The effect of metal ions on the binding of acridine orange to tobacco mosaic virus ribonucleic acid

TMV-RNA has been shown to form two types of complexes with acridine orange depending on whether the ratio of RNA-P: acridine orange is close to 1:1 (Complex I) or 20:1 (Complex II). Cr³⁺ and Cu²⁺ completely suppressed the formation of Complex I at levels around 5·10⁻⁴ M. Partial inhibition was observed with Ni²⁺, Co²⁺, Zn²⁺ and Mg²⁺ at 2.5·10⁻³–10·10⁻³ M, and with Hg⁺² in the range 2.5·10⁻⁴–10·10⁻⁴ M. The inhibitory effectiveness of these metal ions followed the order Cr³⁺, Cu²⁺ > Ni²⁺, Co²⁺ > Zn²⁺ > Mg²⁺ > Hg²⁺. On the other hand, the reaction between TMV-RNA-P and acridine orange to form Complex II was modified by 1·10⁻⁵–10·10⁻⁵ M Cr³⁺, Cu²⁺, Ni²⁺, Mg²⁺ and Hg²⁺ in a different manner indicative of the formation of a new species with altered absorption and fluorescence characteristics. The new species, Complex III, consists of a RNA-acridine orange-metal ternary complex.