Nanoscale Confinement Effects on the Fast Motions of the Backbone and Side Chains of Ubiquitin

The prevailing model for solvent dependence of protein motions, the solvent slaving model, predicts that protein motions should be dramatically dependent on the dynamics of the solvent. We have previously measured the dynamics of water near the surface of ubiquitin when encapsulated in reverse micelles. In order to directly investigate the relationship between protein and solvent motions, we have measured the backbone and side chain ps-ns motions of ubiquitin under various reverse micelle encapsulation conditions using nuclear spin relaxation methods. Confinement of the protein in the aqueous core of the reverse micelle produces a strong, general damping of the fast backbone motions of the protein. In distinct contrast, the side chain methyl motions are only slightly affected by encapsulation in reverse micelles. No strong correlations between the protein motional character and the dynamics of the protein's hydration layer were observed, suggesting that the dynamics of water molecules near the protein surface have minimal consequences for fast dynamic motions of proteins.