Joanna S. Stevens

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.

Immobilization of cell-binding peptides on poly-ε-caprolactone film surface to biomimic the peripheral nervous system.

Cell-material interactions are crucial for cell adhesion and proliferation on biomaterial surfaces. Immobilization of biomolecules leads to the formation of biomimetic substrates, improving cell response. We introduced RGD (Arg-Gly-Asp) sequences on poly-ε-caprolactone (PCL) film surfaces using thiol chemistry to enhance Schwann cell (SC) response. XPS elemental analysis indicated an estimate of 2-3% peptide functionalization on the PCL surface, comparable with carbodiimide chemistry. Contact angle was not remarkably reduced; hence, cell response was only affected by chemical cues on the film surface. Adhesion and proliferation of Schwann cells were enhanced after PCL modification. Particularly, RGD immobilization increased cell attachment up to 40% after 6 h of culture. It was demonstrated that SC morphology changed from round to very elongated shape when surface modification was carried out, with an increase in the length of cellular processes up to 50% after 5 days of culture. Finally RGD immobilization triggered the formation of focal adhesion related to higher cell spreading. In summary, this study provides a method for immobilization of biomolecules on PCL films to be used in peripheral nerve repair, as demonstrated by the enhanced response of Schwann cells.

Incisive probing of intermolecular interactions in molecular crystals: core level spectroscopy combined with density functional theory.

The α-form of crystalline para-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding.

Chemical Speciation and Bond Lengths of Organic Solutes by Core-Level Spectroscopy: pH and Solvent Influence on p-Aminobenzoic Acid

Through X-ray absorption and emission spectroscopies, the chemical, electronic and structural properties of organic species in solution can be observed. Near-edge X-ray absorption fine structure (NEXAFS) and resonant inelastic X-ray scattering (RIXS) measurements at the nitrogen K-edge of para-aminobenzoic acid reveal both pH- and solvent-dependent variations in the ionisation potential (IP), 1s→π* resonances and HOMO–LUMO gap. These changes unequivocally identify the chemical species (neutral, cationic or anionic) present in solution. It is shown how this incisive chemical state sensitivity is further enhanced by the possibility of quantitative bond length determination, based on the analysis of chemical shifts in IPs and σ* shape resonances in the NEXAFS spectra. This provides experimental access to detecting even minor variations in the molecular structure of solutes in solution, thereby providing an avenue to examining computational predictions of solute properties and solute–solvent interactions.

NEXAFS Sensitivity to Bond Lengths in Complex Molecular Materials: A Study of Crystalline Saccharides

Detailed analysis of the C K near-edge X-ray absorption fine structure (NEXAFS) spectra of a series of saccharides (fructose, xylose, glucose, galactose, maltose monohydrate, α-lactose monohydrate, anhydrous β-lactose, cellulose) indicates that the precise determination of IPs and σ* shape resonance energies is sensitive enough to distinguish different crystalline saccharides through the variations in their average C-OH bond lengths. Experimental data as well as FEFF8 calculations confirm that bond length variations in the organic solid state of 10(-2) Å can be experimentally detected, opening up the possibility to use NEXAFS for obtaining incisive structural information for molecular materials, including noncrystalline systems without long-range order such as dissolved species in solutions, colloids, melts, and similar amorphous phases. The observed bond length sensitivity is as good as that originally reported for gas-phase and adsorbed molecular species. NEXAFS-derived molecular structure data for the condensed phase may therefore be used to guide molecular modeling as well as to validate computationally derived structure models for such systems. Some results indicate further analytical value in that the σ* shape resonance analysis may distinguish hemiketals from hemiacetals (i.e., derived from ketoses and aldoses) as well as α from β forms of otherwise identical saccharides.

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.

Intermolecular bonding of hemin in solution and in solid state probed by N K-edge X-ray spectroscopies.

X-ray absorption/emission spectroscopy (XAS/XES) at the N K-edge of iron protoporphyrin IX chloride (FePPIX-Cl, or hemin) has been carried out for dissolved monomers in DMSO, dimers in water and for the solid state. This sequence of samples permits identification of characteristic spectral features associated with the hemin intermolecular bonding. These characteristic features are further analyzed and understood at the molecular orbital (MO) level based on the DFT calculations.

NEXAFS and XPS of p-Aminobenzoic Acid Polymorphs: The Influence of Local Environment

Nitrogen K-edge XPS and NEXAFS of the two polymorphic forms of para- aminobenzoic acid (PABA) are significantly different reflecting variation in hydrogen bonding. Alteration in hydrogen bonding at the amino group leads to a shift to high energy for both the XPS N 1s core level and the 3π* NEXAFS resonance with β-PABA. Participation of the amine group in the aromatic system causes the 1π* resonance to be sensitive to the nature of the intermolecular bonding at the para-carboxylic acid group, and a shift to low energy for α- PABA is observed due to hydrogen-bonded carboxylic acid dimer formation. FEFF calculations also successfully reproduce both the energy and intensity variations observed for the σ* shape resonance associated with the C-N bond, with the majority of the decrease in energy observed for b-PABA arising from the longer C-N bond.