Small molecule-RNA interaction: Spectroscopic and calorimetric studies on the binding by the cytotoxic protoberberine alkaloid coralyne to single stranded polyribonucleotides

Background: RNA has attracted recent attention for its key role in gene expression and hence targeting by small molecules for therapeutic intervention. This study is aimed to elucidate the specificity of the alkaloid coralyne to poly(G), poly(C), poly(l) and poly(U) in the light of its ability in inducing self-structure in poly(A). Methods: Multifaceted experimental techniques like competition dialysis, absorption, fluorescence, circular dichroism and calorimetry were employed. Salt dependence and temperature dependence of the binding was also elucidated. Results: Results of competition dialysis, absorption and fluorescence studies revealed that coralyne binds strongly to the polypurines, poly(G) and poly(I) compared to the polypyrimidines, poly(U) and poly(C). Partial intercalative binding due to the stacking of the molecules between the bases was envisaged. The binding was predominantly enthalpy driven with favourable entropy term with a large favourable non-electrostatic contribution revealed from salt dependent data and the dissection of the free energy. The heat capacity change of -125 and -119 cal/mol K-1 respectively for poly(G) and poly(I) and the partial enthalpy-entropy compensation phenomenon observed confirmed the involvement of multiple weak noncovalent interactions. Circular dichroism studies provided evidence for significant perturbation of the conformation of the RNAs, but no self-structure induction was evident in any of the polymers under the condition of the study. Conclusions: This study presents a complete structural and thermodynamic profile of coralyne interaction to four single stranded RNA polymers. General significance: The study for the first time elucidates the base specificity of coralyne-RNA complexation at the single stranded level. (C) 2009 Elsevier B.V. All rights reserved.