The Free Energy Landscape of Pseudorotation in 3′–5′ and 2′–5′ Linked Nucleic Acids

[Image: see text] The five-membered furanose ring is a central component of the chemical structure of biological nucleic acids. The conformations of the furanose ring can be analytically described using the concept of pseudorotation, and for RNA and DNA they are dominated by the C(2′)-endo and C(3′)-endo conformers. While the free energy difference between these two conformers can be inferred from NMR measurements, a free energy landscape of the complete pseudorotation cycle of nucleic acids in solution has remained elusive. Here, we describe a new free energy calculation method for molecular dynamics (MD) simulations using the two pseudorotation parameters directly as the collective variables. To validate our approach, we calculated the free energy surface of ribose pseudorotation in guanosine and 2′-deoxyguanosine. The calculated free energy landscape reveals not only the relative stability of the different pseudorotation conformers, but also the main transition path between the stable conformations. Applying this method to a standard A-form RNA duplex uncovered the expected minimum at the C(3′)-endo state. However, at a 2′–5′ linkage, the minimum shifts to the C(2′)-endo conformation. The free energy of the C(3′)-endo conformation is 3 kcal/mol higher due to a weaker hydrogen bond and a reduced base stacking interaction. Unrestrained MD simulations suggest that the conversion from C(3′)-endo to C(2′)-endo and vice versa is on the nanosecond and microsecond time scale, respectively. These calculations suggest that 2′–5′ linkages may enable folded RNAs to sample a wider spectrum of their pseudorotation conformations.