Takaaki Ishigami

Chiral Recognition of L-Amino Acids on Liposomes Prepared with L-Phospholipid.

In this study, we demonstrated that liposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) can recognize several l-amino acids, but not their d-enantiomers, by analyzing their adsorptive behavior and using circular dichroism spectroscopy. Changes in liposomal membrane properties, determined based on fluorescent probe analysis and differential scanning calorimetry, were induced by l-amino acid binding. UV resonance Raman spectroscopy analysis suggested that the chiral recognition was mediated by electrostatic, hydrophobic, and hydrogen bond interactions, where the recognition site could therefore be constructed on the DPPC membrane. Our findings clearly indicate the potential function of liposomes in asymmetric recognition.

Chiral Selective Adsorption of Ibuprofen on a Liposome Membrane

We investigated the key factors that affect enantioselective adsorption of ibuprofen (IBU) on a liposome membrane by changing its lipid composition: the liposome membrane shows different membrane fluidity, surface charge, content of chiral components, and heterogeneity (nanodomain). Nonspecific interactions (hydrophobic and electrostatic) were revealed to be an important factor in enhancing the adsorbed amount of IBU, based on adsorption experiments carried out using single lipids (DPPC, DMPC, DOPC, and DLPC) and positively charged liposomes (DOTAP and liposome containing DC-Ch). Furthermore, control of the boundary edge (i.e., the nanodomain size) derived from the membrane heterogeneity was important for enantioselective adsorption; as well as multiple weak interactions between lipid molecules and IBU enantiomers. The above findings provided a good index for constructing liposomal chiral adsorbents.

High performance optical resolution with liposome immobilized hydrogel.

We prepared liposome immobilized hydrogels (LI-gels) for analysis and separation of chiral molecules, to overcome the drawbacks of liposomes such as low stability, and difficulties with handling and isolation from sample solutions. The amounts of liposomes in the hydrogels were larger than those in other solid matrices reported previously. The liposome morphology was intact, and its original properties, such as fluidity and phase transition behaviors, were preserved. We investigated the chiral recognition performance of the LI-gel, as described in our previous paper. Our results indicate that the enantioselectivity of the LI-gel was higher than those of conventional methods and of the liposomes alone. Our prepared LI-gel therefore overcomes the drawbacks of liposomes, and has potential applications in analysis and separation, including chiral separation.

Effect of Boundary Edge in DOPC/DPPC/Cholesterol Liposomes on Acceleration of L-Histidine Preferential Adsorption.

In order to investigate the interaction of hydrophilic molecules with liposomal membranes, we employed 1-(4-(trimethylamino)phenyl)-6-phenyl-1,3,5-hexatriene and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(5-dimethylamino-1-naphthalenesulfonyl) as fluorescent probes to monitor the surface regions of the membrane, and the results for various liposomes were plotted in correlation diagrams. According to the formation of a variety of phase states, different tendencies of decreasing surface hydrophobicity were observed in the liposomes that were modified with high concentrations of cholesterol or in the liposomes that were composed of ternary components. These liposomes, with hydrophobic surfaces, also showed preferential adsorption of l-histidine (l-His), and the hydrophobicity of the liposomal membrane at the surface changed during l-His adsorption regardless of the initial liposomal properties. Furthermore, we revealed that accelerated adsorption of l-His and preferential binding was induced in ternary liposomes forming boundaries between two separate phases.

Homochiral oligomerization of L-histidine in the presence of liposome membranes

We report about a novel method for the homochiral oligomerization of l-amino acids in the presence of liposomes. For this homochiral oligomerization, we took advantage of the superior properties of liposomes as chiral adsorbent, concentrator of reacting molecules, and as effective reaction medium. Indeed, liposomes composed of l-phospholipids could enantioselectively bind and concentrate only l-histidine (l-His) on the liposome membrane, and not D-His, which was confirmed by UV measurements. Furthermore, we demonstrate that the liposomes enabled effective oligomerization of l-His on or in the liposome membrane, while the oligomerization of d-His in the liposome system—or in absence of liposomes—was less efficient. The experimental findings are supported by logP value calculations. These calculations indicate that activated intermediates locate in the center of the bilayer where they are protected from hydrolysis due to the hydrophobic environment of the liposome membrane. Therefore, the use of liposomes from l-phospholipids enables homochiral oligomerization and more efficient oligomerization of l-amino acids as compared to d-amino acids. Thus, our developed method could be used for the synthesis of homochiral polymers from racemic mixtures of monomers and may contribute to the understanding of prebiotic biopolymer formation.