Bicelles at low concentrations.

Bilayered detergent-lipid assemblies known as bicelles have been widely used as model membranes in structural biological studies and are being explored for wider applications, including pharmaceutical use. Most studies to date have involved the use of concentrated bicelle mixtures, such that little is known about the capacity of bicellar mixtures to be diluted without unwanted transitions to nonisotropic phases. Here, different detergent/lipid mixtures have been explored, leading to the identification of two different families of bicelles for which it is possible to lower the total amphiphile (detergent + lipid) concentration to <1% (w/v) while retaining isotropic assemblies. These include a novel family of bicelles based on mixtures of 6-cyclohexyl-1-hexylphosphocholine (Cyclofos-6) and the lipid dimyristoylphosphatidylcholine (DMPC). Bicelles formed by these mixtures can be diluted to <0.5% and also have attractive biochemical properties. However, a caveat of our results is that the diffusion coefficients measured for the lipid component of the different bicelles tested were seen to be dependent on sample history, even though all samples were optically transparent. This suggests that the phase behavior of bicelles at low lipid-to-detergent ratios may be more complex than previously appreciated.