The structural changes of DNA, induced by the antitumour antibiotic nogalamycin, have been studied by atomic force microscopy (AFM). The transformation in the tertiary structure of 4361 bp long plasmid pBR322 DNA, after incubation with nogalamycin at 37 degrees C, has been monitored at the single molecule level. The AFM topographs of free DNA and the DNA-nogalamycin complex, incubated for 6, 12, 24, 36 and 48 h, reveal a gradual change from the circular supercoiled form having strand crossovers to the more compact plectonemic superhelix. With increasing incubation time, the extent of plectonemic coiling increases, indicating increasing level of drug binding via intercalative mode. Supportive evidences are obtained from the CD and UV-vis spectroscopic Studies. To our knowledge, this is the first report on an AFM imaging study of the effects of nogalamycin, an anthracyclin intercalator, on DNA. (C) 2008 Elsevier Inc. All rights reserved.