HIGH-RESOLUTION HETERONUCLEAR NMR OF HUMAN UBIQUITIN IN AN AQUEOUS LIQUID-CRYSTALLINE MEDIUM

A mixture of dihexanoyl phosphatidylcholine and dimyristoyl phosphatid ylcholine in water forms disc-shaped particles, often referred to as b icelles [Sanders and Schwonek (1992) Biochemistry, 31, 8898-8905]. The se 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, exc eeds 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 collinea r with ubiquitin's rotational diffusion tenser. The degree of alignmen t obtained offers unique opportunities for studying the protein's stru cture and dynamics.