Atomic force microscopy analysis of intermediates in cobalt hexammine-induced DNA condensation

The packaging pathway of cobalt hexammine-induced DNA condensation on the s urface of mica was examined by varying the concentration of Co(NH3)(6)(3+) in a dilute DNA solution and visualizing the condensates by atomic force mi croscopy (AFM). Images reveal that cobalt hexammine-induced DNA condensatio n on mica involves well-defined structures. At 30 mu M Co(NH3)(6)(3+), prol ate ellipsoid condensates composed of relatively shorter rods with linkages between them are: formed. At 80 mu M Co(NH3)(6)(3+), the condensed feature s include toroids with average diameter of similar to 240 nm as well as U-s haped and rod-like condensates with nodular appearances. The results imply that the condensates, whether toroids, U-shaped or rod-like structures have similar intermediate state which includes relatively shorter rod-like segm ents. The average size: of the condensed toroids after incubated at room te mperature for 5 h (similar to 240 nm) is much larger than that incubated fo r 0.5 h (similar to 100 nm). The results indicate that the condensation of DNA by Co(NH3)(6)(3+) is a kinetic-controlled process.