Many important biological processes, from the interferon antiviral response to the generation of microRNA regulators of translation, involve duplex RNA. Small molecules capable of binding duplex RNA structures with high affinity and selectivity will be useful in regulating these processes and, as such, are valuable research tools and potentially therapeutic. In this paper, the synthesis and duplex RNA-binding properties of EDTA·Fe-modified peptide-intercalator conjugates (PICs) are described. Peptide appendages at the 4- and 9-positions of the planar acridine ring system render these PICs threading intercalators, directing the substituents into both grooves of double helical RNA simultaneously. Directed hydroxyl radical cleavage experiments conducted with varying RNA stem-loop structures indicate a preferred binding polarity with the N- and C-termini of the PIC in the minor and major grooves, respectively. However, this binding polarity is shown to be dependent on both the structure of the PIC and the RNA secondary structure adjacent to the intercalation site. Definition of the minimal RNA structure required for binding to one of these PICs led to the identification of an intercalation site in a pre-microRNA from Caenorhabditis elegans. Results presented will guide both rational design and combinatorial approaches for the generation of new RNA binding PICs and will continue to facilitate the identification of naturally occurring RNA targets for these small molecules.
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