Purple Membrane Induced Alignment of Biological Macromolecules in the Magnetic Field

Abstract

The general possibility to align biological macromolecules in the magnetic field by the presence of orienting agents such as lipid bicelles has led to a wealth of new structural parameters which can be derived from residual tensorial interactions in high resolution NMR. Here, we report that alignment of water soluble biomacromolecules can be achieved in the presence of the naturally occurring two-dimensional crystals (purple membrane) of the membrane protein bacteriorhodopsin. The extent of the alignment is tunable by the concentration of purple membranes and by the addition of salt which reduces the strength of the electric interaction between solute macromolecule and the membrane. At very high salt concentrations the purple membrane suspension undergoes a phase transition to a very viscous state. In this state, rotation of the membranes is hindered such that the orientation of the membrane patches persists even in the absence of a magnetic field. The induced alignment of solute molecules is shown for the two proteins, ubiquitin and p53, with residual dipolar one-bond 1H−15N couplings in the order of 20 Hz. A concept for the description of the irreducible components of the alignment tensors as a linear vector space is presented. In the case of ubiquitin, the direction of the alignment tensor differs strongly from the alignment tensor observed in DMPC/DHPC bicelles. This offers the possibility of an accurate triangulation of the bond vector direction from a combination of the two alignment experiments.