Noriyuki Ishii

Photoconductive coaxial nanotubes of molecularly connected electron donor and acceptor layers.

Controlled self-assembly of a trinitrofluorenone-appended gemini-shaped amphiphilic hexabenzocoronene selectively formed nanotubes or microfibers with different photochemical properties. In these nanotubes, which are 16 nanometers in diameter and several micrometers long, a molecular layer of electron-accepting trinitrofluorenone laminates an electron-donating graphitic layer of π-stacked hexabenzocoronene. The coaxial nanotubular structure allows photochemical generation of spatially separated charge carriers and a quick photoconductive response with a large on/off ratio greater than 104. In sharp contrast, the microfibers consist of a charge-transfer complex between the hexabenzocoronene and trinitrofluorenone parts and exhibit almost no photocurrent generation.

Chaperonin-mediated stabilization and ATP-triggered release of semiconductor nanoparticles

Various properties of semiconductor nanoparticles, including photoluminescence and catalytic activity, make these materials attractive for a range of applications. As nanoparticles readily coagulate and so lose their size-dependent properties, shape-persistent three-dimensional stabilizers that enfold nanoparticles have been exploited. However, such wrapping approaches also make the nanoparticles insensitive to external stimuli, and so may limit their application. The chaperonin proteins GroEL (from Escherichia coli) and T.th (‘T.th cpn’, from Thermus thermophilus HB8) encapsulate denatured proteins inside a cylindrical cavity; after refolding, the encapsulated proteins are released by the action of ATP inducing a conformational change of the cavity. Here we report that GroEL and T.th cpn can also enfold CdS semiconductor nanoparticles, giving them high thermal and chemical stability in aqueous media. Analogous to the biological function of the chaperonins, the nanoparticles can be readily released from the protein cavities by the action of ATP. We expect that integration of such biological mechanisms into materials science will open a door to conceptually new bioresponsive devices.

Ambipolar-transporting coaxial nanotubes with a tailored molecular graphene–fullerene heterojunction

Despite a large steric bulk of C60, a molecular graphene with a covalently linked C60 pendant [hexabenzocoronene (HBC)–C60; 1] self-assembles into a coaxial nanotube whose wall consists of a graphite-like π-stacked HBC array, whereas the nanotube surface is fully covered by a molecular layer of clustering C60. Because of this explicit coaxial configuration, the nanotube exhibits an ambipolar character in the field-effect transistor output [hole mobility (μh) = 9.7 × 10−7 cm2 V−1 s−1; electron mobility (μe) = 1.1 × 10−5 cm2 V−1 s−1] and displays a photovoltaic response upon light illumination. Successful coassembly of 1 and an HBC derivative without C60 (2) allows for tailoring the p/n heterojunction in the nanotube, so that its ambipolar carrier transport property can be optimized for enhancing the open-circuit voltage in the photovoltaic output. As evaluated by an electrodeless method called flash-photolysis time-resolved microwave conductivity technique, the intratubular hole mobility (2.0 cm2 V−1 s−1) of a coassembled nanotube containing 10 mol % of HBC–C60 (1) is as large as the intersheet mobility in graphite. The homotropic nanotube of 2 blended with a soluble C60 derivative [(6,6)-phenyl C61 butyric acid methyl ester] displayed a photovoltaic response with a much different composition dependency, where the largest open-circuit voltage attained was obviously lower than that realized by the coassembly of 1 and 2.

Systematic Studies on Structural Parameters for Nanotubular Assembly of Hexa-peri-hexabenzocoronenes

Thirteen different hexa-peri-hexabenzocoronenes (HBCs) I-III were newly synthesized, and their self-assembling behaviors were investigated. Taking into account also the reported behaviors of amphiphilic HBCs, some structural parameters of HBC essential for the tubular assembly were revealed. Points to highlight include (1) the importance of two phenyl groups attached to one side of the HBC unit, (2) essential roles of long paraffinic side chains on the other side of the phenyl groups, and (3) no necessity of hydrophilic oligo(ethylene glycol) side chains. The hierarchical nanotubular structure, rendered by virtue of a synchrotron radiation technique, was virtually identical to our previous proposal, where the nanotubes are composed of helically coiled bilayer tapes with a tilting angle of approximately 45 degrees. Each tape consists of pi-stacked HBC units, where the inner and outer HBC layers are connected by interdigitation of paraffinic side chains. The coiled structure is most likely caused by a steric congestion of the phenyl groups attached to the HBC unit, whose tilting direction may determine the handedness of the helically chiral nanotube.

Conductive One‐Handed Nanocoils by Coassembly of Hexabenzocoronenes: Control of Morphology and Helical Chirality

Electroconductive one-handed helical nanofibers are attractive in view of their potential for the realization of nanoscale solenoids. While there are many reported examples of the construction of helical nanofibers by self-assembly of pelectronic molecules, most of these involve twisted ribbons, which do not provide the coiled pathways essential for electromagnetic properties. Some coiled assemblies of aromatic molecules have been reported, although they are still very rare and their conducting properties have not been investigated, mainly because of their insufficient morphological robustness for electrochemical doping. Thus, the design of nanostructures that satisfy the three requisites for electromagnetic properties—coiled pathways, one-handedness, and electroconductivity—is still a highly challenging research topic. A few years ago we found that a Gemini-shaped hexabenzocoronene (HBC) bearing triethylene glycol and dodecyl chains self-assembles into a graphitic nanotube. More recently, we have also found that the incorporation of pendant norbornene groups into the amphiphilic HBC (1) gives rise to a nanocoiled assembly with uniform diameter and helical pitch. The metastable coiled structure exists for a sufficiently long time, probably because of a steric effect of the pendant norbornene groups, and therefore allows post ring-opening metathesis polymerization (ROMP) of the norbornene groups to covalently stabilize the kinetically selected assembly against a thermodynamic coil-to-tube transformation. The polymerized nanocoil consists of a pstacked HBC array that exhibits conductivity upon oxidative doping. However, the resultant nanocoils are a mixture of rightand left-handed ones, which means that further structural elaboration is needed to realize the one-handed helical graphitic array that is essential for the exploration of electromagnetic properties. Herein we report the selective formation of covalently stabilized conductive nanocoils with a one-handed helical chirality by coassembly of norbornene-appended HBC derivatives 1 and 4 (Figure 1). This achievement is the result of a

A Tubular Biocontainer: Metal Ion-Induced 1D Assembly of a Molecularly Engineered Chaperonin

GroEL(SP/MC), prepared by genetic and chemical modifications of group I chaperonin protein GroEL, site-specifically possesses approximately 28 photochromic (spiropyran [SP] and merocyanine [MC]) units in the entrance parts of its cavity. Addition of divalent metal ions such as Mg(2+) to a tris-HCl buffer solution of GroEL(SP/MC) results in one-dimensional (1D) assembly of GroEL(SP/MC), affording cylindrical hollow fibers with a very large aspect ratio; the longest fiber was approximately 2.5 microm long, corresponding to a 170-mer of GroEL(SP/MC) (MW approximately 1.4 x 10(8)). When such long fibers are mixed with EDTA, they are cut into short-chain oligomers and eventually into monomeric GroEL(SP/MC). Similar to GroEL, GroEL(SP/MC) possesses a large binding affinity toward denatured proteins. When GroEL(SP/MC) undergoes 1D assembly after incubation with a denatured protein, guest-containing cylindrical fibers result.

Structure of holo-chaperonin studied with electron microscopy Oligomeric cpn10 on top of two layers of cpn60 rings with two stripes each

A structural model of holo‐chaperonin, known as a protein‐folding control protein comprising 60 kDa (cpn60) and 10 kDa polypeptides (cpn10), is proposed based on the electron microscopic images of holo‐chaperonin from Thermus thermophilus and cpn60 from Paracoccus denitrificans. Isolated Paracoccus cpn60 shows very similar images to those of Escherichia coli tetradecameric cpn60, a seven‐membered ring in the top view and a rectangular shape with four stripes in the side view. However, a small number of half‐thick rectangles with two stripes are also seen which indicates that a single cpn60‐heptamer ring has two stripes parallel to the plane of the ring. Thermus holo‐chaperonin shows a bullet‐like shape in the side view, and antibory against cpn10 binds only to the round side of the bullet. We conclude that a single cpn60‐heptamer ring with two stripes stacks into two layers, and a cpn10 oligomer binds to one side of the layers.

Hexabenzocoronene graphitic nanotube appended with dithienylethene pendants: photochromism for the modulation of photoconductivity.

[*] Dr. Y. Yamamoto, Dr. W. Jin ERATO-SORST Nanospace Project Japan Science and Technology Agency (JST) 2-41 Aomi, Koto-ku, Tokyo 135-0064 (Japan) E-mail: yamamoto@nanospace.miraikan.jst.go.jp jin@nanospace.miraikan.jst.go.jp Prof. T. Aida, Dr. Y. He Department of Chemistry and Biotechnology School of Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan) E-mail: aida@macro.t.u-tokyo.ac.jp Dr. T. Fukushima Advanced Research Institute, RIKEN 2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)

Molecular glues carrying multiple guanidinium ion pendants via an oligoether spacer: stabilization of microtubules against depolymerization.

Dendron G1(Gu(+))(9)R and linear peptide oligomer Asn(TEG-Gu(+))(9), decorated with multiple guanidinium (Gu(+)) ions as sticky pendants via an oligo(oxyethylene) spacer, adhere to BSA and protein assemblies such as microtubules in aqueous buffers. Using fluorescently labeled G1(Gu(+))(9)R with pyrenyl and rhodamine focal cores, the adhesion process can be visualized by FRET or confocal laser scanning microscopy. The adhesion to microtubules leads to their stabilization against depolymerization into alpha/beta-tubulin heterodimer components, where the effects of G1(Gu(+))(9)R and Asn(TEG-Gu(+))(9) are comparable to that of paclitaxel, known as an anticancer drug. Since G1(Gu(+))(9)R and Asn(TEG-Gu(+))(9) are superior to lower-generation G0(Gu(+))(3)OMe and arginine nonamer, respectively, the multivalency of the interaction and a conformational flexibility of the oligoether spacers play a crucial role in the efficient adhesion to proteins.

Use of Side-Chain Incompatibility for Tailoring Long-Range p/n Heterojunctions: Photoconductive Nanofibers Formed by Self-Assembly of an Amphiphilic Donor–Acceptor Dyad Consisting of Oligothiophene and Perylenediimide

To tailor organic p/n heterojunctions with molecular-level precision, a rational design strategy using side-chain incompatibility of a covalently connected donor-acceptor (D-A) dyad has been successfully carried out. An oligothiophene-perylenediimide dyad, when modified with triethylene glycol side chains at one terminus and dodecyl side chains at the other (2(Amphi)), self-assembles into nanofibers with a long-range D/A heterojunction. In contrast, when the dyad is modified with dodecyl side chains at both termini (2(Lipo)), ill-defined microfibers result. In steady-state measurements using microgap electrodes, a cast film of the nanofiber of 2(Amphi) displays far better photoconducting properties than that of the microfiber of 2(Lipo). Flash-photolysis time-resolved microwave conductivity measurements, in conjunction with transient absorption spectroscopy, clearly indicate that the nanofiber of 2(Amphi) intrinsically allows for better carrier generation and transport properties than the microfibrous assembly of 2(Lipo).