Temperature dependence of DMPC/DHPC mixing in a bicellar solution and its structural implications

Bicelle-forming mixtures of short-chain 1,2-dicaproyl-sn-glycero-3-phosphocholin (DHPC) and long-chain 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) exhibit complicated phase behavior. We studied chain-melting and morphological phase changes in mixtures of DMPC/DHPC of 4.5:1 molar ratio in the temperature range of 0-60 degreesC. The phase state of both lipids and the alignment of lipid monolayers in the magnetic field were determined by solid-state H-2 and P-31 NMR. DMPC and DHPC mix in a micellar state at 0-10 degreesC but separate upon heating to 15 degreesC into gel-phase DMPC bilayers and DHPC micelles. Near the gel-to-liquid-crystalline phase transition of DMPC, at 24 degreesC, the DHPC spectra become anisotropic suggesting arrangement of DHPC in a mixed DMPC/DHPC phase. DMPC in this structure possesses the chain order typical of liquid crystalline bilayers, but DHPC chain order remains much lower, with lipid aggregates oriented mostly at random. In the bicelle phase, at 32-36 degreesC, highly anisotropic lipid particles with their DMPC monolayer normal oriented perpendicular to the magnetic field are formed. The angular distribution function of bilayer normals has significant mosaic spread and tails with surprisingly high intensity that is caused by the monolayers that are oriented at random. In the bicelle phase, magic angle spinning nuclear Overhauser effect H-1 NMR spectrometry revealed intensive intermolecular cross-relaxation among DMPC and among DHPC molecules but no detectable magnetization exchange between DMPC and DHPC, confirming that DHPC and DMPC experience limited physical contact. Above 36 degreesC, chain order of DMPC decreased, in particular for the bonds near the terminal methyl groups, while order parameters of DHPC increased, both indicative for DMPC/DHPC mixing in the same monolayer. The degree of this mixed monolayer orientation at temperatures above the bicelle state is significantly lower. Near 60 degreesC a fraction of DHPC converts to a micellar phase that also dissolves a trace of DMPC. Thus, a highly aligned bicelle phase exists only in a narrow range of temperatures. Complex orientational distributions arise at other temperatures, driven by lipid mixing and structural phase changes.