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The interaction between the Sm(III) complex of an ionic Schiff base [HL]^, derived from vanillin and L-tryptophan, and herring sperm DNA at physiological pH (7.40) has been studied by UV-Vis absorption, fluorescence and viscosity methods. The binding ratios <i>n</i>_Sm(III) : <i>n</i>_K[HL] = 1:1 and <i>n</i>_Sm(III)L : <i>n</i>_DNA = 5:1 were confirmed by a mole ratio method. The calculated binding constants of [Sm(III)L]⁺ with DNA at 25 and 37 ^C were 1.19 × 10⁵ M^-1 and 3.96 × 10⁴ M^-1, respectively. The thermodynamic parameters demonstrate that the interaction between [Sm(III)L] ⁺ and DNA is driven mainly by enthalpy. Fluorimetric studies show that [Sm(III)L]+ is able to displace intercalated acridine orange. Combined with Scatchard methods, circular dichroism spectroscopy and viscosity measurements, the results indicate that the interaction between [Sm(III)L]⁺ and herring sperm DNA is mainly by intercalation with some groove binding.