Using pisa pies to resolve ambiguities in angular constraints from PISEMA spectra of aligned proteins

The structures of proteins are mapped onto the patterns of resonances in NMR spectra of aligned samples. This is most clearly illustrated with Pisa wheels of helical membrane proteins, where the distinctive 'wheel-like' patterns of resonances reflect the tilt and rotation of the helices in the bilayers. These patterns contain both structural and assignment information. This Communication describes a simple way of using this information to resolve angular ambiguities inherent in orientational constraints derived from NMR data. This contributes to the use of solid-state NMR of aligned samples for protein structure determination.