Interaction of a platinum terpyridine complex with DNA and anion binding to the trinuclear cluster of Rhus laccase

We present a spectroscopic investigation of interactions of Pt(terpy)OH$\sp+$, where terpy denotes 2,2$\sp\prime$:6,2$\sp{\prime\prime}$-terpyridine, with DNA. Absorption spectroscopy and viscometry indicate that, immediately after mixing, Pt(terpy)OH$\sp+$ intercalates into DNA, preferentially between GC base pairs. The luminescent properties of the complex are greatly affected by the base sequence of the polynucleotide. Intercalation between AT base pairs increases the luminescence, while electron transfer from the guanine residue quenches the luminescence when the complex intercalates between GC base pairs. At room temperature, Pt(terpy)OH$\sp+$ exhibits absorption maxima in the near-UV and visible regions with extinction coefficients on the order of 1,000 M$\sp{-1}$. Since these transitions exhibit solvent dependence and occur well below those of the $\sp1\pi \to \pi\sp\*$ transitions of the terpy ligand, we assign them as metal-to-ligand charge transfer (CT) transitions. At room temperature, the complex exhibits a broad, unstructured emission. The uncorrected emission maxima (and lifetimes) are 580 nm (10 ns) in water and 610 nm (117 ns) in acetonitrile. Temperature dependent luminescence in acetonitrile shows that there is a thermally activated quenching route with a barrier height of ca 1,170 cm$\sp{-1}$. The broad emission signal observed contrasts with the structured high energy emission obtained from metal-perturbed-intraligand states of Pt(terpy)CN$\sp+$ and Pt(terpy)(xylylCN)$\sp{2+}$. We also describe electron paramagnetic resonance (EPR) studies of the binding of cyanide to the derivative of laccase in which the type 1 site contains the EPR silent Hg(II) ion (T1Hg laccase). T1Hg laccase forms a dicyanide adduct that exhibits ligand hyperfine structure from two protein nitrogen and two cyanide carbon donors. We also observe a pH-induced transition in the type 2/type 3 cluster occurring at pH 5.5. On the basis of our results and the newly reported crystal structure of the diazide adduct of ascorbate oxidase (Messerschmidt et al., 1993), we conclude that azide and cyanide anions bind at one of the type 3 copper atoms in laccase. These results illustrate the fluctionality of the trinuclear cluster of laccase.