DNA binding studies of cationic lanthanide complexes bearing a phenanthridinium group

The binding of tetracationic Lambda- and Delta -europium complexes of tetra amide and triamide ligands incorporating an N-methylphenanthridinium group to calf-thymus DNA, poly(dGdC) and super-coiled plasmid DNA has been monito red by absorption, luminescence and CD difference spectroscopy. With the te traamide complexes, preferential binding to (CG) base-pairs was signalled b y pronounced hypochromism, a small red shift and substantial fluorescence q uenching with a limiting base-pair complex ratio of 2:1, consistent with an intercalative interaction and charge neutralisation. For the triamide Eu c omplex; a lower base-pair : complex ratio was found and changes in the form and polarisation of the Eu emission spectrum were consistent with coordina tion of a nucleobase phosphate group to the hydrated Eu centre.