Atomic force microscopic study on lipid bilayer nanoscale phase separation

Abstract

Phase separation of copolymers or lipid membranes in nanoscale has attracted increasing interests for their applicationsin the synthesis of inorganic nanomaterial. The nanoscale phase separation of liquid bilayer as a supported membrane is systematically investigated by atomic force microscope (AFM). Moreover, the position of the fluorescence indicator, which is commonly used in the phase separation study in optical microscope, is also probed in this study to complete the knowledge of tradition microscopy research. Overall, the authors demonstrate the advantages of using AFM for phase separation study in this article. IntroductionLipids bilayer phase separation is always a popular research field as a simplest mimic model for microdomains or rafts in biosystems, which are believed to play a significant role in protein sorting, signal transducting and cell growth regulating.1,2,3 It also develops researchers interested in the nanomaterial research recently for the self-assembly domain formation in nanoscale, which could be potentially used as nanomaterials' template in nanomaterials bottom-up synthesis methods. 4,5 In the liquid crystal lipid domains, the soft hydrocarbon chain of the lipids generally is vertical to the surface of the membrane and their mobility is quite high, which allows their free movement inside of the membrane. In the solid state of the lipid domains, the rigid hydrocarbon chain would be vertical or shown a certain angle to the surface of the membrane. Typical phase separation occurs in the supported membrane formed with more than two lipid types. When the temperature is high enough, both types of liquids are soft and have high enough mobility in the membrane uniformly mixed together. With decreasing temperature, the mobility of the liquid is lowered until one of them achieved the solid phase. As a result, the phase separation is observed in normal cases. This process is studied in both optical microscope and atomic force microscope (AFM) by researchers. 6,7,8 However, the drawback of optical microscope is more and more obvious for the limited observation resolution, lack of details and unavoidable introduction of the fluorescence indicator. AFM study could provide us more detailed information compared with optical microscope with precisely vertical direction monitoring. In this article, the authors show two examples on the advantages of using AFM to study the lipid bilayer phase separation. The first part shows that difference in the same lipid bilayers, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/sphingomyelin (SM), when different SM sources are introduced. Although no difference would be observed from the optical microscope, the AFM could easily find out the difference. In the second case, the commonly used fluorescence indicator, Texas Red, is introduced in the films. The precise position dispersion of the fluorescence indicator is also probed in AFM study. Experimental section Supported membrane synthesisThe lipids used in this study are DOPC, SM from cow brain (BSM) and SM from egg ...

Viscoelastic changes measured in partially suspended single bilayer membranes

“…Furthermore, AFM imaging also revealed that flat lamellar membranes are not always homogeneous: mixtures of phosphatidylcholine and phosphatidylglycerol are prone to domain separation. [20][21][22][23][24][25] Thus, even in the absence of any unopened liposomes, supported membranes can be inhomogeneous.…”

Editorial

“…Liu, in this short communication, demonstrated the superior advantages of AFM over other optical methods to systematically probe the process of phase separation of liquids bilayer as a supported membrane. 8 The approach is important to mimic models for microdomains or rafts in biosystems that play a significant role in sorting of proteins, signal transduction and regulation of cell growth.…”