Portability of the GN(R)A hairpin loop motif between RNA and DNA

Hairpins are common nucleic acid secondary structures that serve many structural and functional roles. Recently, we reported that r(UNCG) and r(GNRA) hairpin families use molecular mimicry and electrostatic factors to attain exceptional thermodynamic stability with a CG closing base pair (cbp). Despite having very different overall folds, these tetraloops present the same functionalities and partial charges to the major groove edge of the CG cbp to achieve stability. Herein, we compare the r(GNRA) tetraloop family to the DNA triloop family d(GNA), which is also exceptionally stable with a CG cbp and possesses the same base pairing between the first and last positions of the loop. Nucleobase and functional group modifications were used to investigate interactions of d(GNA) loops with the cbp, which provided for comparison with similar substitutions in r(GNRA) hairpins. Interruption or deletion of loop-cbp interactions in d(GNA) was consistent with electrostatic interactions identified through nonlinear Poisson-Boltzmann (NLPB) calculations, and loop stability changed in a manner consistent with similar loop-cbp interactions for d(GNA) and r(GNRA) loops. We also compared the relationship of DeltaG degrees (37) and log[Na+] for d(GNA) and r(GNRA) loops and found a decreased dependence of stability on salt for both loop families when a CG cbp was present. The similarity of the loop-cbp interactions shows portability of this loop-cbp motif across polymer type and loop size and indicates convergence on similar molecular solutions for stability in RNA and DNA.