posted on 2015-12-17, 04:22authored byMaïwenn Beaugrand, Alexandre
A. Arnold, Jérôme Hénin, Dror E. Warschawski, Philip T. F. Williamson, Isabelle Marcotte
Bicelles are model
membranes generally made of long-chain dimyristoylphosphatidylcholine
(DMPC) and short-chain dihexanoyl-PC (DHPC). They are extensively
used in the study of membrane interactions and structure determination
of membrane-associated peptides, since their composition and morphology
mimic the widespread PC-rich natural eukaryotic membranes. At low
DMPC/DHPC (q) molar ratios, fast-tumbling bicelles
are formed in which the DMPC bilayer is stabilized by DHPC molecules
in the high-curvature rim region. Experimental constraints imposed
by techniques such as circular dichroism, dynamic light scattering,
or microscopy may require the use of bicelles at high dilutions. Studies
have shown that such conditions induce the formation of small aggregates
and alter the lipid-to-detergent ratio of the bicelle assemblies.
The objectives of this work were to determine the exact composition
of those DMPC/DHPC isotropic bicelles and study the lipid miscibility.
This was done using 31P nuclear magnetic resonance (NMR)
and exploring a wide range of lipid concentrations (2–400 mM)
and q ratios (0.15–2). Our data demonstrate
how dilution modifies the actual DMPC/DHPC molar ratio in the bicelles.
Care must be taken for samples with a total lipid concentration ≤250
mM and especially at q ∼ 1.5–2, since
moderate dilutions could lead to the formation of large and slow-tumbling
lipid structures that could hinder the use of solution NMR methods,
circular dichroism or dynamic light scattering studies. Our results,
supported by infrared spectroscopy and molecular dynamics simulations,
also show that phospholipids in bicelles are largely segregated only
when q > 1. Boundaries are presented within which
control of the bicelles’ q ratio is possible.
This work, thus, intends to guide the choice of q ratio and total phospholipid concentration when using isotropic
bicelles.