Centrosymmetric crystals for the direct assignment of the absolute configuration of chiral molecules. Application to the .alpha.-amino acids by their effect on glycine crystals

Engineering of Pyroelectric Crystals Decoupled from Piezoelectricity as Illustrated by Doped α‐Glycine

Design of pyroelectric crystals decoupled from piezoelectricity is not only a topic of scientific curiosity but also demonstrates effects in principle that have the potential to be technologically advantageous. Here we report a new method for the design of such materials. Thus, the co-doping of centrosymmetric crystals with tailor-made guest molecules, as illustrated by the doping of α-glycine with different amino acids (Threonine, Alanine and Serine). The polarization of those crystals displays two distinct contributions, one arising from the difference in dipole moments between guest and host and the other from the displacement of host molecules from their symmetry-related positions. These contributions exhibit different temperature dependences and response to mechanical deformation. Thus, providing a proof of concept for the ability to design pyroelectric materials with reduced piezoelectric coefficient (d₂₂ ) to a minimal value, below the resolution limit of the method (<0.005 pm/V).

Modification of Drug Crystallization by Cyclodextrins in Pre-formulation Study

Controlling drug crystallization is one of the important issues in pre-formulation study. In recent years, advanced approaches including the use of tailor-made additives have gathered considerable attention to control crystallization behavior of drugs. This review focuses on the use of hydrophilic cyclodextrins (CDs) as additives for controlling drug crystallization. CDs affect the crystallization of drugs in solution and in solid state based on a host-guest interaction. For example, 2,6-di-O-methyl-β-CD and 2-hydroxybutyl-β-CD suppressed solution-mediated transition of drugs during crystallization by the host-guest interaction; as a result, metastable forms selectively precipitated in solution. The use of CDs in crystal engineering provided an opportunity for the detection of a new polymorph by changing the crystallization pathway. It was also possible to modify crystal morphology (i.e., crystal habit) by selective suppression of crystal growth on a certain direction based on the host-gust interaction. For solid formulation, stable amorphous drug/CDs complex under humid conditions was prepared using two different CDs. An overview of some recent progress in the use of CDs in crystal engineering and in amorphous formulation is described in this review.

Assigning the absolute configuration of single aliphatic molecules by visual inspection

Deciphering absolute configuration of a single molecule by direct visual inspection is the next step in compound identification, with far-reaching implications for medicinal chemistry, pharmacology, and natural product synthesis. We demonstrate the feasibility of this approach utilizing low temperature atomic force microscopy (AFM) with a CO-functionalized tip to determine the absolute configuration and orientation of a single, adsorbed [123]tetramantane molecule, the smallest chiral diamondoid. We differentiate between single enantiomers on Cu(111) by direct visual inspection, and furthermore identify molecular dimers and molecular clusters. The experimental results are confirmed by a computational study that allowed quantification of the corresponding intermolecular interactions. The unique toolset of absolute configuration determination combined with AFM tip manipulation opens a route for studying molecular nucleation, including chirality-driven assembly or reaction mechanisms.

Polar Imperfections in Amino Acid Crystals: Design, Structure, and Emerging Functionalities

Crystals are physical arrays delineated by polar surfaces and often contain imperfections of a polar nature. Understanding the structure of such defects on the molecular level is of topical importance since they strongly affect the macroscopic properties of materials. Moreover, polar imperfections in crystals can be created intentionally and specifically designed by doping nonpolar crystals with "tailor-made" additives as dopants, since their incorporation generally takes place in a polar mode. Insertion of dopants also induces a polar deformation of neighboring host molecules, resulting in the creation of polar domains within the crystals. The contribution of the distorted host molecules to the polarity of such domains should be substantial, particularly in crystals composed of molecules with large dipole moments, such as the zwitterionic amino acids, which possess dipole moments as high as ∼14 D. Polar materials are pyroelectric, i.e., they generate surface charge as a result of temperature change. With the application of recent very sensitive instruments for measuring electric currents, coupled with theoretical computations, it has become possible to determine the structure of polar imperfections, including surfaces, at a molecular level. The detection of pyroelectricity requires attachment of electrodes, which might induce various artifacts and modify the surface of the crystal. Therefore, a new method for contactless pyroelectric measurement using X-ray photoelectron spectroscopy was developed and compared to the traditional periodic temperature change technique. Here we describe the molecular-level determination of the structure of imperfections of different natures in molecular crystals and how they affect the macroscopic properties of the crystals, with the following specific examples: (i) Experimental support for the nonclassical crystal growth mechanism as provided by the detection of pyroelectricity from near-surface solvated polar layers present at different faces of nonpolar amino acid crystals. (ii) Enantiomeric disorder in dl-alanine crystals disclosed by detection of anomalously strong pyroelectricity along their nonpolar directions. The presence of such disorder, which is not revealed by accurate diffraction techniques, explains the riddle of their needlelike morphology. (iii) The design of mixed polar crystals of l-asparagine·H₂O/l-aspartic acid with controlled degrees of polarity, as determined by pyroelectricity and X-ray diffraction, and their use in mechanistic studies of electrofreezing of supercooled water. (iv) Pyroelectricity coupled with dispersion-corrected density functional theory calculations and molecular dynamics simulations as an analytical method for the molecular-level determination of the structure of polar domains created by doping of α-glycine crystals with different l-amino acids at concentrations below 0.5%. (v) Selective insertion of minute amounts of alcohols within the bulk of α-glycine crystals, elucidating their role as inducers of the metastable β-glycine polymorph. In conclusion, the various examples demonstrate that although these imperfections are present in minute amounts, they can be detected by the sensitive pyroelectric measurement, and by combining them with theoretical computations one can elucidate their diverse emerging functionalities.

Origin and structure of polar domains in doped molecular crystals

Doping is a primary tool for the modification of the properties of materials. Occlusion of guest molecules in crystals generally reduces their symmetry by the creation of polar domains, which engender polarization and pyroelectricity in the doped crystals. Here we describe a molecular-level determination of the structure of such polar domains, as created by low dopant concentrations (<0.5%). The approach comprises crystal engineering and pyroelectric measurements, together with dispersion-corrected density functional theory and classical molecular dynamics calculations of the doped crystals, using neutron diffraction data of the host at different temperatures. This approach is illustrated using centrosymmetric α-glycine crystals doped with minute amounts of different L-amino acids. The experimentally determined pyroelectric coefficients are explained by the structure and polarization calculations, thus providing strong support for the local and global understanding of how different dopants influence the properties of molecular crystals.

A molecular dynamics simulation of solvent effects on the crystal morphology of HMX

The solvent has a large effect on the crystal morphology of the organic explosive compound octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX, C(4)H(8)N(8)O(8)). The attachment energy calculations predict a growth morphology in vacuum dominated by (020), (011), (102 ), (111 ) and (100) crystal forms. Molecular dynamics simulations are performed for these crystal faces of HMX in contact with acetone solvent. A corrected attachment energy model, accounting for the surface chemistry and the associated topography (step structure) of the habit crystal plane, is applied to predict the morphological importance of a crystal surface in solvent. From the solvent-effected attachment energy calculations it follows that the (100) face becomes morphologically more important compared with that in vacuum, while the (020) and (102 ) are not visible at all. This agrees well with the observed experimental HMX morphology grown from the acetone solution.

Additive-Induced Metastable Single Crystal of Mefenamic Acid

PURPOSE

To utilize additives to develop a strategy and a method to grow single crystals that allow structure determination of a metastable form of a drug.

MATERIALS AND METHODS

The metastable form of mefenamic acid (MFA) was grown in the presence of various amounts of the structurally similar additive flufenamic acid (FFA) in ethanol. Single crystal X-ray analysis was performed on the single crystals of MFA II that were formed. The solubility of MFA in the presence of FFA was measured to elucidate the mechanism of MFA II formation.

RESULTS

A supersaturated solution of MFA in ethanol produced the metastable form using FFA as an additive. Ethanol-water mixtures and toluene were also used to investigate the relationships between form produced and solvent since these two solvent systems do not produce MFA II.

CONCLUSIONS

Additives can be used to obtain the metastable form of pharmaceutical compounds, and the relationships between molecules and solvent as well as between host and guest molecules are critical to obtaining the desired form.

Parity violating energetic difference and enantiomorphous crystalsp-caveats; reinvestigation of tyrosine crystallization

The present article challenges reports claiming to have demonstrated the Parity Violating Energetic Difference (PVED) between enantiomorphous D- and L-crystals. Apart from PVED, the presence of minute quantities and differing profiles of impurities incorporated during their different history of preparation will affect the physical properties of D- and L-crystals. These impurities are anticipated to play a much greater role in affecting crystallization behavior than PVED. The effect of impurities on the growth and dissolution of enantiomorphous crystals is illustrated with some representative examples. Shinitzky et al. (2002) reported recently dramatic differences in the growth and dissolution properties of the D- and L-crystals of tyrosine. We have repeated these experiments using commercial samples from different sources and employing a validated enantioselective gas chromatographic technique. We attribute Shinitzky's findings either to the use of inappropriate analytical techniques for the determination of enantiomeric composition and/or to the presence of unidentified contaminants in the commercial tyrosine samples. Related caveats hold also for the recently published claims by Shinitzky (2006) and Scolnik et al. (2006) to have observed experimentally PVED between enantiomeric helices of poly-glutamic acid composed of 24 repeating units.

Impact of Molecular Speciation on Crystal Nucleation in Polymorphic Systems: The Conundrum of γ Glycine and Molecular ‘Self Poisoning'

The polymorphism of the simple amino acid glycine has been known for almost a century. It is also known that in aqueous solutions, at the isoelectric point (pI 5.9), the metastable alpha polymorph crystallizes, while the stable gamma form of glycine only nucleates at high and low pH. Despite the importance of understanding the process by which crystals nucleate, the solution and solid-state chemistry underlying this simple observation have never been explored. In this contribution, we have combined solution chemistry, crystallization, and crystallographic data to investigate the mechanisms by which this effect occurs. It is concluded that solution speciation and the consequent interactions between charged species and developing crystal nuclei determine the structural outcome of the crystallization process.

Molecular Recognition at Crystal Interfaces

Nucleation, growth, and dissolution of crystals have been studied by stereochemical approach involving molecular recognition at interfaces. A methodology is described for using ''tailor-made'' additives designed to interact stereospecifically with crystal surfaces during growth and dissolution. This procedure was instrumental in controlling crystal morphology and in revising the concept of the structure and symmetry of solid solutions. Consequently, it was applied to the transformation of centrosymmetric single crystals into solid solutions with polar arrangement displaying second-harmonic generation and to the performance of asymmetric synthesis of guest molecules inside centrosymmetric host crystals. The method has led to a discovery of a new ''relay'' mechanism explaining the effect of solvent on crystal growth. Finally, it allowed for the design of auxiliary molecules that act as promoters or inhibitors of crystal nucleation that can be used to resolve enantiomers and crystallize desired polymorphs.