Publications

*Kazuya Saito, Mafumi Hishida, and Yasuhisa Yamamura,
A Possible Critical Point for Nematic Order on the Basis of Landau Free Energy Having Dual Instabilities for Nano-Segregated Smectic Liquid Crystal”,
Soft Matter 11, 8493-8498 (2015).

[Summary] Landau expansion of free energy assuming dual instabilities for nano-segregated SmA phase is analyzed. In addition to known phase sequences (on cooling, disordered isotropic liquid => nematic phase!smectic phase, and disordered isotropic liquid => smectic phase), a new sequence (disordered isotropic liquid => density wave with subsidiary nematic order => smectic phase) and the existence of a critical point are demonstrated in case where the instability for density wave formation occurs at a higher temperature.

Mafumi Hishida, Asami Endo, Koyomi Nakazawa, Yasuhisa Yamamura, and *Kazuya Saito ,
Effect of n-alkanes on lipid bilayers depending on headgroups,
Chemistry and Physics of Lipids 188, 61-67 (2015).

[Summary] 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.

Takuya Yanagimachi, Mafumi HIshida, Yasuhisa Yamamura, and Kazuya Saito,
Ultraslow oscillation of nematic disclination after abrupt switching of DC voltage,
Journal of the Physical Society of Japan 84, 033601/1-4 (2015).

[Summary] A slow damped oscillation of nematic disclination was observed after abrupt (step-wise) change of DC electric field. The oscillation was not observed under high frequency AC field. These results imply that molecular reorientation in-fluence disclination dynamics after abrupt switching. An effective model, different from the elastic theory, was proposed to analyze the results.

Mafumi Hishida, Yasuhisa Yamamura, and *Kazuya Saito,
Salt effects on lamellar repeat distance depending on head groups of neutrally charged lipids,
Langmuir 30, 10583-10589 (2014).

[Summary] Change in lamellar repeat distances of neutrally charged lipids upon addition of monovalent salts was measured with small-angle X-ray scattering for combinations of two lipids (PC and PE lipids) and six salts. Large dependence on lipid head group is observed in addition to those on added cation and anion. The ion and lipid dependences have little correlation with measured surface potentials of lipid membranes. These results indicate that the lamellar swelling by salt is not explained through balance among interactions considered previously (van der Waals interaction, electrostatic repulsion emerged by ion binding, etc.). It is suggested that effect of water structure, which is affected by not only ions but also lipid itself, should be taken into account for understanding membrane−membrane interactions, as in the Hofmeister effect.

Koyomi Nakazawa, Mafumi Hishida, Shigenori Nagatomo, Yasuhisa Yamamura, and *Kazuya Saito,
Interplay between phase transition of DPPC bilayer and photoisomerization of doped stilbene molecule,
Chemistry Letters 43, 1352-1354 (2014).

[Summary] Cis-trans conformational change of incorporated stilbene in lipid bilayer and the phase-transition behavior of the lipid show strong correlation to each other: Phase-transition temperature is lower with cis-stilbene than with trans-stilbene, whereas photoisomerization rate of stilbene is affected by the phase of the lipid.

*Mafumi Hishida, Koichiro Tanaka, Yasuhisa Yamamura, and Kazuya Saito,
Cooperativity between water and lipids in lamellar to inverted-hexagonal phase transition,
Journal of the Physical Society of Japan 83, 044801 (2014).

[Summary] It has been unclear whether the role of water in the self-assembly of soft materials and biomolecules is influential or water is just a background medium. Here we investigate the correlation between hydration state of lipid membrane and structural phase transition of the membrane between lamellar and inverted-hexagonal phases, as an intermediate process of membrane fusion, by using the complementary techniques of X-ray scattering and terahertz (THz) spectroscopy. By comparing two lipid species, our results indicate that the structural changes of the lipid membrane depend on the behavior of the surrounding water, especially in the second hydration layer, in addition to the molecular shape of lipids. The water behaves differently at each membrane surface owing to the different hydrophilicities of the lipid head groups.