DNA-BINDING TO MICA CORRELATES WITH CATIONIC RADIUS - ASSAY BY ATOMIC-FORCE MICROSCOPY

In buffers containing selected transition metal salts, DNA binds to mi ca tightly enough to be directly imaged in the buffer in the atomic fo rce microscope (AFM, also known as scanning force microscope). The bin ding of DNA to mica, as measured by AFM-imaging, is correlated with th e radius of the transition metal cation. The transition metal cations that effectively bind DNA to mica are Ni(II), Co(II), and Zn(II), whic h have ionic radii from 0.69 to 0.74 Angstrom. In Mn(II), ionic radius 0.82 Angstrom, DNA binds weakly to mica. In Cd(II) and Hg(II), respec tive ionic radii of 0.97 and 1.1 Angstrom, DNA does not bind to mica w ell enough to be imaged with the AFM. These results may to relate to h ow large a cation can fit into the cavities above the recessed hydroxy l groups in the mica lattice, although hypotheses based on hydrated io nic radii cannot be ruled out. The dependence of DNA binding on the co ncentrations of the cations Ni(II), Co(II), or Zn(II) shows maximal DN A binding at similar to 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 ion ic radius similar to that of Ni(II). Ni(II), Co(II), and Zn(II) have a nomalously high enthalpies of hydration that may relate to their abili ty to bind DNA to mica. This AFM assay for DNA binding to mica has pot ential applications for assaying the binding of other polymers to mica and other flat surfaces.