Use of Isotropically Tumbling Bicelles to Measure Curvature Induced by Membrane Components
Isotropically tumbling discoidal
bicelles are a useful biophysical tool for the study of lipids and
proteins by NMR, dynamic light scattering, and small-angle X-ray
scattering. Isotropically tumbling bicelles present a low-curvature
central region, typically enriched with DMPC in the lamellar state,
and a highly curved detergent rim, typically composed of DHPC. In this
report, we study the impact of the partitioning and induced curvature
of a few molecules of a foreign lipid on the bicelle size, structure,
and curvature. Previous approaches for studying curvature have focused
on macroscopic and bulk properties of membrane curvature. In the
approach presented here, we show that the conical shape of the DOPE
lipid and the inverted-conical shape of the DPC lipid induce
measurable curvature changes in the bicelle size. Bicelles with an
average of 1.8 molecules of DOPE have marked increases in the size of
bicelles, consistent with negative membrane curvature in the central
region of the bicelle. With bicelle curvature models, radii of
curvature on the order of −100 Å and below are measured, with a
greater degree of curvature observed in the more pliable Lα state
above the phase-transition temperature of DMPC. Bicelles with an
average of 1.8 molecules of DPC are reduced in size, consistent with
positive membrane curvature in the rim, and at higher temperatures,
DPC is distributed in the central region to form mixed-micelle
structures. We use translational and rotational diffusion measurements
by NMR, size-exclusion chromatography, and structural models to
quantitate changes in bicelle size, curvature, and lipid dynamics.