Folding Thermodynamics and Kinetics of YNMG RNA Hairpins:  Specific Incorporation of 8-Bromoguanosine Leads to Stabilization by Enhancement of the Folding Rate

Modified nucleotides allow fundamental energetic and kinetic properties of nucleic acids to be probed. Here, we demonstrate that an RNA hairpin containing the nucleotide analogue 8-bromoguanosine (8BrG or G ), gcUUC G gc, has enhanced stability relative to the unmodified hairpin, with = −0.69 ± 0.15 kcal mol-1 and ΔT M = +6.8 ± 1.4 °C. NMR spectroscopic data suggest that the enhanced stability of gcUUC G gc does not arise from the native state; laser temperature-jump experiments support this notion, as gcUUCGgc and gcUUC G gc have similar unfolding rate constants, but the folding rate constant of gcUUC G gc is 4.1-fold faster at 37.5 °C and 2.8-fold faster under isoenergetic conditions. On the basis of these findings, we propose that 8BrG reduces the conformational entropy of the denatured state, resulting in an accelerated conformational search for the native state and enhanced stability.