Synthesis,self-assembly,and photomechanical actuator performance of a sequence-defined polyviologen crosslinker

ABSTRACT

Although it is well known that viologen radical cations can self-assemble into stacks or complexes on account of radical-radical pairing interactions, it has only recently been demonstrated that reduction of main-chain polyviologens integrated into hydrogel networks can trigger actuation. In these earlier examples, hydrogels comprising oligoethylene glycol-based polyviologens and poly(ethylene glycol) were functionalized with terminal azide groups to prepare ‘click’-based gels. Here, we report a new structural design for the functional polyviologen that consists of main-chain viologen subunits separated by hexamethylene groups instead of glycols and is capped at each end with styrene groups. Activation of this viologen-based macrocrosslinker was achieved using chemical- and photoreduction methods and its ability to undergo intramolecular chain-folding was monitored by absorption spectroscopy. Acrylate-based organogels and hydrogels were also prepared and a comparison was carried out to assess the actuator performance in each gel in terms of the rate of contraction and changes in stiffness.     

Key interaction patterns in proteins revealed by cluster expansion of the partition function

The European Physical Journal E -     

Time delay as a key to apoptosis induction in the p53 network

A feedback mechanism that involves the proteins p53 and mdm2, induces cell death as a controlled response to severe DNA damage. A minimal model for this mechanism demonstrates that the response may be dynamic and connected with the time needed to translate the mdm2 protein. The response takes place if the dissociation constant k between p53 and mdm2 varies from its normal value. Although it is widely believed that it is an increase in k that triggers the response, we show that the experimental behaviour is better described by a decrease in the dissociation constant. The response is quite robust upon changes in the parameters of the system, as required by any control mechanism, except for few weak points, which could be connected with the onset of cancer. Received 8 May 2002 / Received in final form 9 July 2002 Published online 17 September 2002     

Experimental and theoretical charge density studies at subatomic resolution

Analysis of accurate experimental and theoretical structure factors of diamond and silicon reveals that the contraction of the core shell due to covalent bond formation causes significant perturbations of the total charge density that cannot be ignored in precise charge density studies. We outline that the nature and origin of core contraction/expansion and core polarization phenomena can be analyzed by experimental studies employing an extended Hansen-Coppens multipolar model. Omission or insufficient treatment of these subatomic charge density phenomena might yield erroneous thermal displacement parameters and high residual densities in multipolar refinements. Our detailed studies therefore suggest that the refinement of contraction/expansion and population parameters of all atomic shells is essential to the precise reconstruction of electron density distributions by a multipolar model. Furthermore, our results imply that also the polarization of the inner shells needs to be adopted, especially in cases where second row or even heavier elements are involved in covalent bonding. These theoretical studies are supported by direct multipolar refinements of X-ray powder diffraction data of diamond obtained from a third-generation synchrotron-radiation source (SPring-8, BL02B2).     

Optical absorption of a green fluorescent protein variant: environment effects in a density functional study

We present an ab initio study of the optical absorption properties of a particularly interesting fluorescent protein (E2GFP), whose complex photophysics still escapes elucidation. In particular, we focus on the role of the protein environment, showing that the effects of both nearby residues and the external field due to residues not accounted for explicitly are needed to properly reproduce the experimental data. The spectra calculated taking such contributions into account provide for the first time a robust identification of the states relevant for the photophysics of this system.     

Spectroscopic study on the interaction between[Hg（SCN）4]2- and hemoglobin

The interaction between[Hg（SCN）₄]^2- and hemoglobin（Hb） under conditions that simulate a physiological environment was investigated by UV-vis spectroscopy,fluorescence spectroscopy,resonance Rayleigh scattering（RRS） spectroscopy and circular dichroism（CD） spectroscopy.The results obtained from the change of UV-vis and CD spectra,the quenching of Hb fluorescence and the enhancement of RRS intensity proved that a 10:1 type complex was formed between[Hg（SCN）₄]^2- and Hb.The possible mechanism suggested for the interaction was that ten Hg（SCN）₄]^2- anions entered the four subunits of a Hb molecule to react with some residues to form an adduct by coordination and electrostatic forces.The coordination of[Hg（SCN）₄]^2- with Trp was the major cause of the fluorescence quenching of Hb.     

Tunable Trimers: Using Temperature and Pressure to Control Luminescent Emission in Gold(I) Pyrazolate‐Based Trimers

A systematic investigation into the relationship between the solid‐state luminescence and the intermolecular Au???Au interactions in a series of pyrazolate‐based gold(I) trimers; tris(μ₂‐pyrazolato‐N,N′)‐tri‐gold(I) ( 1 ), tris(μ₂‐3,4,5‐ trimethylpyrazolato‐N,N′)‐tri‐gold(I) ( 2 ), tris(μ₂‐3‐methyl‐5‐phenylpyrazolato‐N,N′)‐tri‐gold(I) ( 3 ) and tris(μ₂‐3,5‐diphenylpyrazolato‐N,N′)‐tri‐gold(I) ( 4 ) has been carried out using variable temperature and high pressure X‐ray crystallography, solid‐state emission spectroscopy, Raman spectroscopy and computational techniques. Single‐crystal X‐ray studies show that there is a significant reduction in the intertrimer Au???Au distances both with decreasing temperature and increasing pressure. In the four complexes, the reduction in temperature from 293 to 100 K is accompanied by a reduction in the shortest intermolecular Au???Au contacts of between 0.04 and 0.08 Å. The solid‐state luminescent emission spectra of 1 and 2 display a red shift with decreasing temperature or increasing pressure. Compound 3 does not emit under ambient conditions but displays increasingly red‐shifted luminescence upon cooling or compression. Compound 4 remains emissionless, consistent with the absence of intermolecular Au???Au interactions. The largest pressure induced shift in emission is observed in 2 with a red shift of approximately 630 cm^?1 per GPa between ambient and 3.80 GPa. The shifts in all the complexes can be correlated with changes in Au???Au distance observed by diffraction.     

Performance of Au/Ti and Au/TiO2 Nanotube Array Electrodes for Borohydride Oxidation and Oxygen Reduction Reaction in Alkaline Media

Polarization curves and complex kinetics of oxygen reduction (ORR) and borohydride oxidation (BOR) reaction were evaluated at thin Au layer on Ti and TiO₂ electrodes. TiO₂ electrodes prepared included amorphous TiO₂ (AM?TiO₂) and anatase (A?TiO₂). The electrodes structure and nanotube arrays morphology were observed by XRD and SEM, respectively. All electrodes show activity for both ORR and BOR. The use of Au layer over A?TiO₂ produces the strongest synergistic effect for ORR with exchange of 3 electrons. On the other hand, the strongest effect for BOR was observed in case of Au/Ti.