Phosphorus-31 NMR study of the interaction of cations with purple membrane and of the purple-blue transition

Abstract

The authors have studied by /sup 31/P NMR both the native purple membrane and the deionized membrane obtained by removal of endogenous cations. The latter membrane was shown to undergo a blue to purple color transition with increasing pH. In parallel with this color transition, the /sup 31/P NMR spectrum obtained at high membrane concentration was dramatically changed from a normal bilayer powder pattern to a seemingly inverted line shape. This effect was not observed in native purple membrane and was found to be reversed by addition of cations to the deionized purple membrane. Several data indicated that this inversion of the /sup 31/P NMR line shape is due to orientation of the membrane fragments perpendicular to the magnetic field. Further studies indicated that both native and deionized purple membranes can undergo such magnetic orientation but that the favorable concentration range is greatly increased for the deionized preparation. This effect is attributed to differences in bacteriorhodopsin conformation and/or membrane surface charge in the two membranes. Binding of divalent cations to the purple membranes was shown to promote an increase of the chemical shift anisotropy of phospholipid phosphate groups as revealed by /sup 31/P NMR. Accordingly, binding of a trivalent paramagneticmore » cation promoted strong broadening of the /sup 31/P NMR spectrum. This suggests a close spatial or structural relationship between phospholipid head groups and cation binding sites in the purple membrane.« less