Inorganica Chimica Acta, 21 (1978) L87-L88 @Elsevier Sequoia S.A., Lausanne - Printed in Switzerland Binding of Pentaammineruthenium(II1) to Double- Helical and Single-Stranded DNA MICHAEL .I. CLARKE and MARK BUCHBINDER Department of Chemistry, Boston College, Chestnut Hill, MA 02167, U.S.A. ASHER D. KELMAN Department of Microbiology, Boston University M edical School, Boston, MA 02118, U.S.A. Received January 24, 1978 Recent research into the interaction of square planar Pt(I1) antitumor complexes with DNA has indicated that these ions interact preferentially with N-7 guanine sites, which are available in the major grove of DNA [ 1, 21. Analogous studies with octa- hedral metal ions exhibiting in vitro biochemical behavior similar to the platinum agents has not been forthcoming. In this communication we report on the interaction of an octahedral metal ion, (NH&- RuOHp, with helical and denatured DNA. The aquo ion was prepared by standard methods [3] from (NH3)sRuC12+, which has been shown to exhibit biochemical properties similar to cis-C12- (NH3)2Pt in vitro, namely the inhibition of cellular DNA synthesis [4]. In the case of the platinum com- plex this activity is thought to be initiated by metal coordination to cellular nucleic acids [5]. Direct DNA binding is also suspected as the initial step in DNA synthesis inhibition by the ruthenium species and DNA extracted from cells incubated in the presence of [Cl(Nl~a)sR~‘~~]~+ has been shown to contain radioactivity [7]. Investigations concerning the direct interaction of (NHs)sRuC12+ with DNA constituent bases indicate that complexes are formed slowly over a period of days. Reduction of the ion in solution either wholly or in part allows the complexes to form in a matter of minutes [8, 111. The effect on DNA synthesis inhibi- tion is therefore suspected to occur via a redox mechanism in which a cellular component reduces the Ru(III) to Ru(I1) prior to binding. We have shown the feasibility of this mechanism in studies using SUC- cinate as a reductant in the presence of rat-liver mito- chondria, (NH3)sR~C12+ and DNA. In the absence of the mitochondrial redox catalyst, no binding to DNA is observed. In its presence, spectra similar to those reported here for [(NHs)sRu] -DNA were observed. The amount of metal-DNA binding was a function of the oxygen content of the solutions. When completely anaerobic conditions were employed, the intensity of the Ru-DNA absorptions was approximately an order of magnitude greater than when air was bubbled L87 through the solutions during the course of the reac- tions [4,9]. In the present study the aquo ion was prepared in 0.0 1 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM F phosphate buffer and syringed into DNA solu- tions deaerated with argon. The reactions were allowed to proceed for one hour and produced light yellow solutions. Oxidation by O2 yielded blue to purple solutions. The visible spectra of solutions pre- pared following this procedure with coiled and heat- denatured DNA are shown in Figures 1 and 2 respec- tively. The absorption maxima in Figure 1 around I I I I , 400 600 1 A(nml Fig. 1. Visible spectrum of (NHs)sRu(III)n-DNA in phos- phate buffer. Guanine concentration is 1.4 X 105. Ru/G ratios are 0.24, 0.48, 0.72, 0.96 and 1.31 with the higher ratios yielding the curves of higher absorbance. 400 600 k(lVll) Fig. 2. Visible spectrum of heat-denatured (NHs)sRu(III),- DNA in phosphate buffer. Guanine concentration is 1.4 X 10v3. Ru/G ratios are 0.13, 0.28, 0.42, 0.59 and 0.87. 550 nm nearly coincides with that obtained for the 7-[(5’GMP)(NH3)sRu(III)] ion [ 10, 111. This absorption maximum shifts slightly to higher energy on increasing the Ru/G ratios indicating a small amount of binding to sites other than guanine at the higher ratios. The spectra shown in Figure 2 clearly indicate binding to additional sites in the case of denatured DNA. Acid hydrolysis of these solutions by warming to 85 ‘C at pH l-2 for one hour followed by ion-