Steroid−DNA Interactions Increasing Stability, Sequence-Selectivity, DNA/RNA Discrimination, and Hypochromicity of Oligonucleotide Duplexes

Cholic acid and its deoxy derivatives were found to increase the melting point of oligonucleotide duplexes when coupled to their 5‘-termini. For duplexes of mixed-sequence octamers, the melting point was 8−11 °C higher with the steroid appendage than without. For the self-complementary hexamer TGCGCA, a 21 °C melting point increase was measured in the presence of steroid appendage and −18 kcal/mol in ΔΔ H °. The affinity increases were accompanied by increased discrimination against mismatches at the two terminal base pairs and increased hypochromicity, as well as an improved DNA/RNA discrimination for the non selcomplementary sequence. Cross-peaks in the NOESY spectrum of (chl-T*GCGCA) 2, where chl denotes the cholic acid residue and T* a 5‘-amino-5‘-deoxythymidine residue, point toward stacking interactions in which the methyl groups of the steroid face the nucleobases. Our results indicate that bile acids can form a specific complex with terminal T:A base pairs of double-stranded DNA.