Stability decrease of RNA double helices by phenylalanine-, tyrosine- and tryptophane-amides. Analysis in terms of site binding and relation to melting proteins

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Abstract
The amides of L-phenyla1an1ne, L-tyros1ne and L-tryptophane decrease the melting temperatures t m of poly(A)*poly(U) and poly(I)*poly(C) double helices at low concentrations (1 mM), whereas high concentrations finally lead to an Increase of t m . This dependence of the t m -values upon the ligand concentration can be represented quantitatively by a simple site binding model, providing binding parameters for the Interaction between the amides and the nucleic adds both 1n the double- and the single-stranded conformation. According to these data the affinity to the single strands is higher than that to the double strands and increases in the series Phe < Tyr <Trp. The binding constants decrease with increasing salt concentration as expected for an interaction driven by electrostatic attraction. However, part of the interaction 1s also due to stacking between the aromatic amides and the nucleic add bases. The present results indicate a direct correlation between the presence of aromatic amino acids at the binding site of helix destabilising proteins and the properties of simple derivatives of these amino acids. Furthermore the results suggest that very simple peptides containing aromatic amino acids served as a starting point for the evolution of helix destabilising proteins.