The dynamic structural basis of differential enhancement of conformational stability by 5'- and 3'-dangling ends in RNA.

Unpaired bases at the end of an RNA duplex (dangling ends) can stabilize the core duplex in a sequence-dependent manner and are important determinants of RNA folding, recognition, and functions. Using 2-aminopurine as a dangling end purine base, we have employed femtosecond time-resolved fluorescence spectroscopy, combined with UV optical melting, to quantitatively investigate the physical and structural nature of the stacking interactions between the dangling end bases and the terminal base pairs. A 3'-dangling purine base has a large subpopulation that stacks on the guanine base of the terminal GC or UG pair, either intrastrand or cross-strand depending on the orientation of the pair, thus providing stabilization of different magnitudes. On the contrary, a 5'-dangling purine base only has a marginal subpopulation that stacks on the purine of the same strand (intrastrand) but has little cross-strand stacking. Thus a 5'-dangling purine does not provide significant stabilization. These stacking structures are not static, and a dangling end base samples a range of stacked and unstacked conformations with respect to the terminal base pair. Femtosecond time-resolved anisotropy decay reveals certain hindered base conformational dynamics that occur on the picosecond to nanosecond time scales, which allow the dangling base to sample these substates. When the dangling purine is opposite to a U and is able to form a potential base pair at the end of the duplex, there is an interplay of base stacking and hydrogen-bonding interactions that depends on the orientation of the base pair relative to the adjacent GC pair. By resolving these populations that are dynamically exchanging on fast time scales, we elucidated the correlation between dynamic conformational distributions and thermodynamic stability.