The structure of magnesium stearate trihydrate determined from a micrometre-sized single crystal using a microfocused synchrotron X-ray beam.

Crystalline magnesium stearate has been extensively used as an additive in pharmaceutical and other industries for decades. However, the lack of suitably large crystals has hindered the determination of the crystal structure and thereby a more fundamental understanding of the structure-functionality relationship. Presented here is the structure of magnesium stearate trihydrate as determined from X-ray diffraction data of a micrometre-sized single crystal measured at a fourth-generation synchrotron facility. Despite the small size of the single crystals and the weak diffraction, it was possible to determine the positions of the non-hydrogen atoms reliably. Periodic dispersion-corrected density functional theory calculations were used to obtain the positions of the hydrogen atoms playing an important role in the overall organization of the structure via a hydrogen-bond network.

Effect of pH on the Surface Layer of Molecular Crystals at the Solid-Liquid Interface.

Dissolution of solid matter into aqueous solution is one of the most challenging physicochemical aspects related to drug development. While influenced by several parameters, the effect of pH remains the most important one to be fully understood. The dissolution process is essentially controlled by activity at the surface of the molecular crystals, which is difficult to characterize experimentally. To address this, a combination of in situ atomic force microscopy (AFM) with molecular dynamics (MD) simulation is reported. AFM allows for direct visualization of the crystal surface of basic and acidic model compounds (carvedilol and ibuprofen) in contact with an aqueous medium with varying pH. A dramatic increase in surface mobility in the solid-liquid interface could be observed experimentally as a function of pH. The in situ AFM approach opens up for a more detailed understanding of the behavior of particulate matter in solution with importance at different levels, ranging from engineering aspects related to crystallization, and biological considerations related to bioavailability of the final drug product.

Probing the Secondary Structure of Individual Aß40 Amorphous Aggregates and Fibrils by AFM-IR Spectroscopy.

Structural characterization of aggregates and fibrils of the Aß protein is pivotal to the molecular-level elucidation of Alzheimer's disease (AD). AFM-IR spectroscopy provides nanoscale resolution, and thus allows the interrogation of individual aggregates and fibrils. During aggregation of Aß, we observed mainly disordered Aß at t=15 min, but substantial structural diversity including the co-existence of parallel and antiparallel ß-sheets within a large amorphous aggregate at t=2 hours, while fibrils exhibited the expected signature of parallel ß-sheets at t=1 week. The resonance observed for parallel ß-sheets at t=2 hours coincides with that observed for fibrils (at 1634 cm-1 ), thus indicating that fibril-like species exist within the large aggregates. Therefore, nucleation might occur within such species, in analogy to current theories of protein crystallization in which nucleation occurs within large protein clusters. Cu2+ perturbs Aß aggregation, catalysing rapid formation of amorphous aggregates with diverse secondary structure, but inhibiting fibril growth.

A free-floating mucin layer to investigate the effect of the local microenvironment in lungs on mucin-nanoparticle interactions.

Respiratory tract mucus represents an important barrier for pulmonary drug delivery. Understanding of mucin-nanoparticle interactions is a prerequisite for rational design of inhalable nanoparticles. In the present study, in order to establish a reliable quartz crystal microbalance with dissipation (QCM-D) approach to reveal the effect of the lung microenvironment on the mucin-nanoparticle interactions, we investigated the intrinsic features of the mucin layers immobilized onto sensors via chemical conjugation or physical adsorption by using atomic force microscopy (AFM) and QCM-D. Our results demonstrated that the covalently-grafted mucin layer responded more sensitively than the physically-adsorbed mucin layer to the local microenvironment shifting from PBS (pH 7.35 and ionic strength 30 mM) to PBS (pH 6.25 and ionic strength 150 mM) and resulted in a softer mucin layer with more hydrophobic areas exposed. Furthermore, using the QCM-D approach with the covalently-grafted mucin layer, we demonstrated the significant influence of the local microenvironment on the interaction of mucin with poly (lactic-co-glycolic acid)-based nanoparticles with different surface hydrophilicity. The present work underlines the QCM-D approach with a covalently-grafted mucin layer as a potent tool to elucidate the potential influence of local microenvironment on mucin-nanoparticle interactions. STATEMENT OF SIGNIFICANCE: Studying interactions between nanoengineered materials and biological systems plays a vital role in development of biomedical applications of nanoengineered materials. In this work, by employing a more biologically relevant, 'free-floating' mucin layer model, we demonstrate the significant impact of the lung microenvironment on the nature and the extent of the interaction between the mucin and the nanoparticles with different surface hydrophilicity. To the best of our knowledge, this is the first work describing the nanoscale properties of immobilized mucin layers and investigating the mucin-nanoparticle interactions with emphasis on the impact of local microenvironment in lungs. Thus, it is expected to have important consequences in rational design of inhalable nanoparticle delivery systems.