Application of synchrotron-XRPD to protein powders

Using the methodology and the experimental protocols that Excelsus developed for the detection of  traces of crystalline forms in pharmaceutical compounds, Excelsus team collected outstanding data on protein powders allowing the Biochemistry & Structural Biology team at the University of Patras to achieve a 1.8 Ang structural model of the molecule “octreotide”, surpassing the model the same group achieved in the past (see: earlier result.). This model corresponds to the highest resolution ever achieved for a peptide/ protein using X-ray powder diffraction and crystallographic methods. Furthermore, traces of minority crystalline impurities were detected for the first time. Structural model available here.

Advances in instrumentation, as well as the development of powerful crystallographic software have significantly facilitated the collection of high-resolution diffraction data and have made X-ray powder diffraction (XRPD) particularly useful for the extraction of structural information; this is true even for complex molecules, especially when combined with synchrotron radiation. In this study, in-line with past instrumental profile studies, an improved data collection strategy exploiting the MYTHEN II detector system together with significant beam focusing and tailored data collection options was introduced and optimized for protein samples at the Material Science beamline at the Swiss Light Source. Polycrystalline precipitates of octreotide, a somatostatin analog of particular pharmaceutical interest, were examined with this novel approach. XRPD experiments resulted in high angular and d-spacing (1.87 Å) resolution data, from which electron-density maps of enhanced quality were extracted, revealing the molecule's structural properties. Since microcrystalline precipitates represent a viable alternative for administration of therapeutic macromolecules, XRPD has been acknowledged as the most applicable tool for examining a wide spectrum of physicochemical properties of such materials and performing studies ranging from phase identification to complete structural characterization.

Fruitful discussions with the Materials Science beamline crew (Nicola Casati and Antonio Cervellino) and the PSI detector group (Bernd Schmitt and Anna Bergamaschi) have been instrumental to the success of the project.