DNA binding studies of cationic lanthanide complexes bearinga phenanthridinium group

The binding of tetracationic Λ‐ and Δ‐europium complexes of tetraamide and triamide ligands incorporating an N‐methylphenanthridinium group to calf‐thymus DNA, poly(dGdC) and super‐coiled plasmid DNA has been monitored by absorption, luminescence and CD difference spectroscopy. With the tetraamide complexes, preferential binding to (CG)base‐pairs was signalled by pronounced hypochromism, a small red shift and substantial fluorescence quenching with a limiting base‐pair complex ratio of 2 14;: p14;1, consistent with an intercalative interaction and charge neutralisation. For the triamide Eu complex, alower base‐pair 14;: 14;complex ratio was found and changes in the form and polarisation of the Eu emission spectrum were consistent with coordination of a nucleobase phosphate group to the hydrated Eu centre.