Detection of heavy metal ions using protein-functionalized microcantilever sensors

Microcantilevers functionalized with metal-binding protein, AgNt84-6, are demonstrated to be sensors for the detection of heavy metal ions like Hg(2+) and Zn(2+). AgNt84-6, a protein that has the ability to bind multiple atoms of Ni(2+), Zn(2+), Co(2+), Cu(2+), Cd(2+) and Hg(2+) was attached to the gold-coated side of silicon nitride cantilevers via linker groups. Upon exposure to 0.1 mM HgCl(2) and 0.1 mM ZnCl(2) solutions, the microcantilevers underwent bending corresponding to an expanding gold side. Exposure to a 0.1 mM solution of MnCl(2) solution did not result in a similar bending indicating a weak or no interaction of Mn(2+) ions with the AgNt84-6 protein. The microcantilever bending data were consistent with data from electrophoresis carried out on SDS-PAGE gels containing metal ions that showed protein interaction with Zn(2+) ions but not with Mn(2+) ions. Thus, we demonstrate that microcantilever bending can be used to discriminate between metal ions that bind and do not bind to AgNt84-6 protein in real time.     

Preliminary morphological and X-ray diffraction studies of the crystals of the DNA cetyltrimethylammonium salt.

Double-stranded DNA molecules (molecular weight 2.5 X 10(5) - 5 X 10(5) daltons) have been crystallized from water-salt solutions as cetyltrimethylammonium salts (CTA-DNA). Variation of crystallization conditions results in a production of different types of CTA-DNA crystals: spherulits, dendrites, needle-shaped and faceted rhombic crystals, the latter beeing up to 0.3 mm on a side. X-ray diffraction data indicate that DNA molecules in the crystals form a hexagonal lattice which parameters vary slightly with the morphological type of the crystal. Comparison of the melting curves of the DNA preparation before and after crystallization suggests that DNA molecules are partially fractionated in the course of crystallization. Crystals of the CTA-DNA-proflavine complex have also been obtained.     

Fibre X-ray and conformational study of the binding of metal ions on DNA

Results obtained from X-ray diffraction as well as from conformational analysis of Ag-DNA fibres are presented. For small percentages of Ag+ bound and high humidity, the B-DNA form is maintained. As the percentage of Ag+ is increased, the helical parameters of the B-DNA are modified. These modifications are directly related to the percentage of G-C bases. The periodicity of the DNA fibres are perturbed as Ag+ is mainly bound to G-C pairs and, thus, only the equatorial diffracted intensities can be compared to values calculated from molecular models. It is shown, by this way, that the first binding site is located on N7 of G. A second site is situated between N3 and N1 of the G-C pair, at the place of a hydrogen bond. A molecular model of the Ag-DNA complex is proposed and shown to be in agreement with experimental data. Results obtained allow to get some information on the binding of other ions such as Cu2+ and Hg2+ which give very little modification of the fibre X-ray patterns.     

X-ray diffraction studies on cation-collapsed DNA

The polyamines spermidine, spermine and putrescine are now known to induce tertiary collapse of DNA. In this collapsed state DNA assumes a compact toroidal conformation. However, the structural details of DNA in these compact particles and the forces that stabilize the collapsed state are not clear. We show here that the structural arrangement of DNA in this tertiary conformation is determined by the chemical structure of the agent used to collapse. We have used aliphatic triamines (NH₃⁺-(CH₂)₃-NH₂⁺-(CH₂)_n-NH₃⁺ with n = 3, 4, 5 and 8) and diamines (NH₃⁺-(CH₂)_x-NH₃⁺ with x = 2, 3, 4 and 6) to collapse DNA. We find that the Bragg spacing and the calculated interhelical spacing for a hexagonal packing model vary systematically with the length of the methylene bridge. We also find that the ionic strength of the solution has no effect on the Bragg spacing. This observation suggests that the arrangement of DNA strands in the complexes is determined by the structure of the polycation, and argues against suggestions that the structure of the collapsed state is maintained by the balance of long-range electrostatic repulsive and attractive forces. Instead we propose that DNA helices form a hexagonal array with counterions in the interstices between the helices resulting in a stable three-dimensional phase with high structural order. Arguments are presented favoring such a model in terms of stabilizing and destabilizing thermodynamic forces.     

X-ray diffraction analysis of crystals of bovine platelet factor 4

Bovine platelet factor 4 has been crystallized by “vapor dilution” in space group P2₁2₁2₁, $\text{a = 63.7}\text{A}\text{?}\text{, b = 66.7}\text{A}\text{?}\text{, c = 80.5}\text{A}\text{?}$, with four molecules, each 9505 M_r, in the asymmetric unit. The crystals diffract X-rays to better than 2.8 Å resolution.     

Electron diffraction from single crystals of DNA.

Crystals of DNA have been grown in a form suitable for study by electron diffraction and electron microscopy. Preliminary electron diffraction patterns of any type that have been obtained from single crystals of highly polymerized DNA. The patterns, obtained from frozen, hydrated crystals with the beam approximately parallel to the DNA strand axis, show a hexagonal geometrical arrangement with a (1,0) Bragg spacing of 23.1 A.     

Selectivity and permeation of alkali metal ions in K+-channels

Ion conduction in K⁺-channels is usually described in terms of concerted movements of K⁺ progressing in a single file through a narrow pore. Permeation is driven by an incoming ion knocking on those ions already inside the protein. A fine-tuned balance between high-affinity binding and electrostatic repulsive forces between permeant ions is needed to achieve efficient conduction. While K⁺-channels are known to be highly selective for K⁺ over Na⁺, some K⁺ channels conduct Na⁺ in the absence of K⁺. Other ions are known to permeate K⁺-channels with a more moderate preference and unusual conduction features. We describe an extensive computational study on ion conduction in K⁺-channels rendering free energy profiles for the translocation of three different alkali ions and some of their mixtures. The free energy maps for Rb⁺ translocation show at atomic level why experimental Rb⁺ conductance is slightly lower than that of K⁺. In contrast to K⁺ or Rb⁺, external Na⁺ block K⁺ currents, and the sites where Na⁺ transport is hindered are characterized. Translocation of K⁺/Na⁺ mixtures is energetically unfavorable owing to the absence of equally spaced ion-binding sites for Na⁺, excluding Na⁺ from a channel already loaded with K⁺.     

X-ray diffraction from DNA fibres under tension

When DNA fibres are stretched during drying, the polymer undergoes a conformational transition. We present quantitative results from X-ray diffraction studies on such fibres held at various ambient relative humidities. These indicate that the molecules are arranged in arrays which are crystalline in projection down the fibre axis. The packing can be explained in terms of a hexagonal cell with a lattice parameter, a, of approximately 13 A which varies with humidity. The patterns contain meridional intensities at 1/3.4 A(-1) and 1/6.5 A(-1), a strong off-meridional intensity at Z=1/5.6 A(-1) and diffuse scatter at Z=1/28 A(-1).     

A re-evaluation of X-ray diffraction patterns of DNA

Concentric circular lines are present on X-ray diffraction patterns of DNA. They cannot be accounted for by the double helix model. It is suggested that they may indicate the presence of double helical side chains.     

X-ray diffraction studies on metal deposition in group D streptococci

Tucker, Fayne L. (University of Southern California, Los Angeles), John W. Thomas, Milo D. Appleman, Stewart H. Goodman, and Jerry Donohue. X-ray diffraction studies on metal deposition in group D streptococci. J. Bacteriol. 92:1311-1314. 1966.-Streptococcus faecalis N83 and S. faecium K6A reduced several compounds of Group VI elements to the elemental form, but reduced none of several compounds tested containing elements of other groups. The elemental tellurium deposited by S. faecium K6A was in general of a larger particle size than that deposited by S. faecalis N83 as judged from X-ray diffraction analysis. The particle size of the deposited tellurium was correlated with the blackness of the precipitate produced by cells growing in the presence of tellurite. A black and gray variation was observed in S. faecium K6A which was considered to be due to particle size, the amount of tellurium present, and the location of the deposited tellurium. The gray color of S. faecium K6A was not due to the presence of any oxidized tellurium products.