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The interaction of [Ru(phen) 2 dppz-idzo] 2+ (phen = 1,10-phenanthroline, dppz-idzo = dppz-imidazolone) with triplex RNA poly(U)·poly(A)*poly(U) was carried out by using spectroscopic and viscometric techniques in this work. Luminescent titrations suggest that [Ru(phen) 2 dppz-idzo] 2+ shows better selectivity for poly(U)·poly(A)*poly(U) compared with poly(U)·poly(A) and poly(U), this complex exhibits a "light switch" effect with an emission enhancement factor of about 123 in the presence of poly(U)·poly(A)*poly(U). Significantly, this "light switch" behavior could even be observed by the naked eye under irradiation with UV light. To our knowledge, [Ru(bpy) 2 dppz-idzo] 2+ is the first small molecule able to serve as a colorimetric molecular "light switch" for the triplex poly(U)·poly(A)*poly(U). Combined with the spectral and viscometric results as well as [Ru(phen) 2 dppz-idzo] 2+ stabilizing the template duplex poly(U)·poly(A), we speculate that [Ru(phen) 2 dppz-idzo] 2+ prefers to bind with the Hoogsteen base-paired strand (the third strand) of the triplex, thus the intercalating [Ru(phen) 2 dppz-idzo] 2+ stabilizing the third strand is more marked in comparison with the Watson-Crick base-paired duplex of the triplex. The results obtained here may be useful for understanding the interaction of triplex RNA poly(U)·poly(A)*poly(U) with small molecule, particularly ruthenium(II) complexes., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Graphical abstract Two Ru(II) complexes [Ru(phen) 2 dicnq]2+ and [Ru(bpy) 2 dicnq]2+ have been synthesized and characterized. Binding properties of the two complexes with poly(U)·poly(A)*poly(U) triplex have been studied. Results indicate that the two stabilize the triplex by the stacking interaction, while they prefer to bind with the Watson-Crick base-paired duplex instead of the Hoogsteen base-paired strand of the triplex under the same conditions. Therefore, the two complexes stabilize the Watson-Crick base-paired duplex of the triplex almost without affecting the stability of third-strand. Abstract Using phen or bpy (phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine) as ancillary ligands and dicnq (dicnq = 6,7-dicyanodipyrido[2,2- d :2′,3′- f ]quinoxaline) as the main ligand, two new Ru(II) polypyridyl complexes containing cyano groups, [Ru(phen) 2 dicnq]2+ (Ru 1) and [Ru(bpy) 2 dicnq]2+ (Ru 2) have been synthesized as binders for poly(U)·poly(A)*poly(U) triplex. The binding properties of Ru 1 and Ru 2 with the triplex has been studied by spectroscopic methods and viscosity measurements. Analysis of the electronic absorption spectra suggests that the binding of intercalating Ru 1 with the triplex is greater than that of Ru 2 , which is also supported by spectroscopic titrations and viscosity measurements. Thermal denaturation studies suggest that the two complexes stabilize the Watson-Crick base-paired duplex of the triplex structure almost without affecting the stability of the Hoogsteen base-paired third strand, suggesting that the electron-withdrawing effect of cyano group hardly increases the binding strengths of the third strand and the template duplex of the triplex under study here. Changes in the CD (CD = circular dichroism) spectra of the triplex indicate that the binding-induced CD perturbation of the triplex structure is more marked with Ru 1. The main results obtained here indicate that small differences in the ancillary ligands play important roles in the triplex binding when ruthenium(II) complexes contain the same intercalative ligands. [ABSTRACT FROM AUTHOR]