1H isotropic chemical shift metrics for NMR crystallography of powdered molecular organics

Hydrogen magnetic shielding values from gauge including projector augmented wave (GIPAW) density functional theory (DFT) calculations, when combined with experimental solid-state 1H nuclear magnetic resonance (NMR) chemical shift data collected on powdered microcrystalline organics, have been used to perform various crystal structure characterization tasks (e.g., refinements, verifications, determinations). These tasks fall under the umbrella of 'NMR crystallography'. In several instances, an isotropic 1H chemical shift (δiso) root-mean-squared deviation (RMSD) metric has been applied during these studies (including the first de novo crystal structure determination: M. Baias, J.-N. Dumez, P. H. Svensson, S. Schantz, G. M. Day and L. Emsley, De Novo Determination of the Crystal Structure of a Large Drug Molecule by Crystal Structure Prediction-Based Powder NMR Crystallography, J. Am. Chem. Soc., 2013, 135, 17501-17507). While it is assumed that the 1H δiso RMSD metrics are converged, our study probes the robustness of these metrics. Specifically, we consider how the structure of the δiso(1H) RMSD metric varies depending on: (i) selected GIPAW DFT input parameters; (ii) the number of fitting parameters used during linear mapping; and (iii) the GIPAW DFT computational software. These δiso(1H) RMSD metrics were produced from a set of 24 benchmark crystal structures (428 crystallographically unique hydrogen atom environments). Interestingly, we find that the δiso(1H) RMSD metric structures are very robust to substantial degradation in the quality of the GIPAW DFT computations, which is unexplored in the NMR crystallography literature as prior studies focus on convergence rather than divergence. We then briefly consider the impact of our findings using the structure determination of thymol as an illustrative example and our results strongly suggest that if δiso(1H) RMSD metrics are being used, then the GIPAW DFT computations can be performed much more efficiently than at present. Overall, this should allow for more efficient NMR crystallography characterization tasks of important materials that contain 1H nuclei, such as organic pharmaceuticals.

Molecular insights into kinetic stabilization of amorphous solid dispersion of pharmaceuticals

Poor aqueous solubility of crystalline active pharmaceutical ingredients (APIs) restricts their bioavailability. Amorphous solid dispersions with biocompatible polymer excipients offer a solution to overcome this problem, potentially enabling a broader use of many drug candidate molecules. This work addresses various aspects of the in silico design of a suitable combination of an API and a polymer to form such a binary solid dispersion. Molecular interactions in such bulk systems are tracked at full atomic resolution within molecular-dynamics (MD) simulations, enabling to identify API-polymer pairs that exhibit the most beneficial interactions. Importance of these interactions is manifold: increasing the mutual miscibility, kinetic stabilization of their amorphous dispersions and impedance of the spurious recrystallization of the API component. MD tools are used to investigate the structural and cohesive properties of pure compounds and mixtures, with a special emphasis on molecular interactions, microscopic structures and internal dynamics. This analysis is then accompanied by a macroscopic image of the energetic compatibility and vitrification tendency of the mixtures in terms of their excess enthalpies and glass transition temperatures. Density-functional theory (DFT) and non-covalent interaction (NCI) analysis fortify our computational conclusions and enable us to map the intensities of particular NCI among the individual target materials and relevant molecular sites therein. Three archetypal polymer excipients and four API molecules are included in this study. The results of our computational analysis of molecular interactions in bulk systems agree with the experimentally observed trends of solubility of the given API in polymers. Our calculations confirm PVP as the most potent acceptor of hydrogen bonding among the three considered polymer excipients, whereas ibuprofen molecules are predicted to be the most efficient hydrogen bond donors among our four target APIs. Our simulations also suggest that carbamazepine does not exhibit particularly strong interactions with the considered polymer excipients. Although current MD cannot offer quantitative accuracy of many of the discussed descriptors, current computational models focusing on NCI of APIs with polymer excipients contribute to understanding of the behavior of these materials at the molecular level, and thus also to the rational design of novel efficient drug formulations.

Using synchrotron high-resolution powder X-ray diffraction for the structure determination of a new cocrystal formed by two active principle ingredients

The crystal structure of a new 1:1 cocrystal of carbamazepine and S-naproxen (C15H12N2O·C14H14O3) was solved from powder X-ray diffraction (PXRD). The PXRD pattern was measured at the high-resolution beamline CRISTAL at synchrotron SOLEIL (France). The structure was solved using Monte Carlo simulated annealing, then refined with Rietveld refinement. The positions of the H atoms were obtained from density functional theory (DFT) ground-state calculations. The symmetry is orthorhombic with the space group P212121 (No. 19) and the following lattice parameters: a = 33.5486 (9), b = 26.4223 (6), c = 5.3651 (10) Å and V = 4755.83 (19) Å3.

Crystal structure of 3-phenylpropyl 2-(6-methoxynaphthalen-2-yl)propanoate, C23H24O3

C23H24O3, orthorhombic, P212121 (no 19), a = 5.7576(2) Å, b = 7.7223(4) Å, c = 41.6710(16) Å, V = 1852.77(14) Å3, Z = 4, R gt(F) = 0.0442, wR ref(F 2) = 0.1068, T = 170 K.

Structure and Glass Transition Temperature of Amorphous Dispersions of Model Pharmaceuticals with Nucleobases from Molecular Dynamics

Glass transition temperature (Tg) is an important material property, which predetermines the kinetic stability of amorphous solids. In the context of active pharmaceutical ingredients (API), there is motivation to maximize their Tg by forming amorphous mixtures with other chemicals, labeled excipients. Molecular dynamics simulations are a natural computational tool to investigate the relationships between structure, dynamics, and cohesion of amorphous materials with an all-atom resolution. This work presents a computational study, addressing primarily the predictions of the glass transition temperatures of four selected API (carbamazepine, racemic ibuprofen, indomethacin, and naproxen) with two nucleobases (adenine and cytosine). Since the classical non-polarizable simulations fail to reach the quantitative accuracy of the predicted Tg, analyses of internal dynamics, hydrogen bonding, and cohesive forces in bulk phases of pure API and their mixtures with the nucleobases are performed to interpret the predicted trends. This manuscript reveals the method for a systematic search of beneficial pairs of API and excipients (with maximum Tg when mixed). Monitoring of transport and cohesive properties of API-excipients systems via molecular simulation will enable the design of such API formulations more efficiently in the future.

The nature of hydrogen-bonding interactions in nonsteroidal anti-inflammatory drugs revealed by polarized IR spectroscopy

The influence of hydrogen-bonding interactions in the solid phase on the IR spectroscopic pattern of the ν_OH band of nonsteroidal anti-inflammatory drugs (NSAIDs) was studied experimentally by IR spectroscopy with the use of polarized light at two temperatures (293K and 77K) and in isotopic dilution. The neat and deuterated crystals of (S)-naproxen ((S)-NPX), (R)-flurbiprofen ((R)-FBP), (RS)-flurbiprofen ((RS)-FBP) and (RS)-ketoprofen ((RS)-KTP) were obtained by melt crystallization between the two squeezed CaF₂ plates. The vibrational spectra of selected α-aryl propionic acid derivatives (2APAs) reflected the characteristics of their hydrogen-bond networks, i.e., 2APAs were characterized by the chain ((S)-NPX, (R)-FBP) and by dimeric ((RS)-FBP, (RS)-KTP) arrangement of hydrogen bonds in the crystal lattice. Spectroscopic results showed that the interchain (through-space) exciton coupling, between two laterally-spaced hydrogen bonds, dominates in the crystals of four NSAIDs. The same exciton coupled hydrogen bonds were also responsible for the H/D isotopic recognition mechanism in the crystalline spectra of deuterated 2APAs. The presented spectral results may help to predict the hydrogen bond motifs in the crystalline NSAIDs, which structures are not yet known, based on their IR spectra of hydrogen bond in the crystals.

Exploring polymorphism and stoichiometric diversity in naproxen/proline cocrystals

This work studies a multitude of cocrystals obtained by combining chiral and/or racemic naproxen and proline, 17 in total.

Properties of Self-Assembled Monolayers Revealed via Inverse Tensiometry

Self-assembled monolayers (SAMs) have emerged as a simple platform technology and hence have been broadly studied. With advances in state-of-the-art fabrication and characterization methods, new insights into SAM structure and related properties have been delineated, albeit with some discrepancies and/or incoherencies. Some discrepancies, especially between experimental and theoretical work, are in part due to the misunderstanding of subtle structural features such as phase evolution and SAM quality. Recent work has, however, shown that simple techniques, such as the measurement of static contact angles, can be used to delineate otherwise complex properties of the SAM, especially when complemented by other more advanced techniques. In this article, we highlight the effect of nanoscale substrate asperities and molecular chain length on the SAM structure and associated properties. First, surfaces with tunable roughness are prepared on both Au and Ag, and their corresponding n-alkanethiolate SAMs are characterized through wetting and spectroscopy. From these data, chain-length- and substrate-morphology-dependent limits to the odd-even effect (structure and properties vary with the number of carbons in the molecules and the nature of the substrate), parametrization of gauche defect densities, and structural phase evolution (liquidlike, waxy, crystalline interfaces) are deduced. An evaluation of the correlation between the effect of roughness and the components of surface tension (polar-γ^p and dispersive-γ^d) reveals that wetting, at nanoscale rough surfaces, evolves proportionally with the ratio of the two components of surface tension. The evolution of conformational order is captured over a range of molecular lengths and parametrized through a dimensionless number, χ_c. By deploying a well-known tensiometry technique (herein the liquid is used to characterize the solid, hence the term inverse tensiometry) to characterize SAMs, we demonstrate that complex molecular-level phenomena in SAMs can be understood through simplicity.