Targeting different RNA motifs by beta carboline alkaloid, harmalol: a comparative photophysical, calorimetric, and molecular docking approach

RNA has attracted recent attention for its key role in gene expression and targeting by small molecules for therapeutic intervention. This work focuses towards understanding interaction of harmalol, a DNA intercalator, with RNAs of different motifs viz. single-stranded A-form poly(A), double-stranded A-form of poly(C)poly(G), and clover leaf tRNA(phe) by different spectroscopic, calorimetric, and molecular modeling techniques. Results of this study converge to suggest that (i) binding constant varied in the order poly(C)poly(G)>tRNA(phe)>poly(A), (ii) non-cooperative binding of harmalol to poly(C)poly(G) and poly(A) and cooperative binding with tRNA(phe), (iii) significant structural changes of poly(C)poly(G) and tRNA(phe) with concomitant induction of optical activity in the bound achiral alkaloid molecules, while with poly(A) no induced Circular dichroism (CD) perturbation was observed, (iv) the binding was predominantly exothermic, enthalpy-driven, entropy-favored with poly(C)poly(G), while it was entropy driven with tRNA(phe) and poly(A), (v) a hydrophobic contribution and comparatively large role of non polyelectrolytic forces to Gibbs energy changes with poly(C)poly(G) and tRNA(phe) and (vi) intercalated state of harmalol inside poly(C)poly(G) structure as revealed from molecular docking was supported by the viscometric and ferrocyanide quenching data. All these findings unequivocally pointed out that harmalol prefers binding with poly(C)poly(G), compared to tRNA(phe) and poly(A); this results serve as data for the development of RNA-based antiviral drugs.