Characterization of side-chain dynamics in a microcrystalline protein by solid state NMR spectroscopy

Rotational Resonance has previously been employed for the determination of internuclear distances in peptides and proteins in the solid state. We present an extension to this theory which facilitates the characterization of both torsion angles and rotameric 'hopping' events experienced by branched amino acid side-chains. This technique has been applied to valine residues present in both flexible and structurally constrained regions of the third Immunoglobulin binding domain of Protein G prepared in the microcrystalline state. Generally, distances and torsion angles obtained were in good agreement with those reported by X-ray diffraction. We have concluded that the side-chain of the surface-exposed valine 21 likely experiences dynamic behaviour which cannot be exclusively attributed to an exchange between rotameric states. Despite this result, we remain confident that the technique introduced in this work can be used to characterize the behaviour of branched side-chains in other solid-phase proteins.