The optical anisotropy and spectral properties of protonated double-stranded DNA during its interaction with trans-dichlorodiammineplatinum(II) (trans-DDP) have been studied. No changes in the optical anisotropy of protonated DNA macromolecules were observed during the preparation of protonated DNA complexes with trans-DDP. It is known, however, that the binding of trans-DDP to native DNA in solution at neutral pH increases its optical anisotropy. The spectral properties of the complexes under study correspond to those of protonated DNA. The experimental data show that the site of native DNA protonation also plays an important role in its binding to trans-DDP. In contrast, a decrease in the pH of the solution containing trans-DDP-DNA complexes to the value at which DNA protonation takes place does not lead to changes in the optical anisotropy or absorption spectrum of the macromolecule. These facts indicate that the protonation sites on the macromolecule are blocked by trans-DDP. Copyright (C) 1999 Elsevier Science B.V.