To study the effect of ligand charge on DNA-ligand binding, we measured the difference in volume change of the noncovalent complex formed between calf thymus DNA and the charged and uncharged form of 3,8-diamino-6-phenylphenanthridine (DAPP). We found that the volume change for binding with the charged DAPPH(+) is about 7.8 +/- 1.5 cm(3) mol(-1) more positive than with the neutral DAPP. We hypothesize that this large difference in interaction volume originates from partial desolvation of the charge of DAPPH(+) when it is bound to DNA.