The application of (199)Hg NMR and (199m)Hg perturbed angular correlation (PAC) spectroscopy to define the biological chemistry of Hg(II): a case study with designed two- and three-stranded coiled coils

The use of de novo designed peptides is a powerful strategy to elucidate Hg(II)-protein interactions and to gain insight into the chemistry of Hg(II) in biological systems. Cysteine derivatives of the designed alpha-helical peptides of the TRI family [Ac-G-(L(a)K(b)A(c)L(d)E(e)E(f)K(g))(4)-G-NH(2)] bind Hg(II) at high pH values and at peptide/Hg(II) ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting Hg(II) complexes are good water-soluble models for Hg(II) binding to the protein MerR. We have carried out a parallel study using (199)Hg NMR and (199m)Hg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI L9C/Hg(II) ratios. These studies prove for the first time the formation of [Hg{(TRI L9C)(2)-(TRI L9C-H)}], a dithiolate-Hg(II) complex in the hydrophobic interior of the three-stranded coiled coil (TRI L9C)(3). (199)Hg NMR and (199m)Hg PAC data demonstrate that this dithiolate-Hg(II) complex is different from the dithiolate [Hg(TRI L9C)(2)], and that the presence of third alpha-helix, containing a protonated cysteine, breaks the symmetry of the coordination environment present in the complex [Hg(TRI L9C)(2)]. As the pH is raised, the deprotonation of this third cysteine generates the trigonal thiolate-Hg(II) complex Hg(TRI L9C)(3)(-) on a timescale that is slower than the NMR timescale (0.01-10 ms). The formation of the species [Hg{(TRI L9C)(2)(TRI L9C-H)}] is the result of a compromise between the high affinity of Hg(II) to form dithiolate complexes and the preference of the peptide to form a three-stranded coiled coil.     

Synthesis and photoinduced intramolecular processes of fulleropyrrolidine-oligothienylenevinylene-ferrocene triads

Two new triads based on N-methylfulleropyrolidine, oligothienylenevinylenes (nTV) and ferrocene (Fc), namely C(60)-nTV-Fc (n=2, 4) have been synthesized. A HOMO-LUMO gap as low as 1.09-1.11 eV was experimentally determined by cyclic voltammetry. In both polar and nonpolar solvents, photoinduced charge-separation (CS) processes in C(60)-nTV-Fc predominantly take place from the singlet excited states of C(60) and nTV; this result was indicated by steady and time-resolved emission spectroscopy. In the case of C(60)-4TV-Fc, the CS state was indicated by the nanosecond transient absorption spectra. In C(60)-2TV-Fc, although the CS process was also confirmed by the fluorescence quenching in nonpolar and polar solvents, the lifetimes of the CS states were shorter than those of C(60)-4TV-Fc. It was revealed that the introduction of Fc donor moiety at the end of the longer nTV chain in the C(60)-nTV dyad systems effectively increases the CS efficiency and the lifetimes of CS states.     

Wurster's crowns: a comparative study of ortho- and para-phenylenediamine-containing macrocyclic receptors

Two series of isomeric, redox-responsive azacrown ethers based on ortho- and para-phenylenediamine (Wurster's crowns) have been synthesized and their properties explored through 13C NMR spectroscopy, electrospray ionization mass spectrometry, cyclic voltammetry, and X-ray crystallography. These crowns display strong affinity for alkali metal cations while maintaining comparable selectivity profiles to the parent crown ethers from which they are derived. Like Wurster's reagent (N,N,N',N'-tetramethyl-p-phenylenediamine or para-TMPD), the para-Wurster's crowns undergo two reversible one-electron oxidations. The integrity of the alkali metal ion complexes is maintained in the neutral and singly oxidized ligand states but not after removal of two electrons. In contrast, the oxidation of ortho-Wurster's crowns is scan rate dependent, occurring at potentials substantially higher than their para counterparts, with their complexes oxidizing irreversibly. X-ray crystal structures of representative complexes show, in all cases, participation of the redox-active phenylenediamine subunits in complex formation via direct bonding to the guest cation.     

Prussian blue analogue CsFe[Cr(CN)(6)] as a matrix for the Fe(II) spin-crossover

The origin of the intriguing spin-transition behavior of the Prussian blue analogue cesium iron hexacyanochromate CsFe[Cr(CN)6] has been investigated by means of correlated ab initio CASPT2 calculations. Using the smallest transiting core [Fe(NC)6]4-, the relative importance of the local ligand field and the Madelung field generated by the rest of the crystal was estimated. It is shown that in the presence of a frozen-charge environment, the high-spin state lies lower in energy than the low-spin state, thus excluding the possibility of observing a spin transition. In contrast, the charge reorganization in the environment evaluated from unrestricted periodic Hartree-Fock calculations creates a prerequisite for the spin-transition phenomenon. The influence of the disorder in the cesium ions' positions on the spin transition has been examined as a possible stabilizing factor of the low-spin state of [Fe(NC)6]4-. It is concluded that this experimentally observed disorder cannot account solely for the unprecedented behavior of the CsFe[Cr(CN)6] compound.     

Toward the description of electrostatic interactions between globular proteins: potential of mean force in the primitive model

Monte Carlo simulations are used to calculate the exact potential of mean force between charged globular proteins in aqueous solution. The aim of the present paper is to study the influence of the ions of the added salt on the effective interaction between these nanoparticles. The charges of the model proteins, either identical or opposite, are either central or distributed on a discrete pattern. Contrarily to Poisson-Boltzmann predictions, attractive, and repulsive direct forces between proteins are not screened similarly. Moreover, it has been shown that the relative orientations of the charge patterns strongly influence salt-mediated interactions. More precisely, for short distances between the proteins, ions enhance the difference of the effective forces between (i) like-charged and oppositely charged proteins, (ii) attractive and repulsive relative orientations of the proteins, which may affect the selectivity of protein/protein recognition. Finally, such results observed with the simplest models are applied to a more elaborate one to demonstrate their generality.     

An analytical derivative procedure for the calculation of vibrational Raman optical activity spectra

We present an analytical time-dependent Hartree-Fock algorithm for the calculation of the derivatives of the electric dipole-magnetic dipole polarizability with respect to atomic Cartesian coordinates. Combined with analogous procedures to determine the derivatives of the electric dipole-electric dipole and electric dipole-electric quadrupole polarizabilities, it enables a fully analytical evaluation of the three frequency-dependent vibrational Raman optical activity (VROA) invariants within the harmonic approximation. The procedure employs traditional non-London atomic orbitals, and the gauge-origin dependence of the VROA intensities has, therefore, been assessed for the commonly used aug-cc-pVDZ and rDPS:3-21G basis sets.     

Capillary electrophoresis-high resolution sector field inductively coupled plasma mass spectrometry

The background and applications of high resolution sector field inductively coupled plasma mass spectrometry (HR-ICP-MS) as a detector for capillary (CE) and gel electrophoretic separations are reviewed. Notable progress has been made in the fields of bioinorganic and environmental (geo-) chemistry. Metallomics, the study of metal species interactions and functions in biological systems, puts substantial technical demands on speciation analysis. The combination of high species resolving power (CE) and high sensitivity-high mass resolving power (HR-ICP-MS) provides a solid base to meet such demands.     

Organic solvent nanofiltration for microfluidic purification of poly(amidoamine) dendrimers

A nanofiltration method has been developed in a microfluidic format for the continuous-flow pressure-driven purification of half-generation poly(amidoamine) (PAMAM) dendrimers, a family of macromolecules characterized by highly branching structures radiating from a central core, without additional solvents or buffers. An organic solvent resistant nanofiltration membrane, STARMEM 122, has been fully integrated into a hard polymer microfluidic module by transmission laser welding. The membrane was initially characterized in a bench-top test fixture to determine the solvent permeance and percent rejection of a surrogate molecule, Rhodamine B, at lower than typical operating pressures (P<7 bar). The microfluidic module then underwent similar testing at 1.4 bar with the surrogate and with the generation-0.5 PAMAM dendrimer. This approach to nanofiltration will readily interface to upstream microreactors.     

Wetting and electrical properties of the human hair surface: delipidation observed at the nanoscale

The electrostatic properties and the wetting behaviour of the human hair surface at the nanometric scale have been investigated by using atomic force microscopy (AFM). Surface potential imaging was used to determine the electrostatic properties while non-contact mode AFM was used to investigate the wetting properties of a test liquid, squalane. We have studied natural hair and hair in which different covalently (18-methyleicosanoic acid) and non-covalently bound fatty acids present at the cuticle surface were selectively extracted. This study shows how the removal of these acids causes various profound changes in hair wettability at the cuticle scale.     

Reactivity study of 1H-thieno[3,2-d][1,3]oxazine-2,4-dione toward the synthesis of bicyclic 3,4-dihydro-1H-thieno[3,2-e][1,4]diazepine-2,5-dione analogues

A series of 10 optically pure 3,4-dihydro-1H-thieno[3,2-e][1,4]diazepine-2,5-dione derivatives has been synthesized in 41-75% yields on treatment of 1H-thieno[3,2-d][1,3]oxazine-2,4-dione with different natural alpha-amino acids.