Vibrational CD (VCD) and atomic force microscopy (AFM) study of DNA interaction with Cr3+ ions:: VCD and AFM evidence of DNA condensation

The interaction of natural calf thymus DNA with Cr3+ ions was studied at room temperature by means of vibrational CD (VCD) and infrared absorption (ir) spectroscopy, and atomic force microscopy (AFM). Cr3+ ion binding mainly. to N-7 (G) and to phosphate groups it-as demonstrated. psi-Type VCD spectra resembling electronic CD (ECD) spectra, which appear during psi-type DNA condensation, were observed. These spectra are characterized mainly by an anomalous, severalfold increase of VCD intensity. Such anomalous VCD spectra were assigned to DNA condensation with formation of large and dense particles of a size comparable to the wavelength of the probing ir beam and possessing large-scale helicity. Atomic force microscopy confirmed DNA condensation by Cr3+ ions and the formation of tight DNA particles responsible for the psi-type VCD spectra. Upon increasing the Cr3+ ion concentration the shape of the condensates changed from loose flower-like, structures to highly packed dense spheres. No DNA denaturation was seen even at the highest concentration of Cr3+ ions studied. The secondary structure of DNA remained it? a B-form before and after the condensation. VCD and ir as well as AFM proved to be an effective combination for investigating DNA condensation. In addition to the ability of VCD to determine DNA condensation, VCD and ir can in the same experiment provide unambiguous information about the secondary, structure of DNA contained in the condensed particles. (C) 2002 Wiley Periodicals, Inc.