Stephen J. Byard

Identification of Protonation State by XPS, Solid-State NMR, and DFT: Characterization of the Nature of a New Theophylline Complex by Experimental and Computational Methods

Recent studies suggested that X-ray photoelectron spectroscopy (XPS) sensitively determines the protonation state of nitrogen functional groups in the solid state, providing a means for distinguishing between co-crystals and salts of organic compounds. Here we describe how a new theophylline complex with 5-sulfosalicylic acid dihydrate was established as a salt by XPS prior to assignment with conventional methods. The presence of a C=NH(+) (N9) N1s peak in XPS allows assignment as a salt, while this peak is clearly absent for a theophylline co-crystal. The large low frequency shift for N9 observed by (15)N solid-state nuclear magnetic resonance spectroscopy (ssNMR) and corresponding density functional theory (DFT) calculations confirm that protonation has occurred. The crystal structure and further analytical studies confirm the conclusions reached with XPS and ssNMR. This study demonstrates XPS as an alternative technique for determining whether proton transfer has occurred in acid-base complexes.

Salt or Co-Crystal? Determination of Protonation State by X-Ray Photoelectron Spectroscopy (XPS)

Combined (15)N ssNMR and X-ray photoelectron spectroscopy (XPS) investigations for theophylline, a theophylline co-crystal, and a theophyllinium salt demonstrate that XPS allows direct observation of the degree of proton transfer, and thus identification of whether a salt or a co-crystal has been formed. The presence of a strongly binding-energy-shifted N 1s XPS peak with protonation indicates a salt (C==NH(+)), while this peak is unmistakably absent in the co-crystal. XPS should be considered as an alternative and complementary technique to single crystal X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy (ssNMR).

Crystallography Aided by Atomic Core-Level Binding Energies: Proton Transfer versus Hydrogen Bonding in Organic Crystal Structures

Ionic bond or hydrogen bridge? Brønsted proton transfer to nitrogen acceptors in organic crystals causes strong N1s core-level binding energy shifts. A study of 15 organic cocrystal and salt systems shows that standard X-ray photoelectron spectroscopy (XPS) can be used as a complementary method to X-ray crystallography for distinguishing proton transfer from H-bonding in organic condensed matter.

Bisphosphonate protonation states, conformations, and dynamics on bone mineral probed by solid-state NMR without isotope enrichment

Recognition of bone mineral by bisphosphonates is crucial to their targeting, efficacy, therapeutic and diagnostic applications, and pharmacokinetics. In a search for rapid and simple NMR approaches to assessing the bone recognition characteristics of bisphosphonates, we have studied alendronate, pamidronate, neridronate, zoledronate and tiludronate, in crystalline form and bound to the surface of pure bone mineral stripped of its organic matrix by a simple chemical process. (31)P NMR chemical shift anisotropies and asymmetries in the crystalline compounds cluster strongly into groupings corresponding to fully protonated, monoprotonated, and deprotonated phosphonate states. All the mineral-bound bisphosphonates cluster in the same anisotropy-asymmetry space as the deprotonated phosphonates. In (13)C{(31)P} rotational echo double resonance (REDOR) experiments, which are sensitive to carbon-phosphorus interatomic distances, the strongly mineral-bound alendronate displays very similar conformational and side chain dynamics to its crystalline state. Pamidronate and neridronate, with shorter and longer sidechains, respectively, and generally weaker mineral binding, display more dynamical sidechains in the mineral-bound state. The REDOR experiment provides a simple rationalization of bisphosphonate-mineral affinity in terms of molecular structure and dynamics, consistent with findings from much more labour- and time-intensive isotope labelling approaches.

A Multi-Technique Approach to the Study of Structural Stability and Desolvation of Two Unusual Channel Hydrate Solvates of Finasteride

The dehydration/desolvation of two hydrate solvates of the pharmaceutically important compound finasteride (namely, bisfinasteride monohydrate monotetrahydrofuran and bisfinasteride monohydrate mono-1,4-dioxane) has been studied by solid-state nuclear magnetic resonance, powder X-ray diffraction, thermogravimetric analysis (including coupling with mass spectrometry) and dynamic vapour sorption. The structure is unusual in that water holds the host finasteride molecules together by hydrogen bonding to form channels in which the solvent is sited. Whilst the solvent guest molecules are not strongly bound to the host, their presence is essential for structural stability. Desolvation is not found to occur at a well-defined temperature or even to consistently produce the same anhydrous form (form I vs. form II), but is instead highly dependent on the physical environment and, therefore, on the technique used. This behaviour complicates investigations, but the combination of complementary methods does allow the desolvation to be understood. Water and solvent are shown to be lost simultaneously, with no evidence of an intermediate form or increased mobility of the hydrogen-bonded water molecules. The results are consistent with a model in which structural collapse and rearrangement follows the loss of a small fraction of the solvent molecules from the channel structure, with the final form produced being very sensitive to the presence of water vapour during desolvation.

The conformational behaviour of phosphatidylinositol

The temperature dependence of the 1H-NMR spectrum of phosphatidylinositol (PI) in d6-dimethylsulphoxide (DMSO) shows that the hydroxy groups at C2 and at C6 of the inositol ring are internally hydrogen-bonded. This probably implies a trans/gauche conformation for the phosphate/inositol linkage. The presence of a trans phosphate-alkyl-oxygen bond is confirmed by 31P-NMR studies. If the conformation of PI in membranes is the same as that in DMSO solution, this implies that the inositol ring points out into the aqueous phase with its C1/C4 axis almost perpendicular to the membrane surface. Progress is also reported in attempts to characterise headgroup orientation and dynamics by 2H-NMR using deuterated synthetic PI, prepared by the route devised by Ward, J.G. and Young, R.C. (Tetrahedron Lett. 29 (1988) 6013-6016).

Characterising the role of water in sildenafil citrate by NMR crystallography

A combination of solid-state NMR techniques, including 13C/1H correlation, 2H magic-angle spinning NMR and first principles calculation are employed to characterise the role of water in different hydration states of sildenafil citrate. The 13C spectrum is fully assigned for the first time and direct correlations made with respect to the crystal structure. 2H magic-angle spinning NMR is demonstrated to be a powerful tool for the study of dynamic and exchange processes in complex hydrate systems, allowing the behaviour at multiple solvate sites to be characterised without the need for expensive and selective labelling. Use of the 2H double-quantum frequency allows resolution of the different sites and, consequently, data fitting to determine rates of spin-diffusion between the different sites. The water is shown to be highly dynamic, undergoing C2 rotation, with chemical exchange between different water molecules and also with the host structure. The methods adopted are applicable to the investigation of an extensive range of hydration types found in pharmaceutical drug substances.

Stabilization of Insulin by Adsorption on a Hydrophobic Silane Self-Assembled Monolayer.

The interaction between many proteins and hydrophobic functionalized surfaces is known to induce β-sheet and amyloid fibril formation. In particular, insulin has served as a model peptide to understand such fibrillation, but the early stages of insulin misfolding and the influence of the surface have not been followed in detail under the acidic conditions relevant to the synthesis and purification of insulin. Here we compare the adsorption of human insulin on a hydrophobic (-CH3-terminated) silane self-assembled monolayer to a hydrophilic (-NH3(+)-terminated) layer. We monitor the secondary structure of insulin with Fourier transform infrared attenuated total reflection and side-chain orientation with sum frequency spectroscopy. Adsorbed insulin retains a close-to-native secondary structure on both hydrophobic and hydrophilic surfaces for extended periods at room temperature and converts to a β-sheet-rich structure only at elevated temperature. We propose that the known acid stabilization of human insulin and the protection of the aggregation-prone hydrophobic domains on the insulin monomer by adsorption on the hydrophobic surface work together to inhibit fibril formation at room temperature.

Detection of Free Base Surface Enrichment of a Pharmaceutical Salt by X-ray Photoelectron Spectroscopy (XPS)

Yellow discoloration was observed at the surface of normally white crystals of a development pharmaceutical fumarate salt, tentatively ascribed to the presence of trace amounts of free base. The impact of impurities on sample properties and behavior can be significant, especially if localized at the surface. No conventional bulk analytical technique could readily provide an explanation for the yellow color, so a surface-sensitive technique, X-ray photoelectron spectroscopy (XPS), was employed to characterize the salt. XPS reveals the presence of free base at the surface through the HN(+)/N ratio. A free radical decarboxylation mechanism is proposed to account for the alterations observed with extended irradiation. The lower intensity carboxyl signal and significantly lower HN(+)/N ratio for the yellow surface samples reveal a higher level of free base at the surface than the white samples. The samples with yellow surfaces could not be successfully milled, which was an important part of the production process for providing material of the required physical quality for product formulation. Identification of residual free base at the surface of the crystalline material, by XPS, was significant for optimization of the crystallization process to yield material of required quality for successful milling at plant scale.

An example of how to handle amorphous fractions in API during early pharmaceutical development: SAR114137 – A successful approach

The so-called pharmaceutical solid chain, which encompasses drug substance micronisation to the final tablet production, at pilot plant scale is presented as a case study for a novel, highly potent, pharmaceutical compound: SAR114137. Various solid-state analytical methods, such as solid-state Nuclear Magnetic Resonance (ssNMR), Differential Scanning Calorimetry (DSC), Dynamic Water Vapour Sorption Gravimetry (DWVSG), hot-stage Raman spectroscopy and X-ray Powder Diffraction (XRPD) were applied and evaluated to characterise and quantify amorphous content during the course of the physical treatment of crystalline active pharmaceutical ingredient (API). DSC was successfully used to monitor the changes in amorphous content during micronisation of the API, as well as during stability studies. (19)F solid-state NMR was found to be the method of choice for the detection and quantification of low levels of amorphous API, even in the final drug product (DP), since compaction during tablet manufacture was identified as a further source for the formation of amorphous API. The application of different jet milling techniques was a critical factor with respect to amorphous content formation. In the present case, the change from spiral jet milling to loop jet milling led to a decrease in amorphous API content from 20-30 w/w% to nearly 0 w/w% respectively. The use of loop jet milling also improved the processability of the API. Stability investigations on both the milled API and the DP showed a marked tendency for recrystallisation of the amorphous API content on exposure to elevated levels of relative humidity. No significant impact of amorphous API on either the chemical stability or the dissolution rate of the API in drug formulation was observed. Therefore, the presence of amorphous content in the oral formulation was of no consequence for the clinical trial phases I and II.