DNA binding studies of cationic lanthanide complexes bearing a phenanthridinium group

The binding of tetracationic Lambda- and Delta -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 2:1, consistent with an intercalative interaction and charge neutralisation. For the triamide Eu complex; a lower base-pair : 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.