Morphology of fast-tumbling bicelles: a small angle neutron scattering and NMR study

Abstract

Bilayered micelles, or bicelles, which consist of a mixture of long- and short-chain phospholipids, are a popular model membrane system. Depending on composition, concentration, and temperature, bicelle mixtures may adopt an isotropic phase or form an aligned phase in magnetic fields. Well-resolved (1)H NMR spectra are observed in the isotropic or so-called fast-tumbling bicelle phase, over the range of temperatures investigated (10-40 degrees C), for molar ratios of long-chain lipid to short-chain lipid between 0.20 and 1.0. Small angle neutron scattering data of this phase are consistent with the model in which bicelles were proposed to be disk-shaped. The experimentally determined dimensions are roughly consistent with the predictions of R.R. Vold and R.S. Prosser (J. Magn. Reson. B 113 (1996)). Differential paramagnetic shifts of head group resonances of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC), induced by the addition of Eu(3+), are also consistent with the bicelle model in which DHPC is believed to be primarily sequestered to bicelle rims. Selective irradiation of the DHPC aliphatic methyl resonances results in no detectable magnetization transfer to the corresponding DMPC methyl resonances (and vice versa) in bicelles, which also suggests that DHPC and DMPC are largely sequestered in the bicelle. Finally, (1)H spectra of the antibacterial peptide indolicidin (ILPWKWPWWPWRR-NH(2)) are compared, in a DPC micellar phase and the above fast-tumbling bicellar phases for a variety of compositions. The spectra exhibit adequate resolution and improved dispersion of amide and aromatic resonances in certain bicelle mixtures.

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

“…A resolution sufficient for highresolution NMR studies of membrane-associated peptides was achieved with this system, as discussed in the examples presented below. As mentioned previously, fast-tumbling bicelles are believed to have a disk-shape morphology and several experiments have indicated that DHPC is primarily sequestered to the bicelle rims (46,81).…”

“…2(a). The presence of two well-defined resonances in 31 P NMR spectra, 2 H NMR experiments on deuterated DMPC and DHPC, and the absence of NOEs between DMPC and DHPC in two-dimensional 1 H NMR spectra (18,46,62,72,73), suggest that DMPC molecules compose the planar surface of the disk while the short-chain lipids reside on the edges. Because both phospholipids have the same polar headgroup, the ratio between the areas of the planar surface and the rim would be equivalent to that of the long-and short-chain lipid proportion.…”

Structure Determination of Membrane Proteins by NMR Spectroscopy

“…Small isotropic bicelles are of interest as samples in their own right. 83,84 They also serve as controls for the use of larger, magnetically alignable bicelles as samples for protein structure determination by solid-state NMR methods. 85 The isotropic chemical shift frequency is a highly sensitive qualitative monitor of protein structure.…”

Structure-Functional Insight Into Transmembrane Helix Dimerization

“…A number of bicelle systems have been developed and characterized for their unique liquidcrystal phase behavior. It was shown that bicelles at some conditions have discoidal shape with a bilayer formed by long-chain lipids and a rim of short-chain lipids (Vold et al, 1997;Lee et al, 2008;van Dam et al, 2004;Glover et al, 2001;Luchette et al, 2001). The shape of the particles is controlled by three parameters: the molar ratio q of long-and short-chain lipid (or detergent) concentrations (adjusted for concentrations of free lipids in the bicellar suspension), total lipid concentration c L , and temperature T; and it can be either disc or perforated bilayer the dependence being rather complex (Vold et al, 1997).…”