The effects of ethylene oxide containing lipopolymers and tri-block copolymers on lipid bilayers of dipalmitoylphosphatidylcholine.

A comparative study is conducted on the influence of two types of polymeric compounds on the phase behavior of 1,2-dihexadecanoyl-s,n-glycero-3-phosphotidylcholine (DC16PC) lipid bilayers. The first polymeric compound is a lipopolymer, with two different lengths of a hydrophilic polyethylene oxide moity, anchored to the bilayer by a 1,2-dioctadecanoyl-s,n-glycero-3-phosphoethanolamine (DC18PE) lipid. The second type, which is a novel type of membrane-spanning object, is an amphiphilic tri-block copolymer composed of two hydrophilic stretches of polyethylene oxide separated by a hydrophobic stretch of polystyrene. Hence the tri-block copolymer may act as a membrane-spanning macromolecule mimicking an amphiphilic protein or polypeptide. Differential scanning calorimetry is used to determine a partial phase diagram for the lipopolymer systems and to assess the amount of lipopolymer that can be loaded into DC16PC lipid bilayers before micellization takes place. Unilamellar and micellar phase structures are investigated by fluorescence quenching using bilayer permeating dithionite. The chain length-dependent critical lipopolymer concentration, denoting the lamellar-to-micellar phase transition, compares favorably with a theoretical prediction based on free-energy considerations involving bilayer cohesion and lateral pressure exerted by the polymer chains.

Indirect evidence for lipid-domain formation in the transition region of phospholipid bilayers by two-probe fluorescence energy transfer.

The fluorescence energy transfer between two lipid probes, N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1, 2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (donor) and N-(Lissamine rhodamine B sulfonyl)-1, 2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (acceptor), incorporated into 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine unilamellar and multilamellar lipid bilayers, is studied in the temperature region of the main phase transition. The two probes display different relative solubilities in the gel and fluid lipid-bilayer phases. A distinct maximum in the fluorescence intensity of the donor is observed in the transition region, indicating that the two probes are demixing and hence increasing their average separation. The observation is interpreted in terms of dynamic segregation of the two probes into coexisting gel and fluid lipid domains that are formed dynamically in the transition region due to strong density fluctuations. The interpretation of the experimental observations is supported by a detailed theoretical calculation using computer simulation of a microscopic model that takes full account of diffusion of the two probes and the fluctuations of gel and fluid lipid domains characteristic of the main phase transition.