A novel leaflet-selective fluorescence labeling technique reveals differences between inner and outer leaflets at high bilayer curvature

Understanding the differences in the physical properties of the inner and outer leaflet of membranes and how the leaflets are coupled to each other requires methods that can selectively label both the outer and inner leaflets. In this report we introduce a combined chromatography/cyclodextrin method for selective labeling of the inner leaflet. Combining this method with selective labeling of the outer leaflet, we are able to show that there is a distinct difference in polar headgroup physical properties of the inner and outer leaflet headgroups in small unilamellar vesicles composed of a wide variety of phosphatidylcholines and a phosphaticylcholine/sphingomyelin mixture. It appears that the inner leaflet headgroups are more tightly packed than those of the outer leaflet. This differential packing disappears when vesicle size increases, showing that it is a consequence of membrane curvature. Differential packing is also reduced as acyl chain length is decreased. In the future, selective leaflet labeling is likely to be a powerful tool for investigating the properties of asymmetric lipid vesicles.