High resolution magic angle spinning NMR to investigate ligand–receptor binding events for mass-limited samples in liquids

This work shows that high resolution magic angle spinning (HR-MAS) can be used to elucidate ligand–receptor binding phenomena for mass-limited samples in liquid solution. With respect to conventional 5 mm liquid-state NMR probe heads, HR-MAS allows for a considerable reduction (about one order of magnitude) in sample volume requirement, which represents a considerable advantage when working with biological samples present in low amount. More specifically, using a 20-μL active volume HR-MAS probe head, NMR experiments typically used to describe binding events (e.g. Saturation Transfer Difference, WaterLOGSY, Diffusion Ordered Spectroscopy) were successfully recorded. In fact, contrary to other NMR experiments (e.g. TOCSY), which require special care when performed on liquid samples by HR-MAS, no interferences due to fast sample spinning were observed here. Collectively, the results indicate that HR-MAS allows for a significant mass sensitivity enhancement (a 3-fold increase in signal-to-noise ratio per unit of measuring time and mass was obtained here), hereby improving correspondingly the detection limit of the previously mentioned NMR experiments. More importantly, because MAS removes magnetic susceptibility broadenings in the sample, a good spectral resolution can be easily achieved in HR-MAS even for slightly heterogeneous samples, such as those arising from incomplete purification.