Crystallization-controlled dynamic self-assembly and an on/off switch for equilibration using boronic ester formation

Macrocyclic boronic esters of different sizes can be prepared selectively from the same starting diboronic acid and 1,2-diol by means of an interesting dynamic self-assembly phenomena. More specifically, two kinds of macrocyclic boronic esters could be formed diastereoselectively and nearly quantitatively under neutral conditions by the addition of an appropriate guest molecule that acts as a template. Although a mixture of tetrol 1 and di(boronic acid) 2 in methanol gave only insoluble polymeric boronic esters, a soluble macrocyclic boronic ester, homo-[2+2], was obtained selectively in the presence of toluene as a guest molecule. Furthermore, when benzene was employed as a guest molecule, the selective formation of another macrocyclic boronic ester, hetero-[3+3], occurred. Interestingly, each of these macrocycles could be converted into the other in the presence of methanol and the appropriate guest molecule; however, under aprotic conditions, guest molecules encaged by the macrocyclic boronic ester could be exchanged without affecting its structure. Thus the presence or absence of a protic solvent could be used as a regulator to switch on or off the dynamic equilibrium of the system. In addition, investigation of the effect of reaction time, direct observation of the reaction mixture by NMR spectroscopy, and carrying out the reaction using optically active tetrol suggested that precipitation plays an essentially important role in the selective formation of the macrocyclic boronic esters. Thus, although both of [2+2] and [3+3] were present as solutes in the reaction mixture, the type of added guest molecule induced the selective precipitation of only one form of macrocyclic boronic ester, hence displacing the equilibrium of the system.     

Boronic esters as ionophores for potentiometric discrimination of the cis-trans isomeric dicarboxylic acids

Boronic esters incorporated into a poly(vinyl chloride) (PVC)-supported liquid membrane electrodes have displayed an anionic ionophore properties enabling their use in the potentiometric high-throughput screening procedures. These compounds belong to the class of ligands in which the anion recognition process can be explained on the concept of Lewis type acid-base interactions. Membranes containing boronic esters showed fairly good sensitivity for maleate (cis-isomer) in comparison to fumarate anions (trans-isomer). The potentiometric selectivity coefficients of proposed electrodes proved that common anions did not interfered with the maleate anion determination. The influence of structure of the three boronic esters ionophores on generation of potentiometric signal by developed liquid membrane electrodes was shortly discussed.     

Rational design of a reversible pH-responsive switch for peptide self-assembly

Peptide TZ1H, based on the heptad sequence of a coiled-coil trimer, undergoes fully reversible, pH-dependent self-assembly into long-aspect-ratio helical fibers. Substitution of isoleucine residues with histidine at the core d-positions of alternate heptads introduces a mechanism by which self-assembly is coupled to the protonation state of the imidazole side chain. Circular dichroism spectroscopy, transmission electron microscopy, and microrheology techniques revealed that the self-assembly of TZ1H coincides with a distinct coil-helix conformational transition that occurs within a narrow pH range near the pKa of the imidazole side chains of the core histidine residues.     

Photoisomerization of an individual azobenzene molecule in water: an on-off switch triggered by light at a fixed wavelength

The alpha-hemolysin (alphaHL) pore was used as a nanoreactor for the direct observation of the reversible photoisomerization of individual tethered azobenzene molecules in an aqueous environment. alphaHL pores, PAZO, were used that had been derivatized within the lumen at a single cysteine residue with 4-((4-(2-chloroethanoamido)phenyl)diazenyl)benzenesulfonate. Trans-cis isomerizations were monitored at the single-molecule level by observing the modulation of the current passing through PAZO by electrical recording in planar bilayers. When PAZO was irradiated at 330 nm, continuous interconversion between the trans and cis states was observed. Either the trans or the cis state was maintained in the dark, depending upon which was present when the light source was shuttered. The cis state of PAZO was surprisingly stable in the dark, and no cis --> trans transitions were seen over a total observation period of more than 8 h. Therefore, based on our findings, it might be possible to make fast digital nanoscale switches operated by light of a fixed wavelength.     

A photochemical on-off switch for tuning the equilibrium mixture of H2 nuclear spin isomers as a function of temperature

The photochemical interconversion of the two allotropes of the hydrogen molecule [para-H(2) (pH(2)) and ortho-H(2) (oH(2))] incarcerated inside the fullerene C(70) (pH(2)@C(70) and oH(2)@C(70), respectively) is reported. Photoexcitation of H(2)@C(70) generates a fullerene triplet state that serves as a spin catalyst for pH(2)/oH(2) conversion. This method provides a means of changing the pH(2)/oH(2) ratio inside C(70) by simply irradiating H(2)@C(70) at different temperatures, since the equilibrium ratio is temperature-dependent and the electronic triplet state of the fullerene produced by absorption of the photon serves as an "on-off" spin catalyst. However, under comparable conditions, no photolytic pH(2)/oH(2) interconversion was observed for H(2)@C(60), which was rationalized by the significantly shorter triplet lifetime of H(2)@C(60) relative to H(2)@C(70).     

Design of a selective metal ion switch for self-assembly of peptide-based fibrils

The self-assembling peptide TZ1H, a structural variant of the trimeric isoleucine zipper GCN4-pII, contains histidine residues at core d-positions of alternate heptads that define three trigonal coordination sites within the coiled-coil trimer. Circular dichroism spectropolarimetry indicated that peptide TZ1H undergoes a random coil to alpha-helical conformational change upon binding of 1 equiv of silver(I) ion, but not zinc(II), copper(II), or nickel(II) ions. Isothermal titration calorimetry provided evidence for a single binding-site model in which each peptide contributes one net silver(I) coordination site, in agreement with the proposed structural model. Transmission electron microscopy revealed that TZ1H self-assembles into long aspect ratio helical fibers in the presence of silver(I) ion. These results demonstrate that the rational design of selective metal ion binding sites within de novo designed peptides represents a promising approach to the controlled fabrication of nanoscale, self-assembled materials.     

Biologically active molecules with a "light switch"

Biologically active compounds which are light-responsive offer experimental possibilities which are otherwise very difficult to achieve. Since light can be manipulated very precisely, for example, with lasers and microscopes rapid jumps in concentration of the active form of molecules are possible with exact control of the area, time, and dosage. The development of such strategies started in the 1970s. This review summarizes new developments of the last five years and deals with "small molecules", proteins, and nucleic acids which can either be irreversibly activated with light (these compounds are referred to as "caged compounds") or reversibly switched between an active and an inactive state.     

A new electrochemiluminescent detection system equipped with an electrically heated carbon paste electrode for CE

An electrochemiluminescent (ECL) detection system in CE with an electrically heated carbon paste electrode (CPE) was developed. This CPE could be heated by a 100 kHz alternating current (ac) generated from a function generator, and the temperature of the electrode (Te) could be controlled. To evaluate the feasibility and reliability of this system, the electrochemically generated Ru(bpy)(3) (3+)-based ECL reaction was used for detection of triethylamine (TEA) and tri-n-propylamine (TPrA). Ru(bpy)(3) (2+) was added into the separation buffer solution with precolumn mode. Effects of several important factors were investigated to acquire the optimum conditions. Under the optimum conditions, the heated electrode has been shown to provide advantages by the measurement of ECL intensity in CE at elevated Te. Compared with the conventional electrode at the room temperature, using heated CPE could improve peak shape and gain good reproducibility with lower detection limits and wider linearity ranges. Compared with the room temperature, the linear ranges and detection limits (S/N = 3) for TEA and TPrA were improved about one magnitude when the Te was 39 degrees C. In contrast, the RSD was lower than for the electrode at room temperature.     

A new electrochemiluminescent detection system equipped with an electrically controlled heating cylindrical microelectrode

A new electrochemiluminescent (ECL) detection system equipped with an electrically controlled heating cylindrical microelectrode (HME) was developed in this paper. The cylindrical microelectrode made of platinum wire (25 μm in diameter, 6 mm in long) was used as the working electrode of the ECL detection system, the temperature of the electrode could be controlled electrically. The Ru(bpy)₃²⁺-ECL and Ru(bpy)₃²⁺-C₂O₄²⁻-ECL systems were used to evaluate this ECL detection system. The detection limit for oxalate was found to be 3.0 × 10⁻⁴ mol/L when T_e (temperature of the HME) was 22 °C, and found to be 3.0 × 10⁻⁶ mol/L at 80 °C, which indicates that the detection limit can be improved greatly at higher T_e, based on which, it is possible to establish a more sensitive method for measurement of ECL by using a heated microelectrode.     

Capillary electrophoresis-chemiluminescence detection system equipped with a consecutive sample-injection device.

We have developed a consecutive sample-injection device for capillary electrophoresis, which comprises one four-way cock, two syringe pumps, and an interface part taking advantage of two three-way Teflon joints. Sample introduction into the capillary is made hydrodynamically by pressure, caused by the flow of the sample solution at the tip of the capillary inlet. We combined the injection device with a capillary electrophoresis-chemiluminescence detection system. A mixture solution of N-(4-aminobutyl)-N-ethylisoluminol, isoluminol isothiocyanate, and luminol was analyzed as a model sample by the present system. The sample solution was consecutively injected and detected with about a 230 s interval. The present capillary electrophoresis-chemiluminescence detection system with the consecutive sample injection device features easy and rapid operation, an inexpensive apparatus, high sensitivity, as well as consecutive analysis.     

Boronic acid-appended bis-viologens as a new family of viologen quenchers for glucose sensing

A new family of boronic acid-appended viologen quenchers has been synthesized containing two viologen subunits in a single quencher moiety. Relative to the single viologen-based quenchers previously developed in our laboratory, the bis-viologen ortho-boronic acid (BoB) compounds, in combination with the fluorescent dye, HPTS, display greatly enhanced Stern-Volmer quenching constants and much greater signal modulation in response to glucose. The superior performance is realized at lower quencher-to-dye ratios than are required for the single-viologen systems.     

Deuterium–hydrogen exchange reactions in peptides and polyatomic organic compounds,as studied on an ion cyclotron resonance mass spectrometer equipped with an ion trap with dynamic harmonization

The ultrahigh-resolution mass spectra of a substance P peptide labeled with deuterium have been obtained. The use of an ion trap with dynamic harmonization has made it possible to resolve the hyperfine isotopic structure of peaks and to reliably distinguish the hydrogen isotopes from the isotopes of carbon in the composition of the test molecules. The deuterium–hydrogen exchange reaction of humic acids has been performed in a gas phase, and it has been shown that the resolving power attained is sufficient for reliable interpretation of the results.     

A photochromic fluorescent switch in an organogel system with non-destructive readout ability

A fluorescent organogel based on photochromic dithienylethene was obtained, whose optimal excitation wavelength (470 nm) results in little structural change of both open and closed isomers of diarylethene, thus presenting the first example of a fluorescent switch with non-destructive readout ability in the gel state.     

A coumarin-based fluorescent probe for Hg2+ and Ag+ with an N'-acetylthioureido group as a fluorescence switch

For detection of Hg(2+) and Ag(+), we have developed a coumarin-based probe with an N'-acetylthioureido group as a novel fluorescence switch, in which the hydrogen bond formation between the N-hydrogen and the acetyl O atom markedly increases the susceptibility of the probe toward desulfurization by Hg(2+) and Ag(+).     

Disulfide bond as a cleavable linker for molecular self-assembly and hydrogelation

Here we reported on the first example of using the disulfide bond as a cleavable linker to control molecular self-assembly and the formation of small molecular (SM) hydrogels.     

Membrane selectivity for an aqueous system by dynamic osmometry

Commercial and fractionated samples of poly(ethylene glycol) and a sample of Dextran were used as standards to show the limitation of a cellulose acetate membrane for osmotic pressure determination of molecular weight in an aqueous system. The leakage of this membrane was much greater in the aqueous system than in organic solvents.     

Ruthenium(II) as a novel "labile" partner in thermodynamic self-assembly of heterobimetallic d-f triple-stranded helicates

Unsymmetrical substituted bidentate benzimidazol-2-ylpyridine ligands L2 and L3 react with [Ru(dmso)(4)Cl(2)] in ethanol to give statistical 1:3 mixtures of fac-[Ru(Li)(3)](2+) and mer-[Ru(Li)(3)](2+) (i=2, 3; DeltaGtheta(isomerisation)=-2.7 kJ mol(-1)). In more polar solvents (acetonitrile, methanol), the free energy of the facial<=>meridional isomerisation process favours mer-[Ru(Li)(3)](2+), which is the only isomer observed in solution at the equilibrium (DeltaGtheta(isomerisation)< or = -11.4 kJ mol(-1)). Since the latter process takes several days for [Ru(L2)(3)](2+), fac-[Ru(L2)(3)](2+) and mer-[Ru(L2)(3)](2+) have been separated by chromatography, but the 28-fold increase in velocity observed for [Ru(L3)(3)](2+) provides only mer-[Ru(L3)3](ClO(4))2 after chromatography (RuC(60)H(51)N(9)O(8)Cl(2), monoclinic, P2(1)/n, Z=4). The facial isomer can be stabilised when an appended tridentate binding unit, connected at the 5-position of the benzimidazol-2-ylpyridine unit in ligand L1, interacts with nine-coordinate lanthanides(III). The free energy of the facial<=>meridional isomerisation is reversed (DeltaGtheta(isomerisation)> or =11.4 kJ mol(-1)), and the Ru--N bonds are labile enough to allow the quantitative thermodynamic self-assembly of HHH-[RuLu(L1)(3)]5+ within hours ([RuLu(L1)3](CF(3)SO(3))(4.5)Cl(0.5)(CH(3)OH)(2.5): RuLuC(106)H(109)Cl(0.5)N(21)O(19)S(4.5)F(13.5), triclinic, P(-)1, Z=2). Electrochemical and photophysical studies show that the benzimidazol-2-ylpyridine units in L1-L3 display similar pi-acceptor properties to, but stronger pi-donor properties than, those found in 2,2'-bipyridine. This shifts the intraligand pi-->pi* and the MLCT transitions toward lower energies in the pseudo-octahedral [Ru(Li)(3)](2+) (i=2, 3) chromophores. The concomitant short lifetime of the (3)MLCT excited state points to efficient, thermally activated quenching via low-energy Ru-centred d-d states, a limitation which is partially overcome by mechanical coupling in HHH-[RuLu(L1)(3)]5+.     

Self-location of acceptors as "isolated" or "stacked" energy traps in a supramolecular donor self-assembly: a strategy to wavelength tunable FRET emission

Control over supramolecular assemblies of donor and acceptor arrays in nanoscale dimension that facilitate efficient energy transfer resulting in tunable emission is an outstanding challenge. In pursuit of this goal, we have designed a supramolecular donor-acceptor organogel with tunable emission from green to red through controlled energy transfer by simply varying the acceptor concentration. Temperature-dependent UV/vis absorption, XRD, and AFM studies of the coassembly of 1 (donor) and 2 (acceptor) revealed the intercalation of 2 within the self-assembly of 1. Upon excitation of the decane gels of 1 with 0-2 mol % of 2, quenching of the emission of the former at 509 nm with the formation of the monomer emission of the latter at 555 nm is observed. Upon further addition of 2 (2-20 mol %), the emission was continuously red-shifted to 610 nm, which corresponds to the aggregate emission of 2. Consequently, a 98% quenching of the donor emission was observed at 509 nm. Fluorescence microscopic studies provided visual evidence for the color tuning of the FRET emission. Thus efficient trapping of excitons by "isolated" or "aggregated" acceptors through a subtle control of the self-assembly and the photophysical properties of the donor-acceptor building blocks allowed a continuous shifting of the emission color anywhere between green and red (lambdamax, 509-610 nm) in a supramolecular light harvesting system.