Adsorption of DNA molecules on mica, a highly negatively charged surface, mediated by divalent or trivalent cations is considered. By analyzing atomic force microscope (AFM) images of DNA molecules adsorbed on mica, phase diagrams of DNA molecules interacting with a mica surface are established in terms of concentrations of monovalent salt (NaCl) and divalent (MgCl2) or multivalent (spermidine, cobalt hexamine) salts. These diagrams show two transitions between nonadsorption and adsorption. The first one arises when the concentration of multivalent counterions is larger than a limit value, which is not sensitive to the monovalent salt concentration. The second transition is due to the binding competition between monovalent and multivalent counterions. In addition, we develop a model of polyelectrolyte adsorption on like-charged surfaces with multivalent counterions. This model shows that the correlations of the multivalent counterions at the interface between DNA and mica play a critical role. Furthermore, it appears that DNA adsorption takes place when the energy gain in counterion correlations overcomes an energy barrier. This barrier is induced by the entropy loss in confining DNA in a thin adsorbed layer, the entropy loss in the interpenetration of the clouds of mica and DNA counterions, and the electrostatic repulsion between DNA and mica. The analysis of the experimental results provides an estimation of this energy barrier. We then discuss some important issues, including DNA adsorption under physiological conditions.

Phosphatidylinositol (PI)1 and its phosphorylated derivatives regulate many biological processes, including cell proliferation, cell survival, differentiation, signal transduction, cytoskeleton organization, and membrane trafficking (reviewed in Ref. 1). Various chemical species can be generated by single, double, or triple phosphorylations at the inositol hydroxy groups at positions 3, 4, and 5. Their synthesis and cellular concentrations are regulated by specific lipid kinases and phosphatases. One of the major mechanisms by which PIs regulate cellular processes is by their capacity to serve as membrane signals to affect intracellular localizations of effector proteins.