Predicting the Stability of Lyophilized Human Serum Albumin Formulations Containing Sucrose and Trehalose Using Solid-State NMR Spectroscopy: Effect of Storage Temperature on 1H T1 Relaxation Times.

In a lyophilized protein/disaccharide system, the ability of the disaccharide to form a homogeneous mixture with the protein and to slow the protein mobility dictates the stabilization potential of the formulation. Human serum albumin was lyophilized with sucrose or trehalose in histidine, phosphate, or citrate buffer. 1H T1 relaxation times were measured by solid-state NMR spectroscopy and were used to assess the homogeneity and mobility of the samples after zero, six, and twelve months at different temperatures. The mobility of the samples decreased after 6 and 12 months storage at elevated temperatures, consistent with structural relaxation of the amorphous disaccharide matrix. Formulations with sucrose had lower mobility and greater stability than formulations with trehalose.

Understanding the impact of mannitol on physical stability and aerosolization of spray-dried protein powders for inhalation.

Pulmonary delivery of protein-based therapeutics, including antibodies, is a promising option for treating respiratory diseases. Spray drying is a widely used method for producing dry powder formulations with mannitol being a commonly used excipient for these inhalation formulations. There is limited research available concerning the utilization of mannitol as an excipient in the spray drying of proteins and its impact on aerosol performance. This study highlights the importance to understand mannitol's potential role and impact in this context. To investigate the impact of mannitol on physical stability and aerosolization of spray-dried protein formulations, bovine serum albumin (BSA) was employed as a model protein and formulated with different concentrations of mannitol via spray drying. The spray-dried solids were characterized for their particle size using Malvern mastersizer and aerodynamic particle size using next generation impactor (NGI). Additionally, the solids were characterized with solid-state Fourier-transform infrared spectroscopy (ssFTIR), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and solid-state nuclear magnetic resonance spectroscopy (ssNMR) to analyze the change in their secondary structure, crystallinity, particle morphology, and protein-excipient interaction, respectively. Size exclusion chromatography (SEC) was used to investigate changes in monomer content resulting from storage under stressed condition of 40 °C. Protein formulations containing more than 33 % mannitol by weight showed crystallization tendencies, causing an increase in monomer loss over time. ssNMR data also showed mixing heterogeneity of BSA and mannitol in the formulations with high mannitol contents. Futhermore, fine particle fraction (FPF) was found to decrease over time for the formulations containing BSA: Mannitol in the ratios of 2:1, 1:2, and 1:5, due to particle agglomeration induced by crystallization of mannitol. This study underscores the significant influence of excipients such as mannitol on the aerosol performance and storage stability of spray-dried protein formulations.

Comparison of Differential Scanning Calorimetry, Powder X-ray Diffraction, and Solid-state Nuclear Magnetic Resonance Spectroscopy for Measuring Crystallinity in Amorphous Solid Dispersions - Application to Drug-in-polymer Solubility.

The widespread use of amorphous solid dispersions (ASDs) dictates that analytical methods are required to accurately quantify crystallinity and characterize crystals formed in order to help design a stable ASD. Current crystallinity quantitation methods are limited to ASDs of moderate drug loadings, single polymorphs, and fast crystallization kinetics. The ability of multiple differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), and solid-state nuclear magnetic resonance (SSNMR) techniques were compared for quantifying crystallinity in ASDs in varying conditions. Determining crystallinity by DSC is limited by its ability to measure Tg or heat of fusion. PXRD was relatively robust in determining crystalline and amorphous ratios for drug-polymer systems in the absence of other excipients. SSNMR provides both quantitative information and reveal how crystal quality changes with crystallization conditions and helps to explain the failure of DSC methods. The results of five different methods using three techniques were directly applied to measure drug-in-polymer solubility with four agreeing well with the literature. PXRD and SSNMR are therefore proposed as alternative methods to quantify crystallinity and predict drug-in-polymer solubility when DSC methods do not work. In-situ and ex-situ annealing was also compared, and equivalent crystallinity data was acquired.

Development of hot-melt extruded drug/polymer matrices for sustained delivery of meloxicam.

For effective resolution of regional subacute inflammation and prevention of biofouling formation, we have developed a polymeric implant that can release meloxicam, a selective cyclooxygenase (COX)-2 inhibitor, in a sustained manner. Meloxicam-loaded polymer matrices were produced by hot-melt extrusion, with commercially available biocompatible polymers, poly(ε-caprolactone) (PCL), poly(lactide-co-glycolide) (PLGA), and poly(ethylene vinyl acetate) (EVA). PLGA and EVA had a limited control over the drug release rate partly due to the acidic microenvironment and hydrophobicity, respectively. PCL allowed for sustained release of meloxicam over two weeks and was used as a carrier of meloxicam. Solid-state and image analyses indicated that the PCL matrices encapsulated meloxicam in crystalline clusters, which dissolved in aqueous medium and generated pores for subsequent drug release. The subcutaneously implanted meloxicam-loaded PCL matrices in rats showed pharmacokinetic profiles consistent with their in vitro release kinetics, where higher drug loading led to faster drug release. This study finds that the choice of polymer platform is crucial to continuous release of meloxicam and the drug release rate can be controlled by the amount of drug loaded in the polymer matrices.

Mechanistic Investigation of Drug Supersaturation in the Presence of Polysorbates as Solubilizing Additives by Solution Nuclear Magnetic Resonance Spectroscopy.

The introduction of solubilizing additives has historically been an attractive approach to address the ever-growing proportion of poorly water-soluble drug (PWSD) compounds within the modern drug discovery pipeline. Lipid-formulations, and more specifically micelle formulations, have garnered particular interest because of their simplicity, size, scalability, and avoidance of solid-state limitations. Although micelle formulations have been widely utilized, the molecular mechanism of drug solubilization in surfactant micelles is still poorly understood. In this study, a series of modern nuclear magnetic resonance (NMR) methods are utilized to gain a molecular-level understanding of intermolecular interactions and kinetics in a model system. This approach enabled the understanding of how a PWSD, 17ß-Estradiol (E2), solubilizes within a nonionic micelle system composed of polysorbate 80 (PS80). Based on one-dimensional (1D) 1H chemical shift differences of E2 in PS80 solutions, as well as intermolecular correlations established from 1D selective nuclear Overhauser effect (NOE) and two-dimensional NOE spectroscopy experiments, E2 was found to accumulate within the palisade layer of PS80 micelles. A potential hydrogen-bonding interaction between a hydroxyl group of E2 and a carbonyl group of PS80 alkane chains may allow for stabilizing E2-PS80 mixed micelles. Diffusion and relaxation NMR analysis and particle size measurements using dynamic light scattering indicate a slight increase in the micellar size with increasing degrees of supersaturation, resulting in slower mobility of the drug molecule. Based on these structural findings, a theoretical orientation model of E2 molecules with PS80 molecules was developed and validated by computational docking simulations.

Effect of Storage Humidity on Physical Stability of Spray-Dried Naproxen Amorphous Solid Dispersions with Polyvinylpyrrolidone: Two Fluid Nozzle vs. Three Fluid Nozzle.

In a spray drying operation, a two-fluid nozzle (2FN) with a single channel is commonly used for atomizing the feed solution. However, the less commonly used three-fluid nozzle (3FN) has two separate channels, which allow spray drying of materials in two incompatible solution systems. Although amorphous solid dispersions (ASDs) prepared using a 3FN have been reported to deliver comparable drug dissolution performance relative to those prepared using a 2FN, few studies have systematically examined the effect of 3FN on the physical stability. Therefore, the goal of this work is to systematically study the physical stability of ASDs that are spray-dried using a 3FN compared to those prepared using the traditional 2FN. For the 2FN, a single solution of naproxen and polyvinylpyrrolidone (PVP) was prepared in a mixture of acetone and water at a 1:1 volume ratio because 2FN allows for only one solution inlet. For the 3FN, naproxen and PVP were dissolved individually in acetone and water, respectively, because 3FN allows simultaneous entry of two solutions. Upon storage of the formulated ASDs at different humidity levels (25%, 55% and 75% RH), naproxen crystallized more quickly from the 3FN ASDs as compared with the 2FN ASDs. 3FN ASDs crystallized after 5 days of storage at all conditions, whereas 2FN ASDs did not crystallize even at 55% RH for two months. This relatively higher crystallization tendency of 3FN ASDs was attributed to the inhomogeneity of drug and polymers as identified by the solid-state Nuclear Magnetic Resonance findings, specifically due to poor mixing of water- and acetone-based solutions at the 3FN nozzle. When only acetone was used as a solvent to prepare drug-polymer solutions for 3FN, the formulated ASD was found to be stable for >3 months of storage (at 75% RH), which suggests that instability of the 3FN ASD was due to the insufficient mixing of water and acetone solutions. This study provides insights into the effects of solvent and nozzle choices on the physical stability of spray-dried ASDs.

Understanding the Impact of Protein-Excipient Interactions on Physical Stability of Spray-Dried Protein Solids.

Mannitol, leucine, and trehalose have been widely used in spray-dried formulations, especially for inhalation formulations. The individual contribution of these excipients on protein physical stability in spray-dried solids was studied here using bovine serum albumin (BSA) as a model protein. The spray-dried solids were characterized with scanning electron microscopy, powder X-ray diffraction, and solid-state Fourier-transform infrared spectroscopy to analyze particle morphology, crystallinity, and secondary structure change, respectively. Advanced solid-state characterizations were conducted with solid-state hydrogen-deuterium exchange (ssHDX) and solid-state nuclear magnetic resonance (ssNMR) to explore protein conformation and molecular interactions in the context of the system physical stability. Trehalose remained amorphous after spray-drying and was miscible with BSA, forming hydrogen bonds to maintain protein conformation, whereby this system showed the least monomer loss in the stability study. As indicated by ssNMR, both crystalline and amorphous forms of mannitol existed in the spray-dried BSA-mannitol solids, which explained its partial stabilizing effect on BSA. Leucine showed the strongest crystallization tendency after spray-drying and did not provide a stabilizing effect due to substantial immiscibility and phase separation with BSA as a result of crystal formation. This work showed novel applications of ssNMR in examining protein conformation and protein-excipient interaction in dry formulations. Overall, our results demonstrate the pivotal role of advanced solid-state characterization techniques in understanding the physical stability of spray-dried protein solids.

Probing Microenvironmental Acidity in Lyophilized Protein and Vaccine Formulations Using Solid-state NMR Spectroscopy.

Biophysical and biochemical instability of therapeutic proteins in the solution state may necessitate the development of products in the solid form, due to their enhanced stability. Lyophilization is a widely used method to ensure dry state stabilization of biological products. A commonly encountered issue is the pH shifts that can occur due to undesired crystallization of a buffer component, resulting in loss of protein activities. However, it is technically challenging to noninvasively investigate the physicochemical environment in the lyophile matrix. In this work, we demonstrate an approach based on solid-state NMR to investigate the microenvironmental acidity in lyophilized protein formulations, using histidine, a commonly used buffer agent, as a molecular probe. The solid-state acidity in the lyophilized matrix can be assessed by monitoring the chemical shift changes of histidine. The protonation and tautomeric states of histidine lyophilized at a range of pH values from 4.5 to 11.0 were identified from full 13C and 15N resonance assignments in one-dimensional and two-dimensional NMR experiments. The results demonstrated a pH-dependence of histidine chemical shift in the amorphous state. Moreover, we successfully applied this protocol to investigate the microenvironmental pH in lyophilized formulations of the HPV vaccine and lactate dehydrogenase protein.

The impact of solid-state form, water content and surface area of magnesium stearate on lubrication efficiency, tabletability, and dissolution.

Magnesium stearate (MgSt) is a widely used pharmaceutical lubricant in tablet manufacturing. However, batch-to-batch variability in hydrate form and surface area can lead to inconsistency in tablet performance. In this work, several unique MgSt samples were studied: traditional monohydrate samples with high surface area, dihydrate forms with high and low surface area, and disordered forms with low and medium water content. The effects of solid-state form and particle properties on lubrication efficiency, tabletability and dissolution were studied for tablets in a model direct compression formulation. It was found that the monohydrate and dihydrate forms had good lubrication efficiency compared to the disordered form, while the disordered form had the best tabletability. The dissolution rate correlated with surface area, where slower dissolution rates corresponded with higher MgSt surface areas. The dihydrate sample with lower surface area had the best performance for this model formulation, in terms of lubrication efficiency, tabletability and dissolution. Overall, it is concluded that the choice of the most appropriate grade of MgSt for a particular formulation depends on a comprehensive evaluation of the impact of MgSt properties on lubrication efficiency, tabletability and dissolution.

Amorphous Solid Dispersions of Felodipine and Nifedipine with Soluplus®: Drug-Polymer Miscibility and Intermolecular Interactions.

The objective of this study was to investigate thermodynamic and kinetic miscibility for two structurally similar model compounds nifedipine (NIF) and felodipine (FEL) when formulated as amorphous solid dispersions (ASDs) with an amphiphilic polymer Soluplus®. Thermodynamic miscibility was studied via melting point depression approach for the two systems. The Flory Huggins theory was used to calculate the interaction parameter and generate the phase diagrams. It was shown that NIF was more miscible in Soluplus® than FEL. The nature of drug polymer interactions was studied by fourier transform infra-red spectroscopy (FTIR) and solid-state nuclear magnetic resonance spectroscopy (ssNMR). The data from spectroscopic analyses showed that both the drugs interacted with Soluplus® through hydrogen bonding interactions. Furthermore, 13C ssNMR data was used to get quantitative estimate of the extent of hydrogen bonding for ASDs samples. Proton relaxation measurements were carried out on ASDs in order to evaluate phase heterogeneity on two different length scales of mixing. The data suggested that better phase homogeneity in NIF:SOL systems especially for lower Soluplus® content ASDs on smaller domains. This could be explained by understanding the extent of hydrogen bonding interactions for these two systems. This study highlights the need to consider thermodynamic and kinetic mixing, when formulating ASDs with the goal of understanding phase mixing between drug and polymer.

Quantification of Monomer Units in Insoluble Polymeric Active Pharmaceutical Ingredients Using Solid-State NMR Spectroscopy I: Patiromer.

Although extensive precautions are taken to limit batch-to-batch variation in pharmaceutical manufacturing, differences between lots may still exist, particularly in complex formulations. When polymerization is used in the production process, the potential for varying chain lengths and incorporation of different monomers increases the likelihood of batch-to-batch variation. This poses a significant challenge for demonstrating active pharmaceutical ingredient (API) sameness between the innovator and generic drug under development. Therefore, the ability to accurately analyze and quantify the relative amounts of active ingredients present in a formulated product is critically important. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy was used to identify, quantify, and compare the relative amounts of the three polymer groups in the amorphous block copolymer drug, patiromer (Veltassa®). Techniques such as cross polarization (CP) and magic angle spinning were used to quantify each polymer group while the importance of understanding CP dynamics to obtain quantitative data was also addressed. It was found that the magnetization transfer rate and chemical shift anisotropy for different functional groups present in patiromer play a large role when optimizing parameters for spectral acquisition. Once accounted for, the average patiromer lot contained 90.9%, 7.6%, and 1.5% carboxylate, aromatic, and aliphatic blocks, respectively, with little lot-to-lot variation between different dosage strengths and expiration dates. SSNMR proved to be a sensitive analytical technique for evaluating and quantifying different monomer groups present in patiromer. This procedure may serve as a guide for similar quantitation studies on complex drug products and for demonstrating API sameness during generic drug development.

Investigation into Intermolecular Interactions and Phase Behavior of Binary and Ternary Amorphous Solid Dispersions of Ketoconazole.

Conventionally, amorphous solid dispersions (ASDs) have been formulated as a binary matrix, but in recent years a new class of ASDs has emerged, where generally a second polymer is also added to the formulation. Having the presence of a second polymer necessitates a comprehensive solid-state characterization to study the intermolecular interactions and phase behavior on a molecular level. With this goal in mind, ketoconazole (KET) was selected as a model drug, and hydroxypropyl methyl cellulose (HPMC) and poly(acrylic acid) (PAA) were chosen as polymeric carriers. The binary and ternary ASDs were characterized by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and powder X-ray diffraction (PXRD). The binary KET:HPMC dispersions lacked any specific interactions, whereas binary KET:PAA dispersions and ternary KET:PAA:HPMC dispersions showed evidence for ionic and hydrogen bonding interactions. The 13C SSNMR deconvolution study established a comparison for molecular interactions between the binary KET:PAA and ternary KET:PAA:HPMC dispersions, with the binary KET:PAA system showing higher prevalence of ionic and hydrogen bonds than the ternary KET:PAA:HPMC system. Moreover, individual binary and ternary ASDs were found to be homogeneous on a nanometric level, implying the presence of a second polymer did not impact the phase homogeneity. In addition, a stronger interaction in binary KET:PAA and ternary KET:HPMC:PAA systems translated to better physical stability at different storage conditions. Through this case study it is recommended that a comprehensive investigation is needed to study the impact of using two polymers in ASD formulations in terms underlying intermolecular interactions and physical stability.

Utility of Films to Anticipate Effect of Drug Load and Polymer on Dissolution Performance from Tablets of Amorphous Itraconazole Spray-Dried Dispersions.

Because spray-dried dispersion (SDD) performance depends on polymer selection and drug load, time- and resource-sparing methods to screen drug/polymer combinations before spray drying are desirable. The primary objective was to assess the utility of films to anticipate the effects of drug load and polymer grade on dissolution performance of tablets containing SDDs of itraconazole (ITZ). A secondary objective was to characterize the solid-state attributes of films and SDDs to explain drug load and polymer effects on dissolution performance. SDDs employed three different grades of hypromellose acetate succinate (i.e., either HPMCAS-L, HPMCAS-M, or HPMCAS-H). Solid-state characterization employed differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. Results indicate that films correctly anticipated the effects of drug load and polymer on dissolution performance. The best dissolution profiles were observed under the following conditions: 20% drug loading performed better than 30% for both films and SDDs, and the polymer grade rank order was HPMCAS-L > HPMCAS-M > HPMCAS-H for both films and SDDs. No dissolution was detected from films or SDDs containing HPMCAS-H. Solid-state characterization revealed percent crystallinity and phase miscibility as contributing factors to dissolution, but were not the sole factors. Amorphous content in films varied with drug load (10% > 20% > 30%) and polymer grades (HPMCAS-L > HPMCAS-M > HPMCAS-H), in agreement with dissolution. In conclusion, films anticipated the rank-order effects of drug load and polymer grade on dissolution performance from SDDs of ITZ, in part through percent crystallinity and phase miscibility influences.

Phase Behavior of Amorphous Solid Dispersions of Felodipine: Homogeneity and Drug-Polymer Interactions.

In the current investigation, the role of drug-polymer hydrogen bonding (H-bonding) with respect to the phase behavior of amorphous solid dispersions (ASDs) is studied in depth on a nanometer level. Melt-quenched dispersions of felodipine (FEL) with poly(vinylpyrrolidone), or PVP, poly(vinylpyrrolidone-co-vinylacetate), or PVP/VA, and poly(vinylacetate), or PVAc, were prepared at drug loadings of 50-90% w/w. Modulated differential scanning calorimetry (MDSC) was used to detect microscopic homogeneity for each set of ASDs. A single composition dependent glass transition temperature (Tg) was observed over the entire composition range in MDSC data for each set of ASDs; however some samples within each set of ASDs showed a crystallization exotherm and corresponding melting endotherm in the first heating scan. Solid-state nuclear magnetic resonance spectroscopy (SSNMR) was further employed to understand phase homogeneity in these systems. The proton spin-lattice relaxation times in the laboratory and rotating frame (1H T1 and T1ρ) for the drug and individual polymer for each set of ASDs were measured to evaluate phase homogeneity. On the basis of proton relaxation measurements, it was revealed that FEL:PVP and FEL:PVP/VA ASDs exhibited better compositional homogeneity than FEL:PVAc ASDs. The strength and the extent of H-bonding were studied by using 13C SSNMR spectra. In addition, deconvolution of the carbonyl region of amorphous FEL revealed that 40% of amorphous FEL molecules were hydrogen bonded (H-bonded) through dimers and the remaining 60% were free/non H-bonded. The dimer fraction decreased as the polymer content increased for each set of ASDs, while the free fraction increased. This indicated that the polymers containing hydrogen bond acceptor groups disrupted dimers and formed intermolecular H-bonding interactions with FEL. The strength and extent of FEL:polymer H-bonding was rank ordered as PVP > PVP/VA > PVAc. These findings were also confirmed through DFT calculations on these systems. Our results suggest that drug-polymer H-bonding interaction may impact the phase homogeneity in ASDs formulated by a specific method. The data from the current study further demonstrate that SSNMR is a powerful tool for characterizing phase homogeneity in ASDs with sub-50 nm resolution. In addition, SSNMR can provide insights into drug-polymer interactions and speciation in ASDs.

Highly sulphated cellulose: a versatile, reusable and selective desilylating agent for deprotection of alcoholic TBDMS ethers.

A mild, efficient and rapid protocol was developed for the deprotection of alcoholic TBDMS ethers using a recyclable, eco-friendly highly sulphated cellulose sulphate acid catalyst in methanol. This acid catalyst selectively cleaves alcoholic TBDMS ethers in bis-TBDMS ethers containing both alcoholic and phenolic TBDMS ether moieties.

Exploring Molecular Speciation and Crystallization Mechanism of Amorphous 2-Phenylamino Nicotinic Acid.

PURPOSE: Molecular understanding of phase stability and transition of the amorphous state helps in formulation and manufacturing of poorly-soluble drugs. Crystallization of a model compound, 2-phenylamino nicotinic acid (2PNA), from the amorphous state was studied using solid-state analytical methods. Our previous report suggests that 2PNA molecules mainly develop intermolecular -COOH∙∙∙pyridine N (acid-pyridine) interactions in the amorphous state. In the current study, the molecular speciation is explored with regard to the phase transition from the amorphous to the crystalline state. METHODS: Using spectroscopic techniques, the molecular interactions and structural evolvement during the recrystallization from the glassy state were investigated. RESULTS: The results unveiled that the structurally heterogeneous amorphous state contains acid-pyridine aggregates - either as hydrogen-bonded neutral molecules or as zwitterions - as well as a population of carboxylic acid dimers. Phase transition from the amorphous state results in crystal structures composed of carboxylic acid dimer (acid-acid) synthon or acid-pyridine chains depending on the crystallization conditions employed. CONCLUSIONS: The study underlines the structural evolvement, as well as its impact on the metastability, of amorphous samples from local, supramolecular assemblies to long-range intermolecular ordering through crystallization.

Synthesis and characterization of thermally responsive N-isopropylacrylamide hydrogels copolymerized with novel hydrophobic polyphenolic crosslinkers.

Two series of thermosensitive hydrogels were synthesized by copolymerizing N-isopropylacrylamide (NIPAAm) with various contents of novel hydrophobic crosslinkers, curcumin multiacrylate (CMA) and quercetin multiacrylate (QMA). The compositions of the resulting hydrogels were characterized using solid state-NMR (ss-NMR), and the temperature dependent swelling behavior and lower critical solution temperature (LCST) were characterized using swelling studies and differential scanning calorimetry (DSC). Increasing the crosslinker content resulted in a significant decrease in the LCST and swelling ratio of hydrogels, which could be attributed to the increased hydrophobicity introduced by CMA or QMA. All of the hydrogels demonstrated temperature responsive swelling with the extent of swelling decreasing with increasing crosslinker content. The lower crosslinker content gels displayed sharper phase transitions, while the high crosslinker content gels had broader phase transitions.

Enhancing the resolution of 1H and 13C solid-state NMR spectra by reduction of anisotropic bulk magnetic susceptibility broadening.

We demonstrate that natural isotopic abundance 2D heteronuclear correlation (HETCOR) solid-state NMR spectra can be used to significantly reduce or eliminate the broadening of 1H and 13C solid-state NMR spectra of organic solids due to anisotropic bulk magnetic susceptibility (ABMS). ABMS often manifests in solids with aromatic groups, such as active pharmaceutical ingredients (APIs), and inhomogeneously broadens the NMR peaks of all nuclei in the sample. Inhomogeneous peaks with full widths at half maximum (FWHM) of ∼1 ppm typically result from ABMS broadening and the low spectral resolution impedes the analysis of solid-state NMR spectra. ABMS broadening of solid-state NMR spectra has previously been eliminated using 2D multiple-quantum correlation experiments, or by performing NMR experiments on diluted materials or single crystals. However, these experiments are often infeasible due to their poor sensitivity and/or provide limited gains in resolution. 2D 1H-13C HETCOR experiments have previously been applied to reduce susceptibility broadening in paramagnetic solids and we show that this strategy can significantly reduce ABMS broadening in diamagnetic organic solids. Comparisons of 1D solid-state NMR spectra and 1H and 13C solid-state NMR spectra obtained from 2D 1H-13C HETCOR NMR spectra show that the HETCOR spectrum directly increases resolution by a factor of 1.5 to 8. The direct gain in resolution is determined by the ratio of the inhomogeneous 13C/1H linewidth to the homogeneous 1H linewidth, with the former depending on the magnitude of the ABMS broadening and the strength of the applied field and the latter on the efficiency of homonuclear decoupling. The direct gains in resolution obtained using the 2D HETCOR experiments are better than that obtained by dilution. For solids with long proton longitudinal relaxation times, dynamic nuclear polarization (DNP) was applied to enhance sensitivity and enable the acquisition of 2D 1H-13C HETCOR NMR spectra. 2D 1H-13C HETCOR experiments were applied to resolve and partially assign the NMR signals of the form I and form II polymorphs of aspirin in a sample containing both forms. These findings have important implications for ultra-high field NMR experiments, optimization of decoupling schemes and assessment of the fundamental limits on the resolution of solid-state NMR spectra.

Impact of Supramolecular Aggregation on the Crystallization Kinetics of Organic Compounds from the Supercooled Liquid State.

Despite numerous challenges in their theoretical description and practical implementation, amorphous drugs are of growing importance to the pharmaceutical industry. One such challenge is to gain molecular level understanding of the propensity of a molecule to form and remain as a glassy solid. In this study, a series of structurally similar diarylamine compounds was examined to elucidate the role of supramolecular aggregation on crystallization kinetics from supercooled liquid state. The structural similarity of the compounds makes it easier to isolate the molecular features that affect crystallization kinetics and glass forming ability of these compounds. To examine the role of hydrogen-bonded aggregation and motifs on crystallization kinetics, a combination of thermal and spectroscopic techniques was employed. Using variable temperature FTIR, Raman, and solid-state NMR spectroscopies, the presence of hydrogen bonding in the melt and glassy state was examined and correlated with observed phase transition behaviors. Spectroscopic results revealed that the formation of hydrogen-bonded aggregates involving carboxylic acid and pyridine nitrogen (acid-pyridine aggregates) between neighboring molecules in the melt state impedes crystallization, while the presence of carboxylic acid dimers (acid-acid dimers) in the melt favors crystallization. This study suggests that glass formation of small molecules is influenced by the type of intermolecular interactions present in the melt state and the kinetics associated with the molecules to assemble into a crystalline lattice. For the compounds that form acid-pyridine aggregates, the formation of energy degenerate chains, produced due to conformational flexibility of the molecules, presents a kinetic barrier to crystallization. The poor crystallization tendency of these aggregates stems from the highly directional hydrogen-bonding interactions needed to form the acid-pyridine chains. Conversely, for the compounds that form acid-acid dimers, the nondirectional van der Waals forces needed to construct a nucleus promote rapid assembly and crystallization.