EPR based distance measurement in Cu-porphyrin-DNA

EPR spectroscopy was used to investigate both single and double stranded DNA modified with a variable number of copper(II) porphyrins. The spectra of the porphyrin-DNA complexes resemble those of the Cu(II) porphyrin building blocks, but with appreciable differences in the values for the g(II) and A(II) parameters. In addition, a significant half-field signal is observed, which is interpreted as resulting from copper-copper interactions in both the double strand (dsDNA) and the single strand DNA (ssDNA). Analysis of the EPR spectra gives evidence for cluster formation of three or more DNA strands. From the intensity ratio of the half-field and main transition, the average Cu-Cu distance is estimated to be 6.5-8.9 angstrom. The association of copper centres is consistent with hydrophobic porphyrin stacking, both intra-and intermolecularly, as has previously been observed with other DNA complexes using UV-vis and CD spectroscopy.