How amino acids control the binding of Cu(II) ions to DNA

l -Histidine Cu(II) complex bound to DNA showed broad EPR signals characteristic of the aggregated Cu(II) species, which could be observed even when the molar ratio of l -histidine to Cu(II) ion was smaller than unity. The signal for the DNA fibers changed with the orientation of the fibers in the static magnetic field. Based on these results, the signal was assigned to a mono-histidine Cu(II) complex stereospecifically aggregated in a groove or along a phosphodiester chain of the double helical DNA. In contrast to the l -histidine complex, the d -histidine complex bound to DNA did not show such broad signals and the observed spectra for the complex on B-form DNA fibers at −150 °C were simulated assuming that the g ‖ axis of the mono- d -histidine complex tilts by about 55° from the DNA-fiber axis. Addition of some deoxy-nucleotides, but not deoxy-nucleosides, to a solution of a mono-histidine complex resulted in the formation of a dinuclear ternary complex with different structures for l - or d -histidine, suggesting the possibility that the stereospecific aggregation of the l -histidine complex on a double helical DNA was mediated by the phosphodiester backbones.