Solution structure and dynamics of a complex between DNA and the antitumor bisnaphthalimide LU-79553: Intercalated ring flipping on the millisecond time scale

Using a combination of nuclear magnetic resonance (NMR) spectroscopy experiments and molecular dynamics, we have analyzed the structure and dynamics of a complex between the bisnaphthalimide drug LU-79553 and the DNA duplex d(ATGCAT)(2), LU-79553 is a DNA-binding topoisomerase II inhibitor that is particularly effective against human solid tumors that are refractory to other drugs. We have found that the two naphthalimide chromophores of the drug bisintercalate at the TpG and CpA steps of the DNA hexanucleotide, stacking mainly with the purine G and A bases from opposite strands, The 3,7-diazanonylene linker lies in the major groove of the DNA molecule, with its two amino groups hydrogen-bonded to the symmetry-related guanine bases. Unexpectedly, we have detected an unprecedented exchange process between two equivalent and intercalated states of the naphthalimide rings in the drug-DNA complex. The interconversion process takes place by rotational ring flipping, has an activation energy of 22 kcal mol(-1) for the two rings, and does not affect the aminoalkyl linker region of the drug. The exchange rate is intermediate to fast on the chemical shift time scale at 36 degrees C (1800 s(-1)) but slow at 2 degrees C (20 s(-1)). We have also observed limited flexibility for the drug linker on the picosecond time scale on the basis of NMR data and a time-averaged restrained molecular dynamics simulation. The implications of the structural and dynamic features of the DNA-LU-79553 complex on the binding specificity and on the antitumor activity of bisnaphthalimide agents are discussed.