Yoshio Okahata

Synthetic bilayer membranes prepared from dialkyl amphiphiles with nonionic and zwitterionic head groups

Dialkyl amphiphiles with nonionic and zwitterionic head groups were prepared. The nonionic group is the polyoxyethylene chain and the zwitterionic group is composed of the combinations of ammonium sulfonate, ammonium phosphate, and ammonium amidate. These dialkyl amphiphiles, when dissolved in water, produced very large aggregates as determined by small-angle light scattering: 1–50 million daltons. The critical micelle concentration was 10−5– 10−6 M. The aggregate morphology was examined by electron microscopy. In the case of the nonionic amphiphile, the chain lengths of both the polyoxyethylene and dialkyl groups were shown to affect the morphology. When the alkyl chain length was C12, C14, C16, and C18 (but not C8), the bilayer formation was observed, and vesicles, lamellae, and disks were found depending on the amphiphile structure. The zwitterionic amphiphiles similarly produced vesicles and lamellae. Most of these aggregates showed the phase transition phenomenon (solid-liquid crystal). The transition temperature rose with increasing alkyl chain lengths in general agreement with that of lecithin.

Bilayer characteristics of 1,3-dialkyl- and 1,3-diacyl-rac-glycero-2-phosphocholines☆

Abstract Series of 1,3-dialkyl- and 1,3-diacyl-rac-glycero-2-phosphocholines (alkyl tail, C14, C16, and C18: acyl tail, C12, C14, C16, and C18) were prepared. They form stable bilayer aggregates (vesicle and lamella) in water, as confirmed by electron microscopy. Differential scanning calorimetry showed the crystal-to-liquid crystal phase transition of these bilayers to occur at higher temperatures with increasing chain lengths. The phase-transition behavior of the phosphocholine bilayers was not much affected by the mode of connection at the glycerol unit: dialkyl vs diacyl, or 1,2-disubstitution (lecithin) vs 1,3-disubstitution. Entrapment of glucosamine and ethanolamine by the synthetic 1,3-type phosphatidylcholine vesicle was probed by the fluorescamine method. Fluorescence quenching of trapped riboflavin established that the barrier function of these bilayers against alkali is similar to that of lecithin bilayer vesicles. The barrier function is lost in the liquid crystalline state. Phase separation of azobenzene-containing amphiphiles was examined in the phosphocholine and other bilayer matrices. Different phase-separation behaviors observed were explicable in terms of the head group interaction. All of these data indicate that 1,3-type phosphatidylcholines possess essentially the same bilayer characteristics as those of lecithin (1,2-type).

Multifunctional hydrolytic catalysis: II. Hydrolysis of p-nitrophenyl acetate by a polymer catalyst which contains hydroxamate and imidazole functions

Abstract A water-soluble polymer catalyst was prepared by radical polymerization of a protected hydroxamate monomer, 1-methyl-2-vinylimidazole and acrylamide, and by the subsequent NH 2 NH 2 treatment of the polymer. The hydrolysis of p -nitrophenyl acetate by the bifunctional copolymer obeyed typical burst kinetics: rapid accumulation of acetyl hydroxamate group and its slow decomposition. The acetylation rate of the hydroxamate group was rather close to that of a polymer which does not contain the imidazole unit. However, the deacylation was markedly accelerated by the presence of the imidazole unit, and the difference in rate constants amounted to 60- to 80-fold at pH 8–9. These results indicate that the overall catalytic efficiency of the bifunctional polymer is enhanced due to the complementary action of the imidazole and hydroxamate functions.

Crystal-Liquid Crystal Phase Transformation of Synthetic Bimolecular Membranes

Bimolecular membrane of lipids are in a liquid crystalline state capable of reversible structural modifications and permeation properties of biomembranes depend upon such reversible change. Therefore, attempts have been made to confirm understanding of characteristic behaviors of the solid state, thermotropic and lyotropic mesomorphic states of biomembranes composed of phospholipids. The principal chemical structure of biological phospholipids is classified into the two parts of a polar head group and two longer hydrocarbon chains. This amphiphilic characteristic and the hydrophilic-hydrophobic balance must be taken into consideration to synthesize totally artificial amphiphiles which have membrane structure and permeation property similar to those of biological membranes. Ralston et al. studied the amphiphilic behavior of quaternary ammonium salts containing two long alkyl chains(1). They suggested that the simple positively-charged hydrophilic moiety was sufficient to form bimolecular lamellae instead of complicated polar head groups of biological phospholipids. Kunitake et al. have succeeded in synthesis of totally artificial amphiphiles being capable of formation of monolayer of bilayer membranes(2–4). Their artificial amphiphiles are composed of combinations of the one- or two- headed polar groups and the monoalkyl or dialkyl long chains.

Multifunctional hydrolytic catalysis: VI. Catalytic hydrolysis of p-nitrophenyl acetate by N-(4-imidazolylmethyl)benzohydroxamic acid☆

Abstract A bifunctional catalyst, N -(4-imidazolylmethyl)benzohydroxamic acid, was synthesized from benzohydroxamic acid and chloromethylimidazole, and used for the hydrolysis of p -nitrophenyl acetate. The reaction proceeded via the formation of the acetyl hydroxamate and its subsequent decomposition. The deacylation step was shown to be general base-catalyzed by the intramolecular imidazole group on the basis of the deuterium solvent kinetic isotope effect of 2.0. The efficiency of water attack on the acetyl hydroxamate was enhanced 130-fold by the imidazole group. The catalytic process is compared with the reactions of related monofunctional compounds, and finally its significance as a model of the charge relay system is discussed.

Multifunctional hydrolytic catalyses x. Catalytic hydrolysis of p-Nitrophenyl acetate by a polyethylenimine derivative containing hydroxamate (Zwitterionic) group

Abstract A water-soluble polyethylenimine derivatives was prepared containing the zwittenionic hydroxamate group, 11 mol% dococyl side chain, and a tertiary amino group. This polymer hydrolyzes p -nitrophenyl esters very effeciently (30°C, 3 v/v% EtOHue5f8H 2 O), according to the Michaelis-Menten kinetics. The catalysis involves substrate binding ( K m = (2–5) × 10 −4 M in the case of p -nitrophenyl acetate) due to hydrophobic interaction, and facile acylation and de-acylation at the hydroxamate site. The zwitterionic nature and the hydrophobic microenvironment contribute to the enchanced nucleophilicity to the hydroxamate group. It is suggested that the de-acylation process involves general-base catalysis by the tert-amino group. The overall catalytic effeciency for p -nitrophenyl acetate exceeded that of α-chymotrypsin by a factor of ca. 100 under a comparable condition. Polymers which do not contain the hydroxamate group or contain the imidazole group in its place were much less effective.

Synthetic molecular membranes and their functions

Synthetic bilayer membranes can be formed spontaneously from a variety of single-chain and double-chain(dialkyl) amphiphiles. In the case of single-chain amphiphiles, the presence of rigid segments is required in the hydrophobic portion and the aggregate morphology is strongly affected by the structure of the rigid segment. The phase transition and the phase separation are observed in these membranes and the membrane catalysis is affected by these phenomena.