Z → B transition in poly[d(G‐m 5 C) 2 ] induced by interaction with 4′,6‐diamidino‐2‐phenylindole

The Z form of poly[d(G‐m 5 C) 2 ], in presence of Mg 2+ ion, is found to be transformed into B form upon interaction with 4′,6‐diamidino‐2‐phenylindole (DAPI). The Z → B transformation is complete at a mixing ratio of about 0.07 DAPI per DNA base pairs, i.e., each DAPI molecule may be related to the conversion of 6–7 base pairs. An interaction between DAPI and poly[d(G‐m 5 C) 2 ] in its Z form at low drug: DNA ratios is suggested from optical dichroism and time‐resolved luminescence anisotropy results. The spectroscopic behaviour of DAPI indicates that the Z conformation of DNA does not provide normal binding sites for DAPI, such as groove or intercalation sites, but that the initial association may be of external nature. © 1993 John Wiley & Sons, Inc.