Cation Modulation of Bicelle Size and Magnetic Alignment as Revealed by Solid-State NMR and Electron Microscopy

Abstract

The influence of salts (KCl, NaCl, CaCl 2, and MgCl 2) on bicelles (bilayered micelles) made of dimyristoylphosphatidylcholine (DMPC, molar fraction X = 78%) and dicaproylphosphatidylcholine (DCPC) was investigated by solid-state 31P- and 2H NMR as well as by freeze-fracture electron microscopy. Sizes were determined from 2H- and 31P NMR on the basis of a model that incorporated a planar bilayer and a (half-torus) curved rim representing the DMPC and DCPC regions of the bicelle, respectively. Good agreement was shown with sizes determined independently from freeze-fracture electron microscopy images. In the presence of K + and Na +, bicelles have diameters of ∼300 Å while in the presence of Ca 2+ and Mg 2+; their diameter increases to ∼500 Å. Bicelle magnetic alignment is considerably improved by the presence of salts. The optimum salt concentration for such an effect ranges from 50 to 200 mM. Bicelles are magnetically aligned for temperatures roughly ranging from 30°C to 40°C with monovalent cations; this range is slightly extended in the presence of divalent salts. In this temperature range, the dynamics of the long-chain hydrocarbon region of the bicelle (leading to a bicelle thickness of 38 Å) and of water is about the same independently of cation nature and concentration. However, at higher temperatures, considerable differences in water dynamics are observed between systems with monovalent and divalent cations. In these conditions, the system consists of a mixture of micelles and extended bilayers, which show residual macroscopic alignment in the magnetic field.