Toshiaki Taira

Selective encapsulation of cesium ions using the cyclic peptide moiety of surfactin: Highly efficient removal based on an aqueous giant micellar system

Cyclic peptide of surfactin (SF) is one of the promising environment-friendly biosurfactants abundantly produced by microorganisms such as Bacillus subtilis. SF is also known to act as an ionophore, wherein alkali metal ions can be trapped in the cyclic peptide. Especially, SF is expected to show high affinity for Cs(+) because of the distinctive cavity size and coordination number. In this study, we reported the specific interaction between SF and Cs(+) and succeeded in the highly efficient removal of Cs(+) from water using giant SF micelles as a natural sorbent. The specific interaction between SF and Cs(+) to form their inclusion complex was revealed by nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR) spectroscopy. We found that SF micelles selectively encapsulate Cs(+), which was suggested by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). A highly effective separation of Cs(+) immobilized on the surface of the SF micelles was also achieved through facile centrifugal ultrafiltration in 91% even in coexisting with other alkali metal ions such as Na(+) and K(+). Thus, the use of the giant micellar system of SF with its high Cs(+) affinity and distinctive assembling properties would be a new approach for the treatment of contaminated soil and water.

Interfacial and Emulsifying Properties of Soybean Peptides with Different Degrees of Hydrolysis

In this study, the effects of the degree of hydrolysis on the interfacial and emulsifying properties of soybean peptides were evaluated based on surface and interfacial tension, dynamic light scattering (DLS), and freeze-fracture transmission electron microscopy (FF-TEM) analyses. Of the five evaluated soybean peptides (SP95, SP87, SP75, SP49, and SP23), those with higher degrees of hydrolysis (SP95 and SP87) did not exhibit noticeable surface-active properties in water, whereas those with relatively low degrees of hydrolysis (SP75, SP49, and SP23) exhibited remarkable surface tension-lowering activity. The latter set (SP75, SP49, and SP23) also formed giant associates with average sizes ranging from 64.5 nm to 82.6 nm above their critical association concentration (CAC). Moreover, SP23 with the lowest degree of hydrolysis exhibited excellent emulsifying activity for soybean oil, and FF-TEM analysis demonstrated that the emulsions were stabilized by a lamella-like multilayer peptide structure on the oil droplets that prevented coagulation. The peptide with the lowest degree of hydrolysis (SP23) was effective not only for soybean oil emulsification, but also for the emulsification of liquid paraffin and silicon oil that are generally difficult to emulsify.

Fluorinated polymer surfactants bearing an alternating peptide skeleton prepared by three-component polycondensation

A new species of fluorinated polymer surfactant was developed by three component polycondensation analogous to Ugi four-component condensation. The surfactant exhibited unique surface properties, which made cellulose-based materials hydrophobic and decreased the surface tension of CHCl3. It turned out that the polymer forms micelles in CHCl3.

Screening of a Bacillus subtilis Strain Producing Multiple Types of Cyclic Lipopeptides and Evaluation of Their Surface-tension-lowering Activities

Sixty Bacillus subtilis strains were investigated for their ability to produce cyclic lipopeptides (CLPs). Among them, B. subtilis NBRC 109107 produced at least three types of CLPs by high-performance liquid chromatography (HPLC) analysis, and these CLPs were thought to be surfactin, iturin A, and fengycin by polymerase chain reaction amplification of respective CLP synthetase-encoding genes. However, after HPLC fractionation and purification, structural analysis of the CLPs revealed that these were surfactin homologues, iturin A, and unknown CLPs, whose surface-tension-lowering activities were 29.4, 56.7, and 48.6 mN/m, respectively. By contrast, fengycin was not detected.

Lipid Nanodisc Formation using Pxt-5 Peptide Isolated from Amphibian ( Xenopus tropicalis ) Skin, and its Altered Form, Modify-Pxt-5

Nanodiscs are self-assembled discoidal nanoparticles composed of amphiphilic α-helical scaffold proteins or peptides that accumulate around the circumference of a lipid bilayer. In this study, Pxt-5, which is an antimicrobial peptide isolated from the skin of Xenopus tropicalis, and its modified peptide (Modify-Pxt-5) were synthesized by solid-phase peptide synthesis (SPPS).Their surface properties, which are an important factor in inducing nanodisc formation, were investigated by circular dichroism (CD) spectroscopy, surface tension measurement, phospholipid vesicle clearance assay, and negative-staining transmission electron microscopy (NS-TEM). The α-helicity of Pxt-5 (8.4%) improved drastically to 45.6% by four amino-acid substitutions (Modify-Pxt-5). Both the peptides, having hydrophobic and hydrophilic faces, behaved like general surfactants, and the surface activity of Modify-Pxt-5 (CAC: 9.5×10-5 M, γCAC: 30.3 mN·m-1) was much higher than that of Pxt-5 (CAC: 7.9×10-5 M, γCAC: 38.1 mN·m-1). A turbid L-α-dimyristoylphosphatidylcholine (DMPC) vesicle solution (T = 0.3%) quickly turned transparent upon addition of Pxt-5 or Modify-Pxt-5. After twelve hours, the transmittance of vesicle solution with Modify-Pxt-5 (T = 96.2%) was found to be higher than that of vesicle solution with Pxt-5 (T = 83.5%), and then the micro-solubilized solutions were observed by NS-TEM. Interestingly, nanodisc structures were found in the vicinity of DMPC vesicles in both the images, and the average diameter of the nanodiscs was 11.2 ± 6.0 nm for those containing Pxt-5 and 10.8 ± 5.8 nm for those containing Modify-Pxt-5. It was also found that Modify-Pxt-5 effectively self-assembles into nanodiscs compared to Pxt-5 without any substitutions.

Self-assembling Properties of an N-Heterocyclic Carbene-based Metallosurfactant: Pd-Coordination Induced Formation of Reactive Interfaces in Water

In this study, an N-heterocyclic carbene (NHC)-based metallosurfactant (MS), NHC-PdMS, was synthesized, where Pd(II) was bound to the NHC framework via a robust Pd-carbene bond with NEt3 as a co-ligand. Surface tension measurements revealed that the critical micelle concentration (CMC) of NHC-PdMS (1.8×10-4 M) was one order of magnitude lower than that of its MS precursor (imidazolium bromide). Coordination of the MS precursor and NEt3 to Pd(II) also influenced micelle size; the hydrodynamic diameters of NHC-PdMS and the MS precursor were observed to be 25.8±5.6 nm and 2.5±0.3 nm, respectively. Furthermore, small angle X-ray scattering measurements indicated that NHC-PdMS exhibited liquid crystalline behavior above 26 wt%, with a spacing ratio of 1:2:3 for the first, second, and third Bragg peaks. To understand the role of the reactive interface, NHC-PdMS was also applied to aqueous catalytic reactions. Owing to its low CMC value, a catalytic amount of NHC-PdMS (3 mol%) provided the reactive interface, which facilitated the aqueous Mizoroki-Heck reaction of various aryl iodides and styrene in good yields (72-95%). These results suggest that MS formation results in a drastic change in selfassembling properties, which are important for the development of highly reactive chemical interfaces in water.

Surface Activity and Ca 2+ -Dependent Aggregation Property of Lichenysin Produced by Bacillus licheniformis NBRC 104464

Bacillus licheniformis NBRC 104464 produces a cyclic lipopeptide different from surfactin. After we performed liquid chromatography fractionation and purification, we used structural analyses to identify the cyclic lipopeptide as lichenysin. Surface tension measurements of lichenysin sodium salt in water yielded a critical micelle concentration (CMC) of 1.0×10-5 M. The surface tension at the CMC was 28.9 mN/m. Comparative analysis of Ca2+-influenced micellar aggregation of lichenysin and surfactin revealed that the formation rate of the lichenysin-Ca2+ complex aggregate remained low up to a [Ca2+]/[lichenysin] molar ratio of 80, whereas the surfactin-Ca2+ complex formed micellar aggregates at the same molar ratio. Further excessive addition of Ca2+ to the micellar solution of lichenysin induced higher turbidity than surfactin.

pH-induced conformational change of natural cyclic lipopeptide surfactin and the effect on protease activity

The cyclic lipopeptide surfactin (SF) is one of the promising environmental friendly biosurfactants abundantly produced by microorganisms such as Bacillus subtilis. SF shows excellent surface properties at various pH, together with lower toxicity and higher biodegradability than commonly used petroleum-based surfactants. However, the effect of the dissociation degree of SF on self-assembly is still incompletely understood, even though two acidic amino acid residues (Asp and Glu) are known to influence eventual surface and biological functions. Here, we report changes in the secondary structure of SF induced by increased pH, and the effect on protease activity. We found that the β-sheet and β-turn formation of SF are significantly enhanced through increased dissociation of Asp and Glu as revealed by a titration experiment of SF solution to estimate apparent pK1 and pK2 values together with circular dichroism spectroscopy. We also studied the activity of the common detergent enzyme subtilisin in SF solution at above its pK2 (pH 7.6) to understand the role of the dissociation degree in the interaction with the protein. The mixing of SF having a unique cyclic topological feature with subtilisin suppressed the decrease in protease activity observed in the presence of synthetic surfactants such as sodium dodecyl sulfate and polyoxyethylene alkyl ether. Thus, SF has great potential for use in laundry detergent formulations, to improve the stability and reliability of detergent enzymes.