Electrostatic Assembly of Dendrimer Electrolytes: Negatively and Positively Charged Dendrimer Porphyrins

Carboxylate and ammonium functionalities—32 of each— facilitate electrostatic interaction between oppositely charged dendrimer electrolytes and porphyrin cores, which leads in protic media to a supramolecular, fluorescence-active aggregate of two chromophores that communicate and have a predictable topology (see drawing on the right; A=acceptor, D=donor). In contrast to the complexation of linear polyelectrolytes, the contact area for the two dendrimer molecules is very limited, just as one would expect for an assembly of spherical molecules.     

High oxygen-reduction activity of silk-derived activated carbon

Activated carbon prepared from silk fibroin, which is free of metal elements, showed a high catalytic activity for the oxygen-reduction reaction (ORR). The activated carbon had a very high onset potential of E_onset = 0.83 V (vs. RHE) in oxygen-saturated 0.5 M H₂SO₄ at 60 °C. The ORR on the activated carbon proceeded by a four-electron process in the high-electrode-potential region; this gradually decreased to a 3.5-electron reaction below about 0.6 V (vs. RHE). Only about 1% of nitrogen atoms (mostly quaternary) remained in the activated carbon by heat-treatment at up to 1200 °C are responsible for the high catalytic activity. The open circuit voltage of a polymer electrolyte fuel cell using the activated carbon as the cathode and a platinum/carbon black anode under pure oxygen and hydrogen gases, respectively, both at one atmosphere, was 0.96 V at 27 °C.     

The efficient direct synthesis of N,O-acetal compounds as key intermediates of discorhabdin A: oxidative fragmentation reaction of alpha-amino acids or beta-amino alcohols by using hypervalent iodine(III) reagents

Hypervalent iodine(III) reagents are readily available, easy to handle, and have a low toxicity and similar reactivities to those of heavy metal reagents, and hence they are used for various oxidative reactions. The oxidative cleavage of alkynes or carbonyl compounds by using bis(trifluoroacetoxy)iodo(III) pentafluorobenzene (C(6)F(5)I(OCOCF(3))(2)) has been reported. Herein, the efficient direct synthesis of N,O-acetal compounds as key intermediates of discorhabdin A, by the oxidative fragmentation reaction of alpha-amino acids or beta-amino alcohols by using C(6)F(5)I(OCOCF(3))(2), is described.     

Conformational changes during apoplastocyanin folding observed by photocleavable modification and transient grating

A new method to investigate the initial protein folding dynamics is developed based on a pulsed laser light triggering method and a unique transient grating method. The side chain of the cysteine residue of apoplastocyanin (apoPC) was site-specifically modified with a 4,5-dimethoxy-2-nitrobenzyl derivative, where the CD and 2D NMR spectra showed that the modified apoPC was unfolded. The substituent was cleaved with a rate of about 400 ns by photoirradiation, which was monitored by the disappearance of the absorption band at 355 nm and the increase in the transient grating signal. After a sufficient time from the photocleavage reaction, the CD and NMR spectra showed that the native beta-sheet structure was recovered. Protein folding dynamics was monitored in the time domain with the transient grating method from a viewpoint of the molecular volume change and the diffusion coefficient, both of which reflect the global structural change, including the protein-water interaction. The observed volume decrease of apoPC with a time scale of 270 micros is ascribed to the initial hydrophobic collapse. The increase in the diffusion coefficient (23 ms) is considered to indicate a change from an intermolecular to an intramolecular hydrogen bonding network. The initial folding process of apoPC is discussed based on these observations.     

Systematic structural control of multichromic platinum(II)-diimine complexes ranging from ionic solid to coordination polymer

Reactions of a Pt(II)-diimine-based metalloligand Na(2)[Pt(CN)(2)(4,4'-dcbpy)] (4,4'-H(2)dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) with alkaline-earth metal salts Mg(NO(3))(2)·6H(2)O, CaCl(2), SrCl(2)·6H(2)O, and BaBr(2)·2H(2)O in aqueous solution gave luminescent complexes formulated as [Mg(H(2)O)(5)][Pt(CN)(2)(4,4'-dcbpy)]·4H(2)O (MgPt-4·9H(2)O), {[Ca(H(2)O)(3)][Pt(CN)(2)(4,4'-dcbpy)]·3H(2)O}(∞) (CaPt-4·6H(2)O), {[Sr(H(2)O)(2)][Pt(CN)(2)(4,4'-dcbpy)]·H(2)O}(∞) (SrPt-4·3H(2)O), and {[Ba(H(2)O)(2)][Pt(CN)(2)(4,4'-dcbpy)]·3H(2)O}(∞) (BaPt-4·5H(2)O), respectively. The crystal structures of all MPt-4 complexes were determined by X-ray crystallography. In these structures, the alkaline-earth metal ions are commonly coordinated to the carboxyl groups of the [Pt(CN)(2)(4,4'-dcbpy)](2-) metalloligand. In the case of MgPt-4·9H(2)O, the Mg(II) ion is bound by five water molecules and one oxygen atom of a carboxyl group to form a neutral complex molecule [Mg(H(2)O)(5)][Pt(CN)(2)(4,4'-dcbpy)]. In contrast, the alkaline-earth metal ion and metalloligand form two-dimensional (CaPt-4·6H(2)O) and three-dimensional (SrPt-4·3H(2)O and BaPt-4·5H(2)O) coordination networks, respectively. All fully hydrated complexes exhibited a strong phosphorescence from the triplet π-π* transition state. Luminescence spectroscopy revealed that MgPt-4·9H(2)O exhibited interesting multichromic (i.e., thermo-, mechano-, and vapochromic) luminescence, whereas CaPt-4·6H(2)O showed only thermochromic luminescence. The other two complexes did not exhibit any chromic behaviour. Combination analysis of powder X-ray diffraction, thermogravimetry, and IR spectroscopy suggests that the dimensionality of the coordination network contributes considerably to both the structural flexibility and luminescence properties; that is, the low-dimensional flexible coordination network formed in MPt-4 complexes with smaller alkaline-earth metal ions enables a structural rearrangement induced by thermal and mechanical stimuli and vapour adsorption, resulting in the observed multichromic behaviour.     

Terpyridine platinum(ii) complexes containing triazine di- or tri-thiolate bridges: structures, luminescence, electrochemistry, and aggregation

Dinuclear complexes [{Pt(trpy)}(2)(L)](PF(6))(2) (trpy = 2,2':6',2'-terpyridine, L = 2-octylthio-1,3,5-triazine-4,6-dithiolate ion (1), L = 2-octadecylthio-1,3,5-triazine-4,6-dithiolate ion (2), L = 2-di-n-butylamino-1,3,5-triazine-4,6-dithiolate ion (3)) and a trinuclear complex [{Pt(trpy)}(3)(L)](PF(6))(3) (L = 1,3,5-triazine-2,4,6-trithiolate ion (4)) have been synthesized and characterized. The single crystal X-ray analysis revealed that the two {Pt(trpy)}(2+) fragments in 1 and 3 adopt a syn-configuration. The PtPt distances are around 4.3 ?, suggesting no intramolecular PtPt interactions. Complexes 1-4 in acetonitrile show broad absorption bands at around 470 nm, assigned to mainly the ligand-to-ligand charge transfer ((1)LLCT) from triazine thiolates to trpy based on the comparison to the related complexes and the density functional theory (DFT) calculations. The red luminescence of 1-4 in acetonitrile is attributable to emission predominantly from (3)LLCT. Cyclic voltammograms of 1-3 exhibit four redox couples from -2.0 V to 0 V vs. Ag/AgCl. The two consecutive processes at around -0.70 V are assigned to the sequential reduction of two trpy ligands. This assignment was further supported by the observation of the anion radical of trpy in spectroelectrochemical experiments. The splitting of the redox potentials of two trpy ligands evidences the moderate electronic coupling interactions mediated by the triazine dithiolate bridges. Complex 2 formed a transparent red gel in CH(3)CN, whereas 4 produced a gel-like solid in the mixtures of CH(3)CN and other solvents. The interactions dominating the aggregative behaviours have been discussed based on the results of electronic absorption and emission spectroscopy.     

Gravitational compression dynamics of charged colloidal crystals

We examine the compression of charged colloidal crystals under the influence of gravitational force by monitoring the spatiotemporal variations of Bragg diffraction from the crystal lattice. We use the dilute aqueous dispersions of colloidal silica particles (diameter=216 nm, charge number=733, a particle volume fraction φ=0.06) in the presence of 5-15 μM sodium chloride. The sedimentation profiles of the colloidal crystals along the crystal height are determined by in situ fiber optics reflection spectroscopy. The time evolutions of the sedimentation profiles are calculated by numerical simulations based on a phenomenological continuum model that explicitly incorporates the electrostatic interparticle interactions. The simulation results correctly describe the experiments at sufficiently high ionic strength.