Reflection high-energy electron diffraction intensity oscillations during growth of (Al,Ga)As on GaAs(111)A

We have made a reflection high-energy diffraction (RHEED) intensity oscillation study of the growth of (Al,Ga)As on GaAs and (Al,Ga)As (111)A surfaces. The RHEED intensity oscillations during growth of (Al,Ga)As are dependent on both growth temperature and As₄:Ga flux ratio. In addition, the oscillation period decreases as the Al fraction in the (Al,Ga)As is increased. Changes in the oscillation period during growth of the first few monolayers of both GaAs on (Al,Ga)As and (Al,Ga)As on GaAs may indicate the intermixing of Al and Ga near the heterointerface.     

Studies on interface roughness scattering effects in molecular beam epitaxy grown resonant tunneling structures

Interface roughness (IR) scattering effects in the resonant tunneling (RT) process through molecular beam epitaxy (MBE) grown double barrier (DB) structures have been investigated both theoretically and experimentally. The time dependent perturbation theory was applied to an elastic scattering event which is caused by the IR potential within the dwelling time period of an incident electron inside a DB. The calculated result indicates that the peaked structure of the tunneling transmission coefficient can be degraded drastically when the IR has lateral Fourier components in a specific spatial frequency range, and that the upper and lower limits of the range are related to such structural parameters as the well width and the cavity finesse of the DB. It has been found that the above prediction is possibly supported by the experimental results obtained in tunneling spectroscopy measurements performed on AlAs-GaAs DB structures; measured peak widths of tunneling spectra in samples grown with growth interruption periods are broadened by a factor of more than 2.     

Solid‐Phase Total Synthesis of Yaku'amide B Enabled by Traceless Staudinger Ligation

We report a solid‐phase strategy for total synthesis of the peptidic natural product yaku'amide B ( 1 ), which exhibits antiproliferative activity against various cancer cells. Its linear tridecapeptide sequence bears four β,β‐dialkylated α,β‐dehydroamino acid residues and is capped with an N‐terminal acyl group (NTA) and a C‐terminal amine (CTA). To realize the Fmoc‐based solid‐phase synthesis of this complex structure, we developed new methods for enamide formation, enamide deprotection, and C‐terminal modification. First, traceless Staudinger ligation enabled enamide formation between sterically encumbered alkenyl azides and newly designed phosphinophenol esters. Second, application of Eu(OTf)₃ led to chemoselective removal of the enamide Boc groups without detaching the resin linker. Finally, resin‐cleavage and C‐terminus modification were simultaneously achieved with an ester–amide exchange reaction using CTA and AlMe₃ to deliver 1 in 9.1 % overall yield (24 steps from the resin).     

An enzymatically activated fluorescence probe for targeted tumor imaging

Beta-galactosidase is a widely used reporter enzyme, but although several substrates are available for in vitro detection, its application for in vivo optical imaging remains a challenge. To obtain a probe suitable for in vivo use, we modified our previously developed activatable fluorescence probe, TG-betaGal (J. Am. Chem. Soc. 2005, 127, 4888-4894), on the basis of photochemical and photophysical experiments. The new probe, AM-TG-betaGal, provides a dramatic fluorescence enhancement upon reaction with beta-galactosidase, and further hydrolysis of the ester moiety by ubiquitous intracellular esterases affords a hydrophilic product that is well retained within the cells without loss of fluorescence. We used a mouse tumor model to assess the practical utility of AM-TG-betaGal, after confirming that tumors in the model could be labeled with an avidin-beta-galactosidase conjugate. This conjugate was administered to the mice in vivo, followed by AM-TG-betaGal, and subsequent ex vivo fluorescence imaging clearly visualized intraperitoneal tumors as small as 200 microm. This strategy has potential clinical application, for example, in video-assisted laparoscopic tumor resection.     

Nickel-based oxyphosphide superconductor with a layered crystal structure, LaNiOP

A layered oxyphosphide, LaNiOP, was synthesized by solid-state reactions. This crystal was confirmed to have a layered structure composed of an alternating stack of (La(3+)O(2-))(+) and (Ni(2+)P(3-))(-). We found that the resulting LaNiOP shows a superconducting transition at approximately 3 K. This material exhibited metallic conduction and Pauli paramagnetism in the temperature range of 4-300 K. The resistivity sharply dropped to zero and the magnetic susceptibility became negative at <4 K, indicating that a superconducting transition occurs. The volume fraction of the superconducting phase estimated from the diamagnetic susceptibility reached approximately 40 vol % at 1.8 K, substantiating that LaNiOP is a bulk superconductor.     

Protein assemblies by site-specific avidin-biotin interactions

Exploiting self-assembly systems with biological building blocks is of significant interest in the fabrication of advanced biomaterials. We assessed the potential use of site-specific ligand labeling of protein building blocks in designing functional protein self-assemblies by combining site-specifically biotinylated bacterial alkaline phosphatase (as a bidentate or tetradentate ligand unit) and streptavidin (as a tetrameric receptor).     

Regio- and stereoselective synthesis of 2-amino-1,3-diene derivatives by ruthenium-catalyzed coupling of ynamides and ethylene

A ruthenium-catalyzed hydrovinylation-type cross-coupling of ynamides and ethylene proceeds via ruthenacyclopentene to give 2-aminobuta-1,3-diene derivatives in a highly regioselective manner. It was also demonstrated that 2-aminobuta-1,3-diene derivatives reacted with various dienophiles or singlet oxygen to give a cyclic enamide derivative.     

Synthetic protocol for diarylethenes through Suzuki-Miyaura coupling

The synthesis of a variety of diarylethenes through the Suzuki-Miyaura coupling reaction of 1,2-dichlorohexafluorocyclopentene with arylboronic acids and esters has been developed. Thiophenes with various substituents such as cyano and ester functionalities can be incorporated.     

Ruthenium‐catalyzed cyclizations of enynes via a ruthenacyclopentene intermediate: Development of three novel cyclizations controlled by a substituent on alkyne of enyne

Three novel ruthenium‐catalyzed cyclizations of enynes were developed. In each cyclization, a ruthenacyclopentene derived from enyne and Cp*RuCl(cod) is a common intermediate. When an enyne having an alkyl, an ester, or a formyl group on an alkyne was reacted with Cp*RuCl(cod) under ethylene gas, ethylene was inserted into the ruthenium‐sp² carbon bond of ruthenacyclopentene to afford ruthenacycloheptene, and β‐hydrogen elimination followed by reductive elimination occurred to give a cyclic compound having a 1,3‐diene moiety. When an acyl group was placed on the alkyne, the carbonyl oxygen coordinated to the ruthenium metal of ruthenacyclopentene to produce a ruthenium carbene complex, which reacted with ethylene to give a cyclic compound having a cyclopropane ring on the substituent. On the other hand, when the substituent on the alkyne was pent‐4‐enyl, insertion of an alkene part into ruthenacyclopentene followed by reductive elimination gave a tricyclic compound by a ruthenium‐catalyzed [2 + 2 + 2] cyclization of diene and an alkyne. DOI 10.1002/tcr.201100003     

Theory for trivial trajectory parallelization of multicanonical molecular dynamics and application to a polypeptide in water

Trivial trajectory parallelization of multicanonical molecular dynamics (TTP-McMD) explores the conformational space of a biological system with multiple short runs of McMD starting from various initial structures. This method simply connects (i.e., trivially parallelizes) the short trajectories and generates a long trajectory. First, we theoretically prove that the simple trajectory connection satisfies a detailed balance automatically. Thus, the resultant long trajectory is regarded as a single multicanonical trajectory. Second, we applied TTP-McMD to an alanine decapeptide with an all-atom model in explicit water to compute a free-energy landscape. The theory imposes two requirements on the multiple trajectories. We have demonstrated that TTP-McMD naturally satisfies the requirements. The TTP-McMD produces the free-energy landscape considerably faster than a single-run McMD does. We quantitatively showed that the accuracy of the computed landscape increases with increasing the number of multiple runs. Generally, the free-energy landscape of a large biological system is unknown a priori. The current method is suitable for conformational sampling of such a large system to reduce the waiting time to obtain a canonical ensemble statistically reliable.