DNA condensations on mica surfaces induced collaboratively by alcohol and hexammine cobalt

We performed systematic studies of λ-DNA condensation on mica surfaces induced by alcohol and hexammine cobalt (III) [Co(NH3)63+] using atomic force microscopy (AFM). The critical condensation concentration for [Co(NH3)63+] was found to be about 10 μM; the DNA molecules extended freely on mica when the concentration was below the critical value. The morphology of condensed DNA became more compact with increasing concentration. At about 500 μM [Co(NH3)63+] concentration, no condensation patterns could be observed due to charge inversion of the compact structures resulting in failure of adhesion to the positively charged surfaces. The critical concentration for alcohol was about 15% (v/v). At this concentration, a few intramolecular loops could be observed in the AFM images. With increasing ethanol concentration the condensation pattern became more complicated ranging from flower-like to pancake-like. When the solution contained both alcohol and hexammine cobalt (III), DNA condensation patterns could be observed even when the concentrations of the two condensation agents were lower than their critical values. We observed this phenomenon by adding mixtures of 10% alcohol and 8 μM hexammine cobalt (III) to DNA solutions. The condensation patterns were more compact than those of the condensation agents separately. Typical toroids were found at an appropriate alcohol and hexammine cobalt (III) concentration. The collaborative condensation phenomenon was analyzed by electrostatic interaction and charge neutralization.