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Langmuir 2004, 20, 614-618
Heterogeneous Molecular Distribution in Supported Multicomponent Lipid Bilayers Fuyuki Tokumasu,† Jeeseong Hwang,‡ and James A. Dvorak*,† Laboratory of Malaria and Vector Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892, and Optical Technology Division, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received August 20, 2003. In Final Form: November 19, 2003 Membrane domains contribute important structural and functional attributes to biological membranes. We describe the heterogeneous nanoscale distribution of lipid molecules within microscale membrane domains in multicomponent lipid bilayers composed of dipalmitoylphosphatidylcholine (DPPC), dilauroylphosphatidylcholine (DLPC), and cholesterol (chol). The lipids were labeled with the fluorescent lipid analogues Bodipy-PC and DiI-C20:0 to identify the distribution of individual membrane components. We used a near-field scanning optical microscope (NSOM) at room temperature to identify the nanoscale structures in the membrane. Simultaneous multicolor NSOM imaging at the emission maxima of the fluorescent analogues revealed a patchy distribution of Bodipy-PC and DiI-C20:0 indicative of phase separations in the bilayer. In a cholesterol-free system (DPPC/DLPC ) 1:1), NSOM images proved that the two phosphatidylcholine molecules can coexist in domains at the micrometer level but form nanoscopic patches within the domains; DPPC occurs at the edge of the domains, whereas DLPC is present throughout the domains. In the presence of cholesterol (DPPC/DLPC ) 7:3, chol ) 18.9%), the two lipid molecules were more miscible but incomplete phase separations also occurred. The average domain sizes were 140-200 nm, well below the resolution capabilities of diffraction-limited light microscopy techniques; the domains were unresolvable by confocal microscopy. Our high-resolution NSOM studies of membrane domain behavior provide a better understanding of complex membrane phase phenomena in multicomponent biological membranes.
Introduction A variety of the physical properties of lipid membranes are influenced by phase separation phenomena of constituent lipid molecules. Structural differences in lipid molecules such as acyl chain length, headgroup size and charge, and degree of saturation influence phase properties. In biological systems, the interactions among lipid molecule species induce the formation of membrane domains and phase separations. For example, sphingomyelines, phosphocholines, and cholesterol have been identified in concert with membrane proteins to form detergent-resistant membranes (DRMs) or rafts, which are important to a variety of membrane functions.1,2 At physiological temperature, it has been proposed that rafts are liquid-ordered phase domains.3 Although resistance to detergent solubilization demonstrated the existence of rafts and their functional importance in cell membranes, details regarding the structure and function of rafts remain to be elucidated. One technical problem hampering studies of the structural characteristics of membrane domains is an inability to observe lipid domains smaller than the light microscope diffraction limit. As defined by Abbe, at a wavelength of 488 nm, the maximum point-to-point * Corresponding author. Mailing address: Bldg 4, Rm B2-11, Biochemical and Biophysical Parasitology Section, Laboratory of Malaria and Vector Research, National Institute of Allergy and Infectious Diseases, National Institutes of Health, 4 Center Dr. Bethesda, MD 20892-0425. Phone: 301-496-4880. Fax: 301-4801438. E-mail:
[email protected]. † National Institutes of Health. ‡ National Institute of Standards and Technology. (1) Simons, K.; Ehehalt, R. J. Clin. Invest. 2002, 110, 597-603. (2) Paratcha, G.; Ibanez, C. F. Curr. Opin. Neurobiol. 2002, 12, 5429. (3) London, E.; Brown, D. A. Biochim. Biophys. Acta 2000, 1508, 182-95.
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resolution is approximately 175 nm using an objective and condenser combination at a full aperture of 1.4. Recent studies propose that the size of cell membrane rafts is fitting errors). We also observed a local high fluorescence intensity only of DLPC (Figure 2, region E arrow), demonstrating the existence of strong phase separations at the nanoscopic level (∼210 nm full width, half-maximum). Thus, in an equimolar mixture of DLPC and DPPC, the fluid phase is more discontinuous and heterogeneous than the gel phase at the microscopic level, confirming the CFM data that showed clear discontinuities in the fluid phase, surrounded by a continuous gel phase.10 However, at the nanoscopic level, smaller domain sizes were also found in the fluid phase. Three-Component Lipid Mixtures. A CFM GUV study10 at a resolution limit of ∼300 nm could not directly detect nanoscopic domains in various mixtures of DLPC/ DPPC/chol. However, fluorescence resonance energy transfer (FRET) data suggested that nanoscopic domains did exist. In contrast, an AFM study5 demonstrated that structural nanoscopic domains with an average domain size of ∼48 nm exist in ternary mixtures. We used the NSOM to analyze the phase separation of these lipid mixtures at the nanometer scale. NSOM images (Figure 3) showed that a mixed lipid membrane (mole ratio of DLPC/DPPC ) 3:7, chol ) 18.9 mol % of the total lipids) consists of flat structures with distinguishable domains scattered throughout the membrane. In fluorescence images, DPPC analogues appeared to be distributed in discrete, patchy domains. In contrast, DLPC analogues showed a more continuous distribution of domainlike patches. These results demonstrate the marked differences in continuity between a ternary mixture and an equimolar binary mixture. From the angle(16) Petersen, N. O.; Hoddelius, P. L.; Wiseman, P. W.; Seger, O.; Magnusson, K. E. Biophys. J. 1993, 65, 1135-46.
Heterogeneous Molecular Distribution in Bilayers
Langmuir, Vol. 20, No. 3, 2004 617 Table 1. Comparison of Domain Sizes Observed by Confocal Microscopy and Near-Field Scanning Optical Microscopy two-component systema
CFM NSOM
three-component systemb
nanoscopic domains
microscopic domains
nanoscopic domains
microscopic domains
g300 nmc 123-145 nm
e ∼20 µm 412-505 nm
undetectabled 70-100 nm
undetectable >2 µme
a DLPC/DPPC ) 1:1, no cholesterol. b DLPC/DPPC ) 3:7, chol ) 18.9%. c Nanometer scale domains were only detected above the diffraction limit of the light microscope. d FRET data suggested the existence of nanoscopic domains, but they were not detected by the CFM. e The NSOM image (6 × 6 µm) did not show any microscopic domains. Domain sizes for the NSOM are based on 1/e values of the angle-averaged autocorrelation fit.
Figure 3. Nanoscopic phase separation in a three-component lipid mixture (DLPC/DPPC ) 3:7, chol ) 18.9%). (a) Topographic image. The red channel (DiI) (b) of NSOM images shows marked discontinuities compared with the green channel (Bodipy-PC) (c); the lipid distributions of the two components are more separated (d) than distributions present in equimolar mixtures. (e-g) Corresponding 2D autocorrelation images for images a-c. None of the images show any features larger than the diffraction limit of light. (g) Angle-averaged 2D autocorrelations for both channels also show only ∼80-100 nm nanoscopic phase separations.
averaged 2D autocorrelation images (Figure 3h), the calculated full width of 1/e maximum (2ξ1) of the fluid and gel phases was ∼203 to ∼139 nm, respectively. However, microscopic domain features were not detected and 2D autocorrelation images for both Bodipy-PC and DiI did not reveal any features larger than the diffraction limit of light (Figure 3e-g), confirming the previously reported CFM results. The angle-averaged plots of these images (Figure 3h) show that the long-range 1/e decay lengths (ξ2) are ∼3.8 and ∼4.5 µm (full width of 1/e maximum, 2ξ2) with negligible (
