DNA binding to mica correlates with cationic radius: assay by atomic force microscopy

Abstract

In buffers containing selected transition metal salts, DNA binds to mica tightly enough to be directly imaged in the buffer in the atomic force microscope (AFM, also known as scanning force microscope). The binding of DNA to mica, as measured by AFM-imaging, is correlated with the radius of the transition metal cation. The transition metal cations that effectively bind DNA to mica are Ni(II), Co(II), and Zn(II), which have ionic radii from 0.69 to 0.74 A. In Mn(II), ionic radius 0.82 A, DNA binds weakly to mica. In Cd(II) and Hg(II), respective ionic radii of 0.97 and 1.1 A, DNA does not bind to mica well enough to be imaged with the AFM. These results may to relate to how large a cation can fit into the cavities above the recessed hydroxyl groups in the mica lattice, although hypotheses based on hydrated ionic radii cannot be ruled out. The dependence of DNA binding on the concentrations of the cations Ni(II), Co(II), or Zn(II) shows maximal DNA binding at approximately 1-mM cation. Mg(II) does not bind DNA tightly enough to mica for AFM imaging. Mg(II) is a Group 2 cation with an ionic radius similar to that of Ni(II). Ni(II), Co(II), and Zn(II) have anomalously high enthalpies of hydration that may relate to their ability to bind DNA to mica. This AFM assay for DNA binding to mica has potential applications for assaying the binding of other polymers to mica and other flat surfaces.

Adsorption of Biological Molecules to a Solid Support for Scanning Probe Microscopy

“…This frequently used method of sample preparation is particularly suitable for AFM in buffer solution Hoh et al, 1993;Mü ller et al, 1995aMü ller et al, , 1996bMü ller et al, , 1997aFritz et al, 1995a,b;Vikholm et al, 1995;Walz et al, 1996;Hansma and Laney, 1996). Muscovite mica is one member of the large layered crystals family that includes micas (Bailey, 1984), highly oriented py- rolitic graphite (HOPG), and transition metal dichalcogenides (Wilson and Yoffe, 1969).…”

“…As also shown by other groups, divalent cations are thus especially well suited for attaching native biological macromolecules to a negatively charged support without further treatment of the sample (Butt et al, 1990Hoh et al, 1993;Hansma and Laney, 1996). Divalent cations have been assumed to act as a bridge between two negatively charged surfaces Hansma and Laney, 1996;Shao et al, 1996), but this model does not explain the attachment of negatively charged surfaces in the presence of monovalent cations. As demonstrated here DLVO forces govern the adsorption of biomolecules to mica.…”

Improved chitosan-mediated gene delivery based on easily dissociated chitosan polyplexes of highly defined chitosan oligomers

“…The samples were then dried with a stream of N 2 gas (P ¼ 1.5 bar), followed by vacuum drying (P ¼ 10 À9 bar) for at least 2 h. For the immobilization of uncomplexed DNA on mica, 0.5 mM ZnCl 2 was added to the solution immediately before transferring the solution to the mica. 46 Tapping mode silicon nitride cantilevers TESP (Digital Instruments), with nominal spring constants of 20-100 N/m and nominal resonant frequencies of 200-400 kHz, were used for the imaging. The drive frequency used was less than the free oscillation resonance frequency of the cantilever, yielding 90% of the amplitude at the resonance frequency.…”

Conformational transition in DNA on a cold surface

“…The precise nature of the DNA-surface interactions is still unclear so we set out to compare three different methods: (i) AP-mica (14); (ii) nickel-treated mica (15); and (iii) mica treated with Mg alone (18) without the use of HEPES (17). We found that the magnesium treatment causes linear DNA to condense on the surface (an effect not observed with small circles; 18).…”

“…Several different approaches to binding DNA for imaging under buffer have been described in the literature (14)(15)(16)(17)(18). The precise nature of the DNA-surface interactions is still unclear so we set out to compare three different methods: (i) AP-mica (14); (ii) nickel-treated mica (15); and (iii) mica treated with Mg alone (18) without the use of HEPES (17).…”