Synthesis and structure of iron(iii) diamine-bis(phenolate) complexes

The structures and properties of six new iron(iii) diamine-bis(phenolate) complexes are reported. Reaction of anhydrous FeX(3) salts (where X = Cl or Br) with the diprotonated tripodal tetradentate ligands 2-pyridylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol), H(2)[L(1)], and N,N-dimethyl-N',N'-bis(2-methylene-4-methyl-6-tert-butylphenol)ethylenediamine, H(2)[L(2)], produces the trigonal bipyramidal iron(iii) complexes, [L(1)]FeCl , [L(1)]FeBr , [L(2)]FeCl and [L(2)]FeBr . Reaction of FeX(3) with the related linear tetradentate ligand N,N'-bis(4,6-tert-butyl-2-methylphenol)-N,N'-bismethyl-1,2-diaminoethane, H(2)[L(3)], generates square pyramidal iron(iii) complexes, [L(3)]FeCl and [L(3)]FeBr . Complexes have been characterized using electronic absorption spectroscopy and magnetometry. Single crystal X-ray molecular structures have been determined for complexes 1, 3, 5 and 6.     

High resolution electronic spectroscopy of p-vinylphenol in the gas phase

A recent controversy regarding the proper assignment of two closely spaced bands in the S₁ ← S₀ electronic transition of trans-p-coumaric acid (pCA) has been addressed by recording their spectra at full rotational resolution. The results show unambiguously that the carrier of these two bands is p-vinylphenol (pVP), a thermal decomposition product of pCA. The two bands belong to two conformers of pVP; trans-pVP at 33,207.3 cm⁻¹ and cis-pVP at 33,211.8 cm⁻¹.     

Motion of gold atoms on carbon in the aberration-corrected STEM.

The movement of heavy atoms on a thin carbon substrate is readily observed using a sub-Angstrom electron probe. The observed movement is consistent with an electron beam activation mechanism whereby atoms are occasionally detached from bonding sites, allowing rapid diffusion to new sites that may be quite far from the original. The bonding sites are most often observed to lie at defects, steps, and other asperities in the substrate. Formation of three-dimensional clusters can occur during diffusion of several isolated atoms. Coalescence and dissolution of larger clusters and islands both occur under varying observation conditions, but island coalescence appears most probable for islands that are greater than 2 nm in size.     

Electrochemical reactivity at redox-molecule-based nanoelectrode ensembles

A model describing electrochemical reactivity at nanoelectrode ensembles consisting of redox-molecule-based active sites immobilized on otherwise passivated electrode surfaces is presented. A mathematical treatment in terms of hemispherical diffusion of redox-active solutes to a layer of independent molecule-based nanoelectrode sites is shown to be equivalent to one in terms of a bimolecular diffusion-limited reaction between a layer of immobilized redox molecules and a reservoir of redox-active solutes. This equivalence derives from the fact that in both cases the mass-transfer problem is essentially that of hemispherical diffusion. The model is further developed to consider rate limitation by both the bimolecular redox reaction between the active-site molecule and redox molecules in solution and the heterogeneous redox reaction between the electrode and the active-site molecule. Analytical expressions are derived for the current–voltage relation corresponding to catalyzed electron transfer at an ensemble of redox-molecule-based nanoelectrode sites, and the expressions are used to interpret preliminary data for ultrasensitive electrochemical detection in flow streams via an electrochemical amplification process that is thought to involve redox mediation by individual analyte molecules adsorbed onto monolayer-coated electrodes.     

Effect of agents modifying the membrane dipole potential on properties of syringomycin E channels

We evaluated the effect of agents modifying the membrane dipole potential: phloretin, 6-ketocholestanol and RH 421 on the properties of single channels formed by lipodepsipeptide syringomycin E (SRE) in planar lipid bilayers. SRE forms two conductive states in lipid bilayers: "small" and "large." Large SRE channels are clusters of several small ones, demonstrating synchronous openings and closures. The increase in the membrane dipole potential led to (i) an increase in SRE channel conductance, (ii) an increase in the channel's lifetime, and (iii) a decrease in a number of synchronously operating small channels in the clusters. Overall, the results support the model of the small SRE channel synchronization in the cluster as voltage-dependent orientation of the lipid dipoles associated with the channel pores.     

High-density targeting of a viral multifunctional nanoplatform to a pathogenic, biofilm-forming bacterium

Nanomedicine directed at diagnosis and treatment of infections can benefit from innovations that have substantially increased the variety of available multifunctional nanoplatforms. Here, we targeted a spherical, icosahedral viral nanoplatform to a pathogenic, biofilm-forming bacterium, Staphylococcus aureus. Density of binding mediated through specific protein-ligand interactions exceeded the density expected for a planar, hexagonally close-packed array. A multifunctionalized viral protein cage was used to load imaging agents (fluorophore and MRI contrast agent) onto cells. The fluorescence-imaging capability allowed for direct observation of penetration of the nanoplatform into an S. aureus biofilm. These results demonstrate that multifunctional nanoplatforms based on protein cage architectures have significant potential as tools for both diagnosis and targeted treatment of recalcitrant bacterial infections.     

Oligomers of a 5-carboxy-methanopyrrolidine β-amino acid. A search for order

CD spectra for homooligomers (n = 4, 6, 8) of (1S,4R,5R)-5-syn-carboxy-2-azabicyclo[2.1.1]hexane (MPCA), a methano-bridged pyrrolidine β-carboxylic acid, suggest an ordered secondary structure. Even in the absence of internal hydrogen bonding, solution NMR, X-ray, and in silico analyses of the tetramer are indicative of conformations with trans-amides and C(5)-amide-carbonyls oriented toward the C(4) bridgehead. This highly constrained β-amino acid could prove useful in the ongoing development of well-defined foldamers.     

Oligoether-strapped calix[4]pyrrole: an ion-pair receptor displaying cation-dependent chloride anion transport

A ditopic ion-pair receptor (1), which has tunable cation- and anion-binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([1·F](-) and [1·Cl](-)) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li(+), Na(+), K(+), Cs(+), as their perchlorate salts), ion-dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [1·F](-), no appreciable interaction with the K(+) ion was seen. On the other hand, when this complex was treated with Li(+) or Na(+) ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li(+), Na(+), K(+), treating [1·F](-) with Cs(+) ions gave rise to a stable, host-separated ion-pair complex, [F·1·Cs], which contains the Cs(+) ion bound in the cup-like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [1·Cl](-). Here, no appreciable interaction was observed with Na(+) or K(+). In contrast, treating with Li(+) produces a tight ion-pair complex, [1·Li·Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [1·F](-), treatment of [1·Cl](-) with Cs(+) ions gives rise to a host-separated ion-pair complex, [Cl·1·Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co-transport) and antiport (nitrate-for-chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate-for-chloride anion exchange mechanism.     

Characterization of protein-ligand interactions by high-resolution solid-state NMR spectroscopy

A novel approach for detection of ligand binding to a protein in solid samples is described. Hydrated precipitates of the anti-apoptotic protein Bcl-xL show well-resolved (13)C-(13)C 2D solid-state NMR spectra that allow site-specific assignment of resonances for many residues in uniformly (13)C-enriched samples. Binding of a small peptide or drug-like organic molecule leads to changes in the chemical shift of resonances from multiple residues in the protein that can be monitored to characterize binding. Differential chemical shifts can be used to distinguish between direct protein-ligand contacts and small conformational changes of the protein induced by ligand binding. The agreement with prior solution-state NMR results indicates that the binding pocket in solid and liquid samples is similar for this protein. Advantages of different labeling schemes involving selective (13)C enrichment of methyl groups of Ala, Val, Leu, and Ile (Cdelta1) for characterizing protein-ligand interactions are also discussed. It is demonstrated that high-resolution solid-state NMR spectroscopy on uniformly or extensively (13)C-enriched samples has the potential to screen proteins of moderate size ( approximately 20 kDa) for ligand binding as hydrated solids. The results presented here suggest the possibility of using solid-state NMR to study ligand binding in proteins not amenable to solution NMR.