Die Bestimmung von Spuren von Quecksilber

Ohne Zusammenfassung     

Single-Ion Contributions to Activity Coefficient Derivatives, Second Moment Coefficients, and the Liquid Junction Potential

We discuss several interrelated single-ion thermodynamic properties required to calculate the liquid junction potential between two solutions of the same binary electrolyte. According to a previously reported molecular theory of nonuniform electrolyte solutions in nonequilibrium, is determined by the transport numbers of the ions, and by the second moment coefficients H ⁽²⁾ of the charge densities around the ions. The latter may be viewed as the single-ion contributors to the second moment condition of Stillinger and Lovett. For a solution of a single binary electrolyte, we relate the H ⁽²⁾ (R) to the derivatives of the single-ion activity coefficients with respect to the ionic strength. In the light of these results, we examine, in some detail, the role played by the specific short-range interionic interactions in determining . We investigate this matter by means of integral equation calculations for realistic models of LiCl and NaCl aqueous solutions in the 0–1 mol-dm^–3 range. In addition to the hypernetted-chain (HNC) relation, we perform calculations under a new integral equation closure that is a hybrid between the HNC and Percus–Yevick closures. Like the HNC approximation, the new closure satisfies the Stillinger and Lovett condition. However, for the models considered in this study, the two closures predict different dependence of the H ⁽²⁾ and of on the specific part of the interionic interactions.     

Energy-based reconstruction of a protein backbone from its alpha-carbon trace by a Monte-Carlo method

An automatic procedure is proposed for reconstruction of a protein backbone from its C(alpha)-trace; it is based on optimization of a simplified energy function of a peptide backbone, given its alpha-carbon trace. The energy is expressed as a sum of the energies of interaction between backbone peptide groups that are not neighbors in the sequence, the energies of local interactions within all amino acid residues, and a harmonic penalty function accounting for the conservation of standard bond angles. The energy of peptide group interactions is calculated using the assumption that each peptide group acts as a point dipole. For local interaction energy, use is made of a two-dimensional Fourier series expansion of the energies of model terminally blocked amino acid residues, calculated with the Empirical Conformational Energy Program for Peptides (ECEPP/3) force field in the angles lambda((1)) and lambda((2)) defining the rotation of peptide groups adjacent to a C(alpha) carbon atom about the corresponding C(alpha) em leader C(alpha) virtual-bond axes. To explore all possible rotations of peptide groups within a fixed C(alpha)-trace, a Monte Carlo search is carried out. The initial lambda angles are calculated by aligning the dipoles of the peptide groups that are close in space, subject to the condition of favorable local interactions. After the Monte Carlo search is accomplished with the simplified energy function, the energy of the structure is minimized with the ECEPP/3 force field, with imposition of distance constraints corresponding to the initial C(alpha)-trace geometry. The procedure was tested on model alpha-helices and beta-sheets, as well as on the crystal structure of the immunoglobulin binding protein (PDB code: 1IGD, an alpha/beta protein). In all cases, complete backbone geometry was reconstructed with a root-mean-square (rms) deviation of 0.5 A from the all-atom target structure.     

Electrospray ionization—Fourier transform ion cyclotron resonance mass spectrometry at 11.5 tesla: Instrument design and initial results

Initial results obtained using a new electrospray ionization (ESI) Fourier transform ion cyclotron resonance (FTICR) mass spectrometer operated at a magnetic field 11.5 tesla are presented. The new instrument utilized an electrostatic ion guide between the ESI source and FTICR trap that provided up to 5% overall transmission efficiency for light ions and up to 30% efficiency for heavier biomolecules. The higher magnetic field in combination with an enlarged FTICR ion trap made it possible to substantially improve resolving power and operate in a more robust fashion for large biopolymers compared to lower field instruments. Mass resolution up to 10⁶ has been achieved for intermediate size biopolymers such as bovine ubiquitin (8.6 kDa) and bovine cytochrome c (12.4 kDa) without the use of frequency drift correction methods. A mass resolution of 370,000 has been demonstrated for isotopically resolved molecular ions of bovine serum albumin (66.5 kDa). Comparative measurements were made with the same spectrometer using a lower field 3.5-tesla magnet allowing the performance gains to be more readily quantified. Further improvements in pumping capacity of the vacuum system and efficiency of ion transmission from the source are expected to lead to further substantial sensitivity gains.     

Lattice dynamics with second-neighbor interactions. III. Green's matrix

The theory of pth-order singular differential equations is adaptable to the study of the system of recurrence relations occurring in the problem of a one-dimensional chain with pth-neighbor interactions. By using Green's formula, a mapping is defined between the space V_n of eigenvectors to the dynamical matrix and the symplectic space V_2p of boundary conditions for the recurrence equations. The properties of the resolvent are obtained from an analysis of the solutions of a system of inhomogeneous equations and Green's matrix is constructed for the case of standard Sturm–Liouvilletype boundary conditions. The Weyl surface is discussed and its properties used for the construction of square summable sequences which in turn can be employed in expansion formulas. The generalization of Weyl's m-function in the second-order case (p = 1) becomes for p ≥ 2 a p × p matrix M (λ), where λ is a complex parameter. The imaginary part Im { M (λ)} is related to the spectral properties and serves as basis for the discussion of different concepts of spectral density for the normal modes of lattice dynamical problems. An important practical result is the equation M = ?ΨΦ_a valid in the limit point case, generalizing the corresponding second-order formula.     

Binding at molecule/gold transport interfaces. V. Comparison of different metals and molecular bridges

The geometric and electronic structural properties of symmetric and asymmetric metal cluster-molecule-cluster' complexes have been explored. The metals include Au, Ag, Pd, and Al, and both benzenedithiol and the three isometric forms of dicyanobenzene are included as bridging molecules. Calculated properties such as cluster-molecule interface geometry, electronic state, degree of metal --> molecule charge transfer, metal-molecule mixing in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy region, the HOMO-LUMO gap, cluster --> cluster' charge transfer as a function of external field strength and direction, and the form of the potential profile across such complexes have been examined. Attempts are made to correlate charge transport with the characteristics of the cluster-complex systems. Indications of rectification in complexes that are asymmetric in the molecule, clusters, and molecule-cluster interfaces are discussed. The results obtained here are only suggestive because of the limitations of the cluster-complex model as it relates to charge transport.     

Acoustic micromachining of three-dimensional surfaces for biological applications

We present the use of an accessible micromachining technique (acoustic micromachining) for manufacturing micron-feature surfaces with non-discretely varying depth. Acoustic micromachining allows for non-photolithographic production of metal templates with programmable spatial patterns and involves the use of standard acoustic, cutting and electroplating equipment for mass production of vinyl records. Simple 3D patterns were transferred from an acoustic signal into working nickel templates, from which elastic polymer molds were obtained, featuring deep surface grooves and non-discrete (smooth) variations in the z-dimension. Versatility and applicability of the method is demonstrated in obtaining microfluidics structures, manufacturing high-surface area wavy polymer fibers, assembly of cell networks on scaffolds with 3D topography, and microcontact printing of proteins and cells.     

Simultaneous characterization of the reductive unfolding pathways of RNase B isoforms by top-down mass spectrometry

A novel method for characterization of the simultaneous reductive unfolding pathways of five isoforms of bovine pancreatic ribonuclease B (RNase B) is demonstrated. The results indicate that each isoform unfolds reductively through two three-disulfide-containing structured intermediates before proceeding to the fully reduced form, as in the reductive unfolding pathways of the A variant lacking the carbohydrate chain. The rates of reduction of bovine pancreatic ribonuclease A (RNase A) and RNase B and the formation and consumption of their reductive intermediates are identical, indicating that the unfolding events necessary to expose disulfide bonds for reduction are not affected by the oligosaccharide. The method utilizes top-down mass spectrometry and a naturally occurring tag on the protein, viz. the carbohydrate moiety, to obtain unfolding information of an ensemble of protein isoforms and is a generally applicable methodological advance for conducting folding studies on mixtures of different proteins.     

Development of a novel method to populate native disulfide-bonded intermediates for structural characterization of proteins: implications for the mechanism of oxidative folding of RNase A

RNase A, a model protein for oxidative folding studies, has four native disulfide bonds. The roles of des [40-95] and des [65-72], the two native-like structured three-disulfide-bonded intermediates populated between 8 and 25 degrees C during the oxidative folding of RNase A, are well characterized. Recent work focuses on both the formation of these structured disulfide intermediates from their unstructured precursors and on the subsequent oxidation of the structured species to form the native protein. The major obstacles in this work are the very low concentration of the precursor species and the difficulty of isolating some of the structured intermediates. Here, we demonstrate a novel method that enables the native disulfide-bonded intermediates to be populated and studied regardless of whether they have stable structure and/or are present at low concentrations during the oxidative folding or reductive unfolding process. The application of this method enabled us to populate and, in turn, study the key intermediates with two native disulfide bonds on the oxidative folding pathway of RNase A; it also facilitated the isolation of des [58-110] and des [26-84], the other two native-like structured des species whose isolation had thus far not been possible.