Temperature−Composition Diagram of Dimyristoylphosphatidylcholine−Dicaproylphosphatidylcholine “Bicelles” Self-Orienting in the Magnetic Field. A Solid State ²H and ³¹P NMR Study

Abstract

Mixtures of dimyristoylphosphatidylcholine (DMPC) and dicaproylphosphatidylcholine (DCPC) were investigated by solid-state 31P and 2H NMR. By variation of the mole fraction, X, of DMPC in DCPC, the temperature, and the water content, the conditions at which small discoidal particles, the so-called “bicelles”, self-orient in the magnetic field were determined. The bicellar region has an ellipsoidal shape and is delineated by compositions and temperatures ranging from X = 65−87% and T = 25−45 °C, at 80% hydration in the presence of 100 mM KCl. In the absence of salt, the bicelles still orient in the field but for a narrower range of composition (X ≈ 72−87%). The degree of macroscopic orientation increases in the presence of salt. Reducing hydration to 40% increases the breadth of the bicelle domain. On the contrary, above 95% water content, bicelles are no longer detected by NMR. In the bicelle domain, water shows residual ordering which linearly increases with the decrease in water content. Analysis of the water behavior in terms of bound and free water suggests that water bound to bicelles has the same surface properties as in pure lipid membranes and that it promotes swelling. Perdeuteration of the DMPC lipid chains indicates a reduction in local ordering in the bicelle core, relative to pure lipid dispersions, for corresponding temperatures. As a consequence, the bilayer would be slightly thinner in the bicelle.

Bicelles as model membranes for solid‐ and solution‐state NMR studies of membrane peptides and proteins

“…This orientation occurs above the DMPC gel-to-fluid transition temperature (43), typically between 30°and 50°C for DMPC/DHPC molar ratios (often referred to as q) above 2.3:1 and lipid concentrations of 3-40% w/v in aqueous solution (17,38,44,45). At small q ratios, isotropic bicelles are formed with a small diameter and rapidly tumble in solution.…”

“…The temperature range of stability for Bic/PE is also reduced (29). Interestingly, the presence of salts such as KCl, NaCl, CaCl 2 , and MgC 2 , in a concentration range of 50 -200 mM, has been shown to improve the magnetic orientation (45,47). Bicelles are also stabilized by dehydration, which Figure 1 Applications of bicelles in nuclear magnetic resonance, assuming a discoid model.…”

Applications of neutron and X-ray scattering to the study of biologically relevant model membranes

“…12a) -as deduced from early NMR data -where the long-chain lipids formed the planar portion of the disk and the short-chain lipids segregated into the disk's rim (Sanders and Landis, 1995;Raffard et al, 2000;Sternin et al, 2001). Although NMR is a powerful technique for probing the molecular local environment, SANS and SAXS techniques are capable of determining the aggregate morphologies.…”

A review of factors affecting the success of membrane protein crystallization using bicelles

“…In PC lipids, the temperature range for the discoidal shape is extended by the presence of monovalent salts, which suggests an effect opposing anions on zwitterionic lipids 6 (48). Therefore the presence of ions can be used to modulate and stabilize the fluidity and phase of the bicelle during crystallization, in addition to precipitating and stabilizing the protein.…”