ring system and the phenyl ring is 82.76 (11)°. Two of the sulfonate O atoms form C-H···O and N-H···O hydrogen bonds with the H atoms on the N and C atoms in the 3- and 8-positions, respectively, of the adenine moiety, leading to a zigzag chain. Two antiparallel zigzag chains are linked by the remaining sulfonate O atom through Hoogsteen-site H atoms (i.e. those on the N atoms in the 6- and 7-positions) of the adenine moiety, leading to a double chain. An annulus formed by a pair of inversion-related anions and cations has been identified. An intramolecular toluenesulfonate-phenyl C-H···π interaction is also present.

Compared structure and morphology of nylon-12 and 10-polyurethane lamellar crystals

A comparative study of the structure and morphology of nylon-12 and 10-polyurethane (10-PUR) lamellar crystals, was carried out. Lamellar crystals were obtained by isothermal crystallization from diluted solution. Electron diffraction of lamellae combined with WAXS data recorded from crystal sediments indicated that nylon-12 crystallized in either α-form or γ-form according to the solvent chosen for crystallization. The α-form was the crystal modification predominant in doubly oriented films of nylon-12 prepared by epitaxial crystallization. On the contrary, 10-PUR invariably crystallized in α-form regardless crystallization conditions. The α-form of nylon-12 and 10-PUR shared the same crystal structure with hydrogen-bonded sheets made of antiparallel chains and stacked with progressive shifting along both b and c directions. Lamellar crystals of nylon-12 in γ-form and 10-PUR in α-form displayed similar morphological features but only the former appeared to be sensitive to temperature. Upon heating the nylon-12 crystals near to melting, the real-time WAXS analysis evidenced the occurrence of a partial γ-to-α crystal transition, and in situ AFM observations revealed the appearing of more or less regular ridges on the crystal surface. None of these changes were observed in 10-PUR crystals when subjected to similar treatment.

Polymorphism in a Symmetrical Dipeptidyl Urea with Z′ > 1

Two polymorphic forms of a symmetrical Aib containing urea derivative, DIPUR(α) and DIPUR(β), have been structurally characterized using synchrotron/X-ray diffraction data with a detailed analysis of Hirshfeld surfaces and fingerprint plots facilitating a comparison of the type and nature of intermolecular interactions in the supramolecular architectures. Structural study of DIPUR(α) crystallizing in a Sohnke space group P21 reveals four crystallographically independent molecules (A, B, C, and D) in the asymmetric unit with the pairs of molecules A, B and C, D interlinked via N−H···O hydrogen bonds forming antiparallel one-dimensional chains having a C22(8)R21(6)R21(6) sequence. The combination of intermolecular N−H···O and C−H···O hydrogen bonds generates a three-dimensional framework in DIPUR(α). In the polymorph DIPUR(β) with space group P212121 and Z′ = 2, the N−H···O-bonded R22(11) rings propagate along the [001] direction through C22(8) chains to form a one-dimensional molecular assembly. The Aib re...

Thermal and X-ray powder diffraction structural characterization of two benfluorex hydrochloride polymorphs

Two polymorphic forms of benfluorex hydrochloride, phases I and II, were isolated as monophasic polycrystalline samples, and structurally characterized using ab initio X-ray powder diffraction methods and a global optimization strategy (simulated annealing). Form I crystallizes in monoclinic system, space group P2 1/ n, with Z = 4, a = 21.0719(10) Å, b = 7.0563(4) Å, c = 14.8684(7) Å, β = 116.998(3)°, V = 1969.8(2) Å 3, while Form II crystallizes in the orthorhombic space group Pbca, with Z = 8, a = 33.8031(2) Å, b = 15.1451(8) Å, c = 7.6138(6) Å, V = 3897.9(4) Å 3. Crystals of Form I and Form II of benfluorex hydrochloride are based upon an ionic packing of protonated benfluorex molecules at the most basic site, the N1 atoms, and chloride anions. Form I shows the presence of μ-Cl ions, generating centrosymmetric dimers with a N 2Cl 2 moiety, while Form II contains antiparallel chains of C–H⋯O hydrogen-bonded molecules running along c axis. DSC and thermodiffractometric measurements showed that heating progressively Form II from ambient temperature to 160 °C causes a phase transition to the thermodynamically stable Form I, immediately followed by the sample melting, near 165 °C. Recrystallization directly to Form I is observed when the melt is cooled back to ambient temperature, with a significant hysteresis (this event being centered near 130 °C).

Formation and Assembly−Disassembly Processes of ZnO Hexagonal Pyramids Driven by Dipolar and Excluded Volume Interactions

ZnO hexagonal pyramids were obtained in hydrophilic media without any traditional stabilizers (capping agents). The absence of a thick organic shell reducing the anisotropy of nanoparticle (NP) interactions, oxide nature of the materials, and new geometry of the nanocrystals makes possible the observation of new self-organization phenomena. Several new features not present in the previous cases of NP self-organization were identified and discussed. The formation of ZnO pyramids involved recrystallization of larger amorphous NPs followed by the multistage disassembly of intermediate aggregates into individual virtually perfectly shaped nanocrystals. The evolution of NPs begins with crystallization of clustered plates within the original amorphous spherical colloids, and then agglomerated truncated pyramids are formed. These agglomerates further transform into chained pyramids, which eventually separate from each other. The crystallization and disassembly processes can be associated with the decrease of potential and anisotropy of the attractive force field around the crystallites represented in part by dipole moments. The reassembly of the pyramids can still be attained via engaging excluded volume interaction after adding similarly charged polymer. Overall, in this system, we see the first examples of (1) coupled crystallization and disassembly process; (2) induced assembly of nanoscale particles using excluded volume interactions, which were previously used only for aggregation of microscale colloids; and (3) nanoparticle assemblies with variable and experimentally verifiable relative orientation of dipoles including head-to-tail, tail-to-tail pairs, and antiparallel chains. Described assemblies of ZnO pyramids with collective behavior of individual building blocks as well as distinct and experimentally controlled stages of assembly and disassembly present a fundamentally interesting nanoparticle system with rich dynamic behavior.

2,2′-o-Phenylenediacetonitrile

In the title compound, NCCH2C6H4CH2CN, the bond lengths and angles are within normal ranges. The benzene ring makes dihedral angles of 4.94 (8) and 77.04 (8)° with the C—C—C—N mean planes. Weak non-conventional C—H⋯N hydrogen bonds are effective in the stabilization of the crystal structure. The weak C—H⋯N contacts form anti­parallel chains running in the a + c direction, and ring systems with two N-atom acceptors and four H-atom donors.

S-2-Amino-5-(dimethylammonio)phenyl sulfothioate

The title compound, C8H12N2O3S2, has been isolated as an inter­mediate in the synthesis of methyl­ene blue dye, the best known phenothia­zine dye, and structurally characterized as a zwitterion. The crystal structure is dominated by inter­molecular N—H⋯O hydrogen bonds between the amine and sulfothio­ate groups, with graph-set motif C(9)R 2 2(8), involving anti­parallel chains and a centrosymmetric eight-membered ring. A hydrogen bond with graph-set motif R 2 2(14) between the ammonium and sulfothio­ate groups completes the two-dimensional network in the ab plane. Inter­molecular C—H⋯O hydrogen bonds are also present in the crystal.

Non-strict strand orientation of the Ca 2+-induced dimerization of a conantokin peptide variant with sequence-shifted γ-carboxyglutamate residues

We have previously found a new mode of metal ion-induced helix–helix assembly for the γ-carboxyglutamate (Gla)-containing, neuroactive conantokin (con) peptides that is independent of the hydrophobic effect. In these unique “metallo-zipper” assemblies of con-G and con-T[K7γ], interhelical Ca 2+ coordination induces dimer formation with strictly antiparallel chain orientation in conantokin peptides in which Gla residues are positioned at “i, i + 4, i + 7, i + 11” intervals. In order to probe the property of self-assembly in conantokin peptides with an extended Gla network, a con-T variant (con-T-tri) was synthesized that contains five Gla residues spaced at “i, i + 4, i + 7, i + 11, i + 14” intervals. Sedimentation equilibrium analyses showed that Ca 2+, but not Mg 2+, was capable of promoting con-T-tri self-assembly. Oxidation and rearrangement assays with Cys-containing con-T-tri variants revealed that the peptide strands in the complex can orient in both parallel and antiparallel forms. Stable parallel and antiparallel dimeric forms of con-T-tri were modeled using disulfide-linked peptides and the biological viability of these species was confirmed by electrophysiology. These findings suggest that small changes within the helix–helix interface of the conantokins can be exploited to achieve desired modes of strand alignment.

X-ray crystallographic, scanning microprobe X-ray diffraction, and cross-polarized/magic angle spinning 13C NMR studies of the structure of cellulose III(II).

The X-ray crystallographic structure of cellulose III(II) is characterized by disorder; the unit cell (space group P2(1); a = 4.45 A, b = 7.64 A, c = 10.36 A, alpha = beta = 90 degrees, gamma = 106.96 degrees) is occupied by one chain that is the average of statistically disordered antiparallel chains. 13C CP/MAS NMR studies reveal the presence of three distinct molecular conformations that can be interpreted as a mixture of two different crystal forms, one equivalent to cellulose III(I), and another with two independent glucosyl conformations in the asymmetric unit. Both X-ray crystallographic and 13C NMR spectroscopic results are consistent with an aggregated microdomain structure for cellulose III(II). This structure can be generated from a new crystal form (space group P2(1); a = 4.45 A, b = 14.64 A, c = 10.36 A, alpha = beta = 90 degrees, gamma = 90.05 degrees; two crystallographically independent and antiparallel chains; gt hydroxymethyl groups) by multiple dislocation defects. These defects produce microdomains of the new crystal form and cellulose III(I) that scanning microprobe diffraction studies show are distributed consistently through the cellulose III(II) fiber.

Synthesis, characterization and structure of the first rhenium compound of di-2-pyridyl ketone thiophene-2-carboxylic acid hydrazone (dpktah), fac-[Re(CO) 3( N, N-κ 2-dpktah)Cl

fac-[Re(CO) 3(κ 2- N, N -dpktah)Cl], isolated from the reaction between [Re(CO) 5Cl] and di-2-pyridyl ketone thiophene-2-carboxylic acid hydrazone (dpktah) in refluxing toluene, exhibits rich physico-chemical properties. The formulation of fac-[Re(CO) 3(κ 2- N, N -dpktah)Cl] was established from the results of its elemental analysis and spectroscopic measurements, and confirmed using X-ray crystallography. The 1H NMR spectrum of fac-[Re(CO) 3(κ 2- N, N-dpktah)Cl] revealed the coordination of dpktah and exchange of the amide proton. Electronic absorption measurements show two intra-ligand charge transfer transitions (ILCT) and established inter-conversion between fac-[Re(CO) 3(κ 2- N, N-dpktah)Cl] and its conjugate base. Thermo-optical studies confirmed the facile inter-conversion between fac-[Re(CO) 3(κ 2- N, N-dpktah)Cl] and its conjugate base. Optosensing measurements show [MCl 2] (M = Zn, Cd or Hg) in concentrations as low as 1.00 × 10 −7 M can be detected and determined using protophilic solutions of fac-[Re(CO) 3(κ 2- N, N-dpktah)Cl]. X-ray structural analysis done on a single crystal of fac-[Re(CO) 3(κ 2- N, N-dpktah)Cl] confirmed its identity and divulge two symmetry-independent molecules in the asymmetric unit. The supramolecular structure of fac-[Re(CO) 3(κ 2- N, N-dpktah)Cl] disclosed anti-parallel chains locked via a network of hydrogen bonds. Non-classic hydrogen bonds of the type C–H…Cl connect molecules in the chain and classic hydrogen bonds of the type N–H…O connect adjacent chains.

Solid state molecular assemblies of five bile acid derivatives

Abstract The crystal structures of five bile acid derivatives are reported: 1: 6α,7α-Dihydroxy-5β-cholan-24-oic acid; 2: 3α,6α-Dihydroxy-5β-cholan-24-oic acid (Hyodeoxycholic acid); 3: 3α,7α,12α-Trihydroxy-5β-cholan-24-hydrazide hemihydrate; 4: 3-Dimethyl ketal,7,12-dioxo-5β-cholan-24-oic acid and 5: 3α,7α-Di-O-acetyl-12-oxo-5β-cholan-24-oic acid methyl ester. In all the structures the four saturated cycles, forming the common alicyclic steroidal skeleton, have the same conformation, while the flexible side chain adopts different conformations. Whereas all the crystal structures of Cholic Acid inclusion compounds are characterized by the formation of a bilayer-type structure with channels within the lipophilic layers, the chemical variations of the substituents on the cholanic framework induce remarkable changes in the H-bond scheme and in the crystal packing arrangements. For instance, the structures 1, 2, having a different number and position of the hydroxyl groups on the rigid skeleton, display a variety of supramolecular architectures dominated by networks of cooperative …O—H…O… H-bonds which do not produce typical molecular layers and channels of Cholic Acid inclusion compounds. Cholic hydrazide hemihydrate, 3, where both the hydrazide group and water molecule are strongly involved in the H-bond system, forms an overall structural motif similar to that observed in CA inclusion compounds containing hydrophilic and lipophilic layers and small channels. Compound 4, containing only the carboxylic OH group as H-bond donor, makes simple antiparallel chains of molecules, while compound 5, lacking of H-bond donors, forms a crystal aggregation dominated by weak C—H…O contacts and van der Waals interactions leading to a crystal packing where neither molecular layers nor channels are recognizable.

Structure, mechanism and regulation of pyruvate carboxylase.

PC (pyruvate carboxylase) is a biotin-containing enzyme that catalyses the HCO(3)(-)- and MgATP-dependent carboxylation of pyruvate to form oxaloacetate. This is a very important anaplerotic reaction, replenishing oxaloacetate withdrawn from the tricarboxylic acid cycle for various pivotal biochemical pathways. PC is therefore considered as an enzyme that is crucial for intermediary metabolism, controlling fuel partitioning toward gluconeogenesis or lipogenesis and in insulin secretion. The enzyme was discovered in 1959 and over the last decade there has been much progress in understanding its structure and function. PC from most organisms is a tetrameric protein that is allosterically regulated by acetyl-CoA and aspartate. High-resolution crystal structures of the holoenzyme with various ligands bound have recently been determined, and have revealed details of the binding sites and the relative positions of the biotin carboxylase, carboxyltransferase and biotin carboxyl carrier domains, and also a unique allosteric effector domain. In the presence of the allosteric effector, acetyl-CoA, the biotin moiety transfers the carboxy group between the biotin carboxylase domain active site on one polypeptide chain and the carboxyltransferase active site on the adjacent antiparallel polypeptide chain. In addition, the bona fide role of PC in the non-gluconeogenic tissues has been studied using a combination of classical biochemistry and genetic approaches. The first cloning of the promoter of the PC gene in mammals and subsequent transcriptional studies reveal some key cognate transcription factors regulating tissue-specific expression. The present review summarizes these advances and also offers some prospects in terms of future directions for the study of this important enzyme.

(E)-3-(4-Chlorophenyl)-1-(2,4-dichloro-5-fluorophenyl)prop-2-en-1-one

In the title chalcone derivative, C15H8Cl3FO, the dihedral angle between the two benzene rings is 43.35 (8)°. Weak C—H⋯O and C—H⋯Cl intra­molecular inter­actions involving the enone group generate S(5) and S(6) ring motifs, respectively. In the crystal structure, mol­ecules are linked into anti­parallel chains along the a axis. These chains are stacked along the b axis and short Cl⋯F contacts of 3.100 (1) Å link adjacent mol­ecules of the anti­parallel chains into dimers.

Two-Dimensionally Well-Ordered Multilayer Structures in Thin Films of a Brush Polypeptide

In this study, we report the first production of two-dimensionally well-ordered molecular multilayers (i.e., with a well-defined molecular lamellar structure) based on the antiparallel beta-sheet chain conformation in thin films of a brush polypeptide, poly(S-n-hexadecyl-dl-homocysteine) (PHHC), through the use of a simple spin-coating process and the quantitative structural and property analysis of the thin films using a grazing incidence X-ray scattering technique combined with Fourier transform infrared spectroscopy and differential scanning calorimetry. These analyses provide detailed information about the structure and molecular conformation of the self-assembled lamellae in the PHHC thin film, which is not easily obtained using conventional techniques. Moreover, we used the in situ measurements carried out at various temperatures and the data analyses to establish mechanisms for the evolution of the self-assembled lamellar structures in the film and for their melting. In addition, we propose molecular structure models of the PHHC polymer molecules in the thin film at various temperatures.

Hypothesis of initiation of DNA methylation de novo and allelic exclusion by small RNAs

We have established that 5′-CG-3′ dinucleotide and 5′-CNG-3′ trinucleotide are found in published sequences of small interfering RNA and microRNA more often than they should be in random DNA sequences. This circumstance indicates the important biological role played by 5′-CG-3′ dinucleotides and 5′-CNG-3′ trinucleotides in small RNA sequences. We suggest that small RNAs containing these di- and trinucleotides participate in the creation of chromatin marks of epigenetic information through a highly specific search for repressible DNA sequences and through the initiation of the methylation de novo of 5′-CG-3′ and 5′-CNG-3′ sites in DNA fragments appearing to be bound complementary to small RNAs. Several genes can be inactivated simultaneously if they contain the motif recognized by small RNA. Allelic exclusion appears, in our opinion, as a result of initiation by small RNAs of DNA methylation de novo of all but one of the alleles that exist in the cell. The predecessor of this small RNA is transcribed from the antiparallel allele chain. Alleles whose antiparallel chains are less actively read by RNA polymerase, which, as we suggest, in the process of transcribing, releases DNA from small RNA bound to it, are inactivated. However, the quantity of small RNA transcribed from only one allele is insufficient to overcome the level above which the repression process of this allele is initiated de novo.

OH–π and halogen–π interactions as driving forces in the crystal organisations of tri-bromo and tri-iodo trityl alcohols

The trityl alcohols bearing three bromine or three iodine atoms at the para-positions of the aromatic units, have been known for more than a hundred years. In our case these compounds have been synthesized in one-pot sequence starting from the 1,4-dihalogenobenzenes via mono-lithiation and the successive reaction with diethylcarbonate. The compounds have been crystallized from different solvent mixtures leading to one structure of bromo- (A) and three structures of iodo trityl alcohols (B–D). The inclusion of dichloromethane (C) or benzene (D) in the crystalline lattices has been observed. In all cases the OH–π and halogen–π (and in one case the halogen-halogen and CH-O weak) contacts play crucial role in the crystal organisation. The common motif of the packing is the anti parallel arrangement of the trityl alcohol molecules into infinite chains. There is the gradual change in the intermolecular interactions going from the structures A and B, where only OH–π contacts between anti-parallel molecules are observed, to C with OH–π bifurcated contacts between anti-parallel molecules and further to the structure D, where practically no OH–π contacts are observed any longer. However in the case of D the splitting of the chain with anti-parallel molecules into two anti-parallel chains with parallelly assembled molecules takes place. These OH–π chains interact further with each other either via halogen–π or/and halogen–halogen, CH–O weak interactions. The shortest contacts found are 3.429 Å (1/A, Br–π) and 3.486 Å (2/D, I–π) while the longest contacts are 3.558 Å (1/A, Br–π) and 3.668 Å (2/B, I–π). No short contacts between the included solvent molecules and triiodo trityl alcohol within the structures C and D, have been observed. The quantum chemical calculations (DFT on the M05-2X/6-311G(d,p) level) gave the following interaction energies (kcal mol−1) between two neighbouring molecules of one “OH–π chain”: −7.96 (A), −6.60 (B), −6.43 (C) and −6.45 (D), while the interaction energies via the halogen–π contacts vary from −2.17 to −3.58 kcal mol−1 (per one contact).

Halogen/methyl exchange in a series of isostructural 1,3-bis(m-dihalophenyl)ureas

The solid state structures of a family of 1,3-bis(m-di-X-phenyl)ureas where X = Cl, Br, I and their mono- and di-tolyl analogues were determined. Crystal growth from a variety of solutions yielded isostructural and isomorphous P21212 phases for all compounds. The molecules associate into infinite [R12(6)] hydrogen bonded urea chains which are reinforced through π–π stacking interactions of neighboring phenyl rings within the chain. Short halogen–halogen contacts between anti-parallel urea chains are also observed. Ab initio calculations indicate that the rotational barrier about the amide bond is small, and that the molecular conformations observed in the crystal structures are close in energy to the calculated minima. These calculations suggest that a variety of low energy geometric conformations may be present in solution. Using a small scale conventional polymorph screening protocol (∼9 crystallization solvents and combinations thereof), analysis of the phase purity of powders obtained from a variety of solutions revealed the orthorhombic phase to be the predominant form for all compounds in all solvents. However, additional peaks were observed in select powder X-ray diffraction patterns, leaving open the possibility that concomitant polymorphism or solvate formation occurs for some compounds under particular crystallization conditions.