The interaction of natural calf thymus DNA with Cr(3+) ions was studied at room temperature by means of vibrational CD (VCD) and infrared absorption (ir) spectroscopy, and atomic force microscopy (AFM). Cr(3+) ion binding mainly to N(7) (G) and to phosphate groups was 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 Cr(3+) ions and the formation of tight DNA particles responsible for the psi-type VCD spectra. Upon increasing the Cr(3+) 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 Cr(3+) ions studied. The secondary structure of DNA remained in 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.

Interaction of natural calf thymus DNA with Mn(2+) ions was studied at room temperature and at elevated temperatures in the range from 23 degrees C to 94 degrees C by means of IR absorption and vibrational circular dichroism (VCD) spectroscopy. The Mn(2+) concentration was varied between 0 and 1.3M (0 and 10 [Mn]/[P]). The secondary structure of DNA remained in the frame of the B-form family in the whole ion concentration range at room temperature. No significant DNA denaturation was revealed at room temperature even at the highest concentration of metal ions studied. However at elevated temperatures, DNA denaturation and a significant decrease of the melting temperature of DNA connected with a decrease of the stability of DNA induced by Mn(2+) ions occurred. VCD demonstrated sensitivity to DNA condensation and aggregation as well as an ability to distinguish between these two processes. No condensation or aggregation of DNA was observed at room temperature at any of the metal ion concentrations studied. DNA condensation was revealed in a very narrow range of experimental conditions at around 2.4 [Mn]/[P] and about 55 degrees C. DNA aggregation was observed in the presence of Mn(2+) ions at elevated temperatures during or after denaturation. VCD spectroscopy turned out to be useful for studying DNA condensation and aggregation due to its ability to distinguish between these two processes, and for providing information about DNA secondary structure in a condensed or aggregated state.

The intrastrand cisplatin adduct of the duplex d(CCTG*G*TCC)‚d (GGACCAGG), where the asterisks denote the 1,2-intrastrand crosslink, was synthesized and studied in solution in D 2 O by vibrational circular dichroism (VCD) and infrared absorption spectroscopy. Comparison of the spectra of the platinated and unmodified complexes confirmed that the entire structure is considerably distorted, as a result of the platinum coordination to the octamer duplex. The major changes in the VCD spectra were detected in the fourth base step, where the cisplatin cross-links the adjacent guanine bases G4 and G5. Platinum coordination shifts the G4-C13 and G5-C12 base pairs apart, with a concomitant disruption of the stacking between the neighboring bases T3-G4 and G5-T6. The resultant new VCD couplets were assigned to stacking interactions between A11 and A14, which implies a substantially distorted structure. Slow isomerization is indicated by diagnostic changes in VCD, converting the intrastrand adduct to other adducts. The corresponding absorption spectra remain essentially unchanged. This DNA octamer was previously studied by NMR in great detail when the isomerization was first detected after one day. Our measurements show that the conversion began soon after both strands were annealed and the first signs were detected in 2 h. This investigation again demonstrated convincingly that VCD is very sensitive to DNA structure modified by complex formation with cisplatin and may become more widely applicable for other similar applications.

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