High-resolution heteronuclear NMR of human ubiquitin in an aqueous liquid crystalline medium

A mixture of dihexanoyl phosphatidylcholine and dimyristoyl phosphatidylcholine in water forms disc-shaped particles, often referred to as bicelles [Sanders and Schwonek (1992) Biochemistry, 31, 8898-8905]. These adopt an ordered, liquid crystalline phase, which can be maintained at very low concentrations of the bicelles (down to 3% w/v). At this concentration the spacing between individual bicelles, on average, exceeds 300 Angstrom. The bicelles are shown to have a negligible effect on the rotational diffusion of ubiquitin as judged by the N-15 T-1 rho values of the backbone amides relative to those in isotropic aqueous solution. The protein exhibits a residual degree of alignment which is proportional to the bicelle concentration, and approximately collinear with ubiquitin's rotational diffusion tenser. The degree of alignment obtained offers unique opportunities for studying the protein's structure and dynamics.