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The selenium analogues of anti-thyroid drugs exhibit their anti-thyroid action by a mechanism different from that of MMI. The selenium analogue of MMI and related selenium compounds exhibit high GPx activity, providing a novel method for the reversible inhibition of thyroid hormone biosynthesis.
In the pursuit of advanced biomaterials with combined strength, toughness, and elasticity, a new class of well-defined modular polymers has been synthesized, and their nanomechanical properties have been studied using atomic force microscopy. These polymers are based on a peptidomimetic beta-sheet-based double-closed loop (DCL) module, which was designed to overcome the limitation of the modular polymers we reported previously (J. Am. Chem. Soc. 2004, 126, 2059). Single-molecule force-extension experiments revealed the sequential unfolding of these modules as the polymer is stretched, resulting in more regular sawtooth-patterned curves similar to those seen in titin and other biopolymers. The single-molecule data agreed well with computer modeling, which suggested that hydrogen bonding and pi-stacking are both involved in the formation of small DCL clusters along the polymer chain.
The total synthesis of naphthoquinone natural products isolated from the Bignoniaceae plant family is described. Pinnatal, isopinnatal, sterekunthals A and B, pyranokunthones A and B, and anthrakunthone have been prepared along the lines of a biosynthetic proposal involving pericyclic reactions as key steps. The first case of catalysis in oxa 6pi electrocyclizations is reported.
Synthesis of the South 7 hemisphere of cephalostatin 7 is described applying a second-generation synthetic strategy, which retains all the existing carbons in hecogenin acetate 1. TFAT (trifluoroacetyl trifluoromethanesulfonate)-assisted E-ring opening yields the key D-ring cyclopentadiene. A facially selective [4 + 2] cycloaddition between a series of steroidal D-ring dienes and singlet oxygen was conformationally directed by the C21-Me stereochemistry of the side chain. Sharpless asymmetric dihydroxylation of the terminal olefin provides (S)-C25-OH. An acid-catalyzed SN2' intramolecular alkylation of an acetal oxygen was followed by a one-pot sequence of beta-elimination and spiroketalization. The new route provides a practical 16-operation synthesis of the South 7 hemisphere (20% overall), as well as a model for construction of the crucial North 1 hemisphere.
The coordination chemistry of Zn in an N(3)ArOH environment has been explored. The ligands are based on calix[6]arenes that present two imidazole arms and an amino phenol moiety at the narrow rim. Three different types of complexes have been characterized. One is dicationic with Zn(2+) coordinated to the three nitrogen atoms and to the oxygen of the phenol group of the calix[6]ligand. This complex is very sensitive to exogenous coordinating molecules and exists as a 5-coordinate species due to the endo-complexation of a guest. The second species is a monocationic complex for which the phenol group has been deprotonated. The resulting N(3)ArOZn complex can also bind a guest ligand albeit with a lower affinity than the dicationic complex. The third species is neutral. It can be obtained upon reaction with a base to yield a hydroxo complex or with an anion such as a chloride that coordinates the metal center from the outside of the calixarene cavity. The simultaneous binding of two anionic donors decreases the Zn Lewis acidity, allowing an impressive conformational reorganization of the system. One imidazole arm is released by the metal center. The other one undergoes self-inclusion into the pi-basic calixarene cavity because the low affinity of the metal center for neutral ligand does not allow the endo-coordination of an exogenous guest. Hence, the calix[6]N(3)ArOH-based Zn complexes act as an acid-base switch for guest binding. Several aspects of this system appear reminiscent of Zn-peptidases of the astacin and serralisin families.
We describe a simple and inexpensive method to produce super-hydrophobic surfaces on aluminum and its alloy by oxidation and chemical modification. Water or aqueous solutions (pH = 1-14) have contact angles of 168 +/- 2 and 161 +/- 2 degrees on the treated surfaces of Al and Al alloy, respectively. The super-hydrophobic surfaces are produced by the cooperation of binary structures at micro- and nanometer scales, thus reducing the energies of the surfaces. Such super-hydrophobic properties will greatly extend the applications of aluminum and its alloy as lubricating materials.
A simple approach to prepare and characterize biomaterial-based electrocatalysts for oxygen reduction was carried out. Poly-l-histidine was used as a matrix and ligand to complex Cu2+ to mimic the active sites of laccases. A modified glassy carbon (GC) electrode with Cu2+-poly-l-histidine complex decreases the oxygen reduction overpotential as compared with the bare GC electrode. An array of Cu2+-poly-l-histidine spots with different compositions was deposited on a GC substrate, and their catalytic activity for oxygen reduction was evaluated by a scanning electrochemical microscopy-based screening technique. The electrocatalytic activities of complexes for oxygen reduction strongly depended on the mole ratio of Cu2+ to poly-l-histidine and the applied potential of the substrate.
Siderophores are natural iron chelators that have been evolutionarily selected to bind to Fe ions with very high binding constants. We utilize these unique properties to bind to metal oxide surfaces using a fragment of the cyanobacterial siderophore anachelin. The resulting poly(ethylene glycol) conjugate forms stable adlayers on TiO2 as has been shown by variable angle spectroscopic ellipsometry and X-ray photoelectron spectroscopy. Moreover, these coated surfaces are highly protein-resistant against the adsorption of full human serum.
The mechanism by which [Cu(II)(L)](OTf)2 and [Cu(II)N3(L)](OTf) (L = TEPA: tris(2-pyridylethyl)amine or TMPA: tris(2-pyridylmethyl)amine; OTf = trifluoromethanesulfonate) react with superoxide (O2*-) to form [Cu(I)(L)](OTf) and O2 is described. Evidence for a CuO2 intermediate is presented based on stopped-flow experiments and competitive oxygen (18O) kinetic isotope effects on the bimolecular reactions of (16,16)O2*- and (18,16)O2*- ((16,16)k/(18,16)k). The (16,16)k/(18,16)k fall within a narrow range from 0.9836 +/- 0.0043 to 0.9886 +/- 0.0078 for reactions of copper(II) complexes with different coordination geometries and redox potentials that span a 0.67 V range. The results are inconsistent with a mechanism that involves either rate-determining O2*- binding or one-step electron transfer. Rather a mechanism involving formation of a CuO2 intermediate prior to the loss of O2 in the rate-determining step is proposed. Calculations of similar inverse isotope effects, using stretching frequencies of CuO2 adducts generated from copper(I) complexes and O2, suggest that the intermediate has a superoxo structure. The use of 18O isotope effects to relate activated oxygen intermediates in enzymes to those derived from inorganic compounds is discussed.
As-synthesized single-walled carbon nanotubes (SWNTs) are bundled mixtures of different species. The current challenge in the field of carbon nanotube research lies in the processing and separation of SWNTs, which first require efficient dispersion of individual SWNTs in solvents. We report DNA-mimicking polysoap surfactants that disperse SWNTs in aqueous solutions more effectively than DNA. The polysoaps are synthesized by functionalizing the side chain of poly(styrene-alt-maleic acid) with aminopyrene. The synthetic nature of the polysoap opens a new approach to further optimization of not only SWNT dispersion efficiency but also multi-functional SWNT dispersing surfactant.
The relative rates of acid-catalyzed rearrangements of epoxy esters to [3.2.1]bicyclic orthoesters, the subsequent rearrangements of these ortho esters to substituted tetrahydrofurans, and the rates of orthoester hydrolysis at pH 4.75 were measured in NMR kinetics experiments. The ease of formation and stabilities of these orthoesters compared favorably with the OBO-type [2.2.2]bicyclic orthoesters typically used as protecting groups of carboxylic acids. Studies with 13C NMR-detected 18O-labeling show that epoxy ester rearrangement takes place preferentially via 6-exo cyclization, although the 7-endo process competes when the distal center of the epoxide is disubstituted. The ortho ester-cyclic ether rearrangement was shown by 18O-labeling to occur exclusively via intermediacy of a five-membered dioxonium ion. The structures of the hydrolysis products also indicate the intermediacy of a dioxolanium ion during hydrolysis. The implications for a hypothetical biosynthesis of marine polyether toxins are discussed.
An asymmetric total synthesis of (+)-chatancin was achieved via a transannular Diels-Alder (TADA) reaction of an in situ generated macrocyclic pyranophane pseudobase. The presented route constitutes the second of two proposed biosynthetic pathways that involves a TADA reaction. It links this diterpene biogenetically to the cembranoids. A set of TADA selection rules that rationalize the formation of (+)-chatancin from a dynamic equilibrium of four 2-hydroxy-2H-pyrane bicycles and their 16 potential TADA transition states are also outlined. Beyond the TADA reaction, highlights of the synthetic work include the assembly of a chiral acyclic macrocyclization substrate from (S)-citronellol and an efficient macrocyclization via a beta-ketosulfoxyde/enone Michael addition.
Two new artificial photolyase models that recognize pyrimidine dimers in protic and aprotic organic solvents as well as in water through a combination of charge and hydrogen-bonding interactions and use a mimic of the flavine to achieve repair through reductive photoinduced electron transfer are presented. Fluorescence and NMR titration studies show that it forms a 1:1 complex with pyrimidine dimers with binding constants of approximately 10(3) M(-1) in acetonitrile or methanol, while binding constants in water at pH 7.2 are slightly lower. Excitation of the complex with visible light leads to clean and rapid cycloreversion of the pyrimidine dimer through photoinduced electron transfer catalysis. The reaction in water is significantly faster than in organic solvents. The reaction slows down at higher conversions due to product inhibition.
[structure: see text] The recognition of dopamine in water has been achieved with tripodal oxazoline-based artificial receptors, capable of providing a preorganized hydrophobic environment by rational design, which mimics a hydrophobic pocket predicted for a human D2 receptor. The receptors show an amphiphilic nature owing to the presence of hydrophilic sulfonate groups at the periphery of the tripodal oxazoline ligands, which seems to contribute in forming the preorganized hydrophobic environment. The artificial receptors recognized dopamine hydrochloride in water with reasonable selectivity among various organoammonium guests examined. The observed binding behavior of the receptors was explained by evoking guest inclusion in the preorganized hydrophobic pocket-like environment and not by simple ion-pairing interactions. The rationally predicted 1:1 inclusion binding mode was supported by binding studies such as with a reference receptor that cannot provide a similar binding pocket, Job and VT-NMR experiments, electrospray ionization mass analysis, and guest selectivity data. This study implies that an effective hydrophobic environment can be generated even from an acyclic, small molecular artificial receptor. Such a preorganized hydrophobic environment, as being utilized in biological systems, can be effectively used as a complementary binding force for the recognition of organoammonium guests such as dopamine hydrochloride in water.
We report the synthesis and characterization of a hydroxo-bridged dinuclear phenanthroline cupric complex, [(phen)2Cu-OH-Cu(phen)2](ClO4)) (HPC, phen = phenanthroline), dispersed in molecular sieves: MCM-41's and sodium zeolite Y. We employed spectroscopic techniques (FT-IR, UV-visible, EPR, and EXAFS) to characterize and study the catalytic activities of immobilized HPC in the oxidation of 3,5-di-tert-butylcatechol (DTBC) to the corresponding quinone (3,5-di-tert-butylquinone, DTBQ) to mimic the functionality of catechol oxidases. HPC complexes can adsorb only on the outside surface of the Y zeolite due to its smaller pore size. The EXAFS spectrum gives 3.51 A for the Cu...Cu distance in HPC encapsulated in the nanochannels of Al-MCM-41 mesoporous solids, which is comparable to the O...O distance of the two hydroxyl groups of DTBC, and this made a simultaneous coordination of the diol group to the dicupric center possible. The resultant complex then allows the transfer of two electrons from DTBC to the dicupric center leading to the production of DTBQ. The nanochannels of calcined Al-MCM-41 mesoporous solids provide stability, due to confined space and surface charge, which could prevent excessive separation of the dinuclear cupric centers after removal of the hydroxo bridge in the catalytic process. A catalytic reaction scheme is proposed based on the spectroscopic data obtained in the characterization. The study demonstrates that HPC encapsulated in the nanochannels of Al-MCM-41 mesoporous materials could be a viable system for a broad range of catalytic oxidation to mimic natural occurring enzymes.
Photophysical properties of 3-acetyl-4-oxo-6,7-dihydro-12H-indolo[2,3-a]quinolizine (AODIQ), a bioactive molecule, has been investigated in well-characterized, monodispersed biomimicking nanocavities formed by sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane using steady-state and picosecond time resolved fluorescence and fluorescence anisotropy. The emission behavior of AODIQ is very much dependent upon the water/surfactant mole ratio (W), i.e., on the water pool size of the reverse micellar core. AODIQ exhibits a sharp decrease in fluorescence anisotropy with increasing W, implying that the overall motional restriction experienced by the molecule is decreased with increased hydration. Some of the depth-dependent relevant fluorescence parameters, namely, fluorescence maxima and fluorescence anisotropy (r), have been monitored for exploiting the distribution and microenvironment around the probe in the reverse micelles. Fluorescence spectral position and fluorescence quenching studies suggest that the probe does not penetrate into the reverse micellar core; rather it binds at the interfacial region. Quantitaive estimates of the micropolarity and microviscosity at the binding sites of the probe molecule have been determined as a function of W.
This feature article summarizes our recent accomplishments in the field of carbon nanostructures, fullerenes and carbon nanotubes, as integrative components in multifunctional hybrid cells that bear large promise for applications as photochemical energy conversion systems.
In this paper, we report a simple polypeptide-directed strategy for fabricating large spherical assembly of CaCO(3) nanoparticles. Stepwise growth and assembly of a large number of nanoparticles have been observed, from the formation of an amorphous liquidlike CaCO(3)-polypeptide precursor, to the crystallization and stabilization of polypeptide-capped nanoparticles, and eventually, the spherical assembly of nanoparticles. The "soft" poly(aspartate)-capping layer binding on a nanoparticle surface resulted in the unusual soft nature of nanoparticle assembly, providing a reservoir of primary nanoparticles with a moderate mobility, which is the basis of a new strategy for reconstructing nanoparticle assembly into complex nanoparticle architectures. Moreover, the findings of the secondary assembly of nanoparticle microspheres and the morphology transformation of nanoparticle assembly demonstrate a flexible and controllable pathway for manipulating the shapes and structures of nanoparticle assembly. In addition, the combination of the polypeptide with a double hydrophilic block copolymer (DHBC) allows it to possibly further control the shape and complexity of the nanoparticle assembly. A clear perspective is shown here that more complex nanoparticle materials could be created by using "soft" biological proteins or peptides as a mediating template at the organic-inorganic interface.