Photo-control of a bilayer-to-nonbilayer phase transition (the liquid-crystalline Lalpha phase to the inverted hexagonal HII phase) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) by photoisomerization of incorporated stilbene molecules was examined utilizing differential scanning calorimetry (DSC), small-angle X-ray diffraction (SAXD), ultraviolet (UV)/visible absorption, and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopies. cis-Stilbene lowered the transition temperature, Th, to a greater extent than trans-stilbene, and the difference was at most ca. 10 degree C. At temperatures higher than Th of POPE/cis-stilbene but lower than that of POPE/trans-stilbene, photo-isomerization from the trans to the cis form of the stilbene molecules by irradiation with UV light caused a Lalpha-HII phase transition. The UV irradiation partially induced the HII phase at a constant temperature due to the incomplete photoisomerization of stilbene (ca. 60%). The reduction of Th by the incorporation of stilbenes was caused mainly by reducing the spontaneous radius of curvature of the lipid monolayer, R0. The greater bulkiness of cis-stilbene as compared to the trans form resulted in a more effective reduction of R0 and stabilization of the HII phase.
(Langmuir, 32, 7647-7653 (2016))
Incorporated molecules in biomembranes often have both a core and an alkyl chain in a single molecule like sterols. To clarify effects of these two parts in a molecule, phase behaviors of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer containing 4-n-alkyl-6-cyanobiphenyl (nCB) (n = 0 - 8) are investigated. Both the trends of the main transition temperature (Tm) with respect to n and the pretransition temperature (Tp) with respect to nCB content change at n = 3. It is suggested that two parts have contrary effects on the phase behavior of DPPC bilayer. The core part likely perturbs the molecular ordering in the gel phase and lowers Tm as cholesterol, whereas an alkyl chain orders the lipids in the gel phase and raises Tm as n-alkanes. Tm shows the odd-even effect depending on the alkyl chain length of minor component, nCB. Depending on n, effects of the additives on Tp show the content dependences, although the pretransition is rarely observed at the high contents.
(Langmuir, 32, 5966-5972 (2016))
Towards a greater understanding of the effects of organic molecules in biomembranes, the effects of a flexible alkyl chain on the morphologies of phospholipid vesicles are investigated. Vesicles composed of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine and tetradecane (TD) rupture during cooling from the liquid-crystalline phase to the gel phase. A model calculation based on the size-dependent rupture probability indicates that the bending rigidity of the bilayer in the gel phase is more than 10 times higher than that without TD, resulting in the rupture arising from elastic stress. The rigidification is caused by the denser molecular packing in the hydrophobic region by TD. There is little change of the rigidity in the liquid-crystalline phase. Additionally, the rigidification produces a characteristic morphology of the ternary giant vesicles including TD. Reported thermal behaviors imply that molecules with a linear and long alkyl chain, such as trans fatty acids, universally exhibit a similar effect, in contrast to rigid and bulky molecules, such as cholesterol.
(J. Chem. Phys., 144, 041103 (2016))
Phase behavior and structural properties were examined for bilayers of phospholipids with
different headgroups (DMPC, DMPS and DMPE) while adding n-alkanes to study effect of flexible additives. Change in the temperatures of main transition of the lipid/alkane mixtures against the length of added alkanes depends largely on the headgroup. Theoretical analysis of the change of the temperature of transition indicates that the headgroup dependence is dominantly originated in the strong dependence of total enthalpy on the headgroups. The results of X-ray diffraction show that the enthalpic stabilization due to enhanced packing of lipid molecules by alkanes in the gel phase causes the headgroup-dependent change in the phase transition behavior. The enhanced packing in the gel phase also lead to easy emergence of the subgel phase with very short relaxation times at room temperature in the DMPE-based bilayers.
(Chem. Phys. Lipids, 188, 61-67 (2015))
1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer with stilbene was examined using phase-contrast and fluorescence microscopies, differential scanning calorimetry, and Fourier transform infrared and UV/visible absorption spectroscopies. The results indicate that the temperature of gel/liquid-crystalline phase transition of DPPC bilayer with cis-stilbene is much lower than that with trans-isomer. Photoirradiation on trans-stilbene in DPPC bilayer suppresses the pretransition (Lbeta' to Pbeta'), while the gel/liquid-crystalline phase transition (Pbeta' to Lalpha) changes little due to the poor photoisomerization rate in both gel and liquid-crystalline phases. It is shown that photoisomerization of incorporated stilbene and the phase behavior of the lipid bilayer depend on each other.
(Chem. Lett., 43, 1352-1354 (2014))
To clarify the influence of cis and trans double bonds on conformational disordering of unsaturated hydrocarbon-chain of lipids in bilayer formed in the binary system with water, calorimetric study was conducted for systems of unsaturated monoacylglycerols; monoolein (MO), monovaccenin (MV), and monoelaidin (ME). Heat capacities of the binary systems were measured by adiabatic calorimetry. The observed entropies of transition (DeltatrsS) from lamellar phase (Lalpha) to fluid isotropic (FI) phase are very small and depend on lipids: DeltatrsS of MO/water, MV/water, and ME/water were ca. 0.8 J K-1 (mol of lipid) -1, ca. 0.9 J K-1 (mol of lipid) -1, and ca. 0.4 J K-1 (mol of lipid) -1, respectively. These show that the conformational disordering of the hydrocarbon chain over gauche and trans conformations is suppresed in Lalpha phase. Through the comparison of DeltatrsS among the binary systems, the suppresed conformational disordering of hydrocarbon chain in the bilayer is discussed.
(J. Phys. Chem. B, 115, 14963-14968 (2011))
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