Molecular Self-Assembly Strategy for Generating Catalytic Hybrid Polypeptides.

Recently, catalytic peptides were introduced that mimicked protease activities and showed promising selectivity of products even in organic solvents where protease cannot perform well. However, their catalytic efficiency was extremely low compared to natural enzyme counterparts presumably due to the lack of stable tertiary fold. We hypothesized that assembling these peptides along with simple hydrophobic pockets, mimicking enzyme active sites, could enhance the catalytic activity. Here we fused the sequence of catalytic peptide CP4, capable of protease and esterase-like activities, into a short amyloidogenic peptide fragment of Aß. When the fused CP4-Aß construct assembled into antiparallel ß-sheets and amyloid fibrils, a 4.0-fold increase in the hydrolysis rate of p-nitrophenyl acetate (p-NPA) compared to neat CP4 peptide was observed. The enhanced catalytic activity of CP4-Aß assembly could be explained both by pre-organization of a catalytically competent Ser-His-acid triad and hydrophobic stabilization of a bound substrate between the triad and p-NPA, indicating that a design strategy for self-assembled peptides is important to accomplish the desired functionality.

Biomimetic Crystallization of MnFe2O4 Mediated by Peptide-Catalyzed Esterification at Low Temperature.

Enzymes are some of the most efficient catalysts in nature. If small catalytic peptides mimic enzymes, there is potential for broad applications from catalysis for new material synthesis to drug development, due to the ease of molecular design. Recently a hydrogel-based combinatory phage display library was developed and protease-mimicking peptides were identified. Here we advanced the previous discovery to apply one of these catalytic peptides for the synthesis of bimetal oxide nanocrystals through the catalytic ester-elimination pathway. Conventional bimetal oxide crystallization usually requires high temperatures above several hundred °C; however, this catalytic peptide could grow superparamagnetic MnFe2O4 nanocrystals at 4°C. Superconducting quantum interference device (SQUID) analysis revealed that MnFe2O4 nano-crystals grown by the catalytic peptide exhibit superpara-magnetism. This study demonstrates the usefulness of protease-mimicking catalytic peptides in the field of material synthesis.

Peptide-Metal Organic Framework Swimmers that Direct the Motion toward Chemical Targets.

Highly efficient and robust chemical motors are expected for the application in microbots that can selectively swim toward targets and accomplish their tasks in sensing, labeling, and delivering. However, one of major issues for such development is that current artificial swimmers have difficulty controlling their directional motion toward targets like bacterial chemotaxis. To program synthetic motors with sensing capability for the target-directed motion, we need to develop swimmers whose motions are sensitive to chemical gradients in environments. Here we create a new intelligent biochemical swimmer by integrating metal organic frameworks (MOFs) and peptides that can sense toxic heavy metals in solution and swim toward the targets. With the aid of Pb-binding enzymes, the peptide-MOF motor can directionally swim toward PbSe quantum dots (QD) by sensing pH gradient and eventually complete the motion as the swimmer reaches the highest gradient point at the target position in solution. This type of technology could be evolved to miniaturize chemical robotic systems that sense target chemicals and swim toward target locations.

Peptide assembly-driven metal-organic framework (MOF) motors for micro electric generators.

Peptide-metal-organic framework (Pep-MOF) motors, whose motions are driven by anisotropic surface tension gradients created via peptide self-assembly around frameworks, can rotate microscopic rotors and magnets fast enough to generate an electric power of 0.1 µW. A new rigid Pep-MOF motor can be recycled by refilling the peptide fuel into the nanopores of the MOF.

Autonomous motors of a metal-organic framework powered by reorganization of self-assembled peptides at interfaces.

A variety of microsystems have been developed that harness energy and convert it to mechanical motion. Here we have developed new autonomous biochemical motors by integrating a metal-organic framework (MOF) and self-assembling peptides. The MOF is applied as an energy-storing cell that assembles peptides inside nanoscale pores of the coordination framework. The nature of peptides enables their assemblies to be reconfigured at the water/MOF interface, and thus converted to fuel energy. Reorganization of hydrophobic peptides can create a large surface-tension gradient around the MOF that can efficiently power its translational motion. As a comparison, the velocity normalized by volume for the diphenylalanine-MOF particle is faster and the kinetic energy per unit mass of fuel is more than twice as great as that for previous gel motor systems. This demonstration opens the route towards new applications of MOFs and reconfigurable molecular self-assembly, possibly evolving into a smart autonomous motor capable of mimicking swimming bacteria and, with integrated recognition units, harvesting target chemicals.

Nanoscale shuffling in a template-assisted self-assembly of binary colloidal particles.

We have fabricated a square lattice array of sub-micrometer fluorescent (red and green) polystyrene particles. The particles were each embedded into small pits fabricated on a silicon substrate by electron beam lithography, through the drying process of an aqueous suspension containing equal amounts of the two species. We indexed 0 and 1 for each red and green particle, respectively, and then obtained a one-dimensional bit sequence by the successive reading of the indices in a predetermined manner. We evaluated the randomness of the bit sequence by using the improved FIPS 140-2 statistical test suite. Consequently, we found that the bit sequences do not have any non-randomness. The particle array was obtained by a very simple process, i.e., the drying of a suspension, but the particle distribution pattern was definitely unpredictable and irreproducible, and the number of possible patterns was tremendously large. The signal--i.e., the color of the particle--does not deteriorate within a practical timescale under various conditions, such as in an electric field, in a magnetic field, in air or water, on a solid matrix, and so on, which means that a small tip with the particle pattern can be installed in miscellaneous object, including electronic products, plastic credit cards, currency bills, and so on. Therefore, this particle array is applicable to a nanoscale identification tag or a one-time pad encryption tip.

Temperature-modulated adsorption of poly(N-isopropylacrylamide)-grafted ferritin on solid substrate.

Ferritin grafted with temperature-responsive poly(N-isopropylacrylamide) (PIPAAm-ferritin) was synthesized by a coupling reaction using PIPAAm and ferritin for obtaining stimuli-responsive biomaterials. The hydrodynamic diameter of PIPAAm-ferritins in aqueous solution increased at 37 °C at a higher protein concentration (>0.2mg/mL) because of the intermolecular aggregation through the hydrophobic interaction of PIPAAm chains. On the other hand, PIPAAm-ferritins at a lower concentration (<0.2mg/mL) were unable to increase their size even at 37 °C. The adsorption kinetics of PIPAAm-ferritins on hydrophobically modified Si substrate were evaluated with a quartz crystal microbalance in 10 mmol/L Bis-Tris/HCl buffer (pH 5.8) with and without poly(oxyethylene) sorbitan monolaurate (TWEEN 20) (0.05 wt%) as a surfactant. Although the adsorption of PIPAAm-ferritins on hydrophobic Si substrate at 25 °C in the buffer with TWEEN 20 was hardly observed, PIPAAm-ferritins were considerably adsorbed on the substrate at 37 °C, indicating that the hydrophobic interaction between the substrate and PIPAAm grafts on the ferritins after the destruction of the hydrophobic interaction between the protein and the substrate by TWEEN 20.

Nucleation reaction dynamics of Pt nanoparticles observed by the heterodyne transient grating method.

The nucleation reaction dynamics of platinum nanoparticles in the photoreduction process of H(2)Pt(IV)Cl(6) solution were investigated by the heterodyne transient grating (HD-TG) method. The formation mechanism of platinum nanoparticles was considered, supported by information obtained from UV/VIS absorption spectroscopy during the reaction and SEM images of the generated nanoparticles. In particular, the roles of poly(N-vinyl-2-pyrrolidone) (PVP) as a protective polymer and ethanol as a solvent were studied. The chemical species involved in the reaction can be identified from the diffusion coefficients obtained from HD-TG measurements; the species observed by UV pulse irradiation were assigned to H(2)Pt(IV)Cl(6) as a reactant species and H(2)Pt(II)Cl(4) and Pt nuclei as product species. It was observed that the amounts of the reactant and product species increased, and many homogeneous nanoparticles were generated, by an increase in PVP concentration. The addition of ethanol to the solvent showed a larger effect on the enhancement of the reduction of H(2)Pt(IV)Cl(6) than that of PVP; however, it did not lead to Pt nuclei formation in the order of seconds. Nevertheless, because nanoparticle formation was confirmed by UV/VIS absorption spectroscopy and SEM images, the formation of nanoparticles following nuclei formation must have proceeded via a slow reaction. Therefore, nucleation and nanoparticle formation are considered to occur on a longer time scale than 10 s in water/ethanol solvent.

Collective plasmon modes excited on a silver nanoparticle 2D crystalline sheet.

This paper describes unique plasmonic characteristics of two dimensional (2D) crystalline sheets composed of homogeneous Ag nanoparticles (AgNPs) fabricated by the Langmuir-Schaefer method at an air-water interface. The localized surface plasmon resonance (LSPR) band of the Ag nanosheet was tuned by changing the interparticle distance of AgNPs via the length of the organic capping molecules. Red shift of the LSPR band of the AgNPs sheet followed an exponential law against the interparticle distance in a similar manner to the previous reports of metal nanodisc pairs. However, the shift was much larger and less dependent on the interparticle separation gap. This phenomenon is reasonably interpreted as the long-range interaction of LSPR in the 2D sheet ('delocalized' LSPR) confirmed by simulation using the finite difference time domain (FDTD) method. The FDTD simulation also revealed additional enhancement of local electric fields on the 2D sheet compared to those on the single or paired particles.

Ag nanoparticle sheet as a marker of lateral remote photocatalytic reactions.

The lateral remote photocatalytic activity of TiO(2) nanotubes fabricated by anodic oxidation, was characterized by use of a 'silver nanosheet' as a marker. The silver nanosheet is a two-dimensional crystalline film composed of myristate-capped silver nanoparticles (d = 5 nm), which has a sharp localized plasmon absorption band at lambda(max) = 470 nm. A quartz substrate was coated with TiO(2) nanotubes to a coverage of 10% and then covered with a silver nanosheet. Upon UV irradiation a rapid decrease in the intensity of the 470 nm plasmon band and an increase in the long-wavelength absorption (550-1000 nm) band were clearly confirmed, indicating fusion of the nanoparticles due to decomposition of the myristate capping agent. Surface morphology measured by high-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed the decomposition and fusion of silver nanoparticles even in regions of the surface a micron away from the TiO(2) nanotubes.

Random number generation by a two-dimensional crystal of protein molecules.

We discuss 2D and binary self-assemblies of protein molecules using apo-ferritin and holo-ferritin, which have identical outer-shell structures but different inner structures. The assemblies do not show any phase separation but form 2D monomolecular-layer crystals. Statistical analyses showed a random molecular distribution in the crystal where the molar ratio was conserved as it was in the solution. This molecular pattern is readily prepared, but it is neither reproducible nor predictable and hence can be used as a nanometer-scale cryptographic device or an identification tag.

Growth of giant two-dimensional crystal of protein molecules from a three-phase contact line.

A novel method to fabricate a two-dimensional (2D) crystal of protein molecules has been developed. The method enables us to control both the position of nucleation and the direction of the crystal growth. The crystal obtained using a protein molecule, ferritin, was found to be composed of a number of densely packed single crystal domains with an unprecedentedly large size of approximately 100 microm(2). This method also reveals characteristic behavior of the spatiotemporal evolution of the crystal; for example, "fusion" of the crystal domains, which is never observed in an ordinary crystal composed of atoms or ions, was demonstrated. Our approach could have potential in fabricating extraordinarily large and highly ordered nanoparticle arrays of organic or inorganic materials.

Dynamic interfacial properties of poly(ethylene glycol)-modified ferritin at the solid/liquid interface.

Poly(ethylene glycol)-modified ferritins (PEG-ferritins) with various molecular weights were synthesized by the grafting method, and their dynamic interfacial properties at the solid/liquid interface were investigated. The number of PEG grafted to ferritins was controlled by the amount of 1,1'-carbonyldiimidazole-modified PEG adding to the reaction solution. The adsorption kinetics and energy dissipation of PEG-ferritins onto bare Si substrate and amino-modified Si substrate were investigated with a quartz crystal microbalance (QCM) in 10 mM bis-Tris/HCl buffer (pH 5.8), while their morphologies were characterized by scanning electron microscopy (SEM). The adsorption dynamics of PEG-ferritins onto amino-modified Si substrate were quite different from those of unmodified ferritin, which can be reasonably interpreted by the desorption capability of PEG-ferritins on the surface attributed to amphiphilicity and the high-chain mobility of PEG chains.

Dynamic behaviors of molecular assemblies at liquid/liquid interfaces studied by time-resolved quasi-elastic laser scattering spectroscopy.

The dynamic behaviors of molecular assemblies at two immiscible liquid interfaces are intriguing topics in many fields of science and technology. However, it is generally difficult to investigate the dynamic behaviors of such molecular assemblies because of the buried nature of liquid/liquid interfaces. In the present paper, our recent investigations on dynamic behaviors of various molecular self-assemblies at liquid/liquid interfaces are reviewed. We monitored dynamic behaviors of the molecular assemblies by time-resolved quasi-elastic laser scattering (TR-QELS) and fluorescent spectroscopy. The former method allows us to monitor the change in interfacial tension with millisecond time-resolution. As molecular assemblies, bis(2-ethylhexyl)sulfosuccinate (AOT) microemulsion, phospholipid biomembrane models, and liposome-DNA complexes have all been studied, since they are relevant in material sciences and biological technologies. At liquid/liquid interfaces, these molecular assemblies showed characteristic behaviors. We review the finding of rebound response of the interfacial tension at the liquid/liquid interface induced by the adsorption of the AOT microemulsion. We monitored the hydrolysis reaction of phospholipid biomembrane models formed at oil/water interfaces, observing the different types of behavior of liposome-DNA complexes at biomembrane models with different kinds of phospholipids.

Chemical oscillation with periodic adsorption and desorption of surfactant ions at a water/nitrobenzene interface.

Chemical oscillations with periodic adsorption and desorption of surfactant ions, alkyl sulfate ions, at a water/nitrobenzene interface have been investigated. The interfacial tension was measured with a quasi elastic laser scattering (QELS) method and the interfacial electrical potential was obtained. We found that this oscillation consists of a series of abrupt adsorptions of ions, followed by a gradual desorption. In addition, we observed that each abrupt adsorption was always accompanied by a small waving motion of the liquid interface. From the analysis of the video images of the liquid interface or bulk phase, we could conclude that each abrupt adsorption is caused by nonlinear amplification of mass transfer of ions from the bulk phase to the liquid interface by a Marangoni convection, which was generated due to local adsorption of the surfactant ions at the liquid interface that resulted in the heterogeneity of the interfacial tension. In the present paper, we describe the mechanism of the chemical oscillation in terms of the hydrodynamic effect on the ion adsorption processes, and we also show the interfacial chemical reaction with ion exchange during the ion desorption process.

Hydrodynamically induced chemical oscillation at a water/nitrobenzene interface.

By measuring a time course of interfacial tension and interfacial electrical potential, we successfully observed oscillatory phenomena that were based on alternatively appearing adsorption and desorption processes of anionic surfactant molecules (sodium dodecyl sulfate (SDS)) at the water/nitrobenzene interface. These oscillation patterns were drastically modified by slightly changing the rate of SDS aqueous solution injection into the water phase. When 10 mM of SDS aqueous solution was injected at a low rate, for example, at less than 1 microl/min, abrupt adsorption was repeatedly followed by slow desorption of DS- ions; in other words, the sequence of the oscillation and relaxation processes was repeated. However, when it was injected at a higher rate, no remarkable periodic phenomenon occurred after the first oscillation. In addition, the rapid adsorption process was observed to be accompanied by a flip motion of the liquid/liquid interface and a flow along the interface. This is caused by a Marangoni convection that is brought about by the generation of heterogeneity of interfacial tension. Furthermore, by estimating the flow speed, it was determined that the faster flow tends to quench the periodic oscillation patterns.

Direct observation of chemical oscillation at a water/nitrobenzene interface with a sodium-alkyl-sulfate system.

The oscillation of the interfacial tension and electrical potential at a water/nitrobenzene interface was observed with homologous anionic surfactant molecules, sodium-alkyl-sulfates. Concerning small molecules with a short hydrophobic carbon chain, the oscillation period and amplitude decreased with a decrease of the length of the alkyl chain. On the other hand, when surfactant molecules with a long hydrophobic carbon chain were used, no remarkable periodic oscillation occurred after the first oscillation. In all systems, an interfacial flow by Marangoni convection was observed when the oscillation took place. By monitoring the movement of carbon powder scattered on the liquid/liquid interface with a CCD camera, we could observe that the liquid/liquid interface expanded outward from the area on which the surfactant molecules adsorbed when the oscillation occurred. When the small molecule was used, the speed of expansion of the interface (flow speed) was small and shrinkage followed by expansion of the interface repeatedly occurred. However, when the large molecule was used, the flow speed was large and expansion occurred only one time. These results show that hydrodynamic factors and surface activities are important in chemical oscillation systems.

Real-time observation for the enzymatic reaction of phospholipid membrane: application of the time-resolved quasi-elastic laser scattering method.

An analytical technique to measure reactions in biological membranes was developed and applied to monitoring the hydrolysis reaction of phospholipids (dipalmitoylphosphatidylcholine, DPPC) by phospholipase A(2). The technique uses the time-resolved quasi-elastic laser scattering (TR-QELS) method to measure an oil/phospholipid monolayer/water membrane system by monitoring the change of interfacial tension under a noncontact condition and in real time. When the TR-QELS method is used with the newly developed oil/phospholipid monolayer/water membrane system, measurement of the hydrolysis reaction of phospholipids with long alkyl chains (C >or=16), which are the major components in biological membranes, becomes possible. The reaction progress is monitored by the increase of interfacial tension at the oil/water interface caused by the decrease of surface-active DPPC molecules due to the reaction. The characteristic phases, namely, lag, burst, and equilibrium, are observed. The relationship between the duration of the lag phase (the rate-limiting step of the reaction) and the concentration of calcium ion (an essential cofactor of the reaction) is also investigated. Increase of calcium ion concentration in the subphase is found to shorten the duration of the lag phase. In addition, the real-time measurement simplifies the estimation process for the reaction activation energy.