Multistate B- to A- transition in protein-DNA Binding - How well is it described by current AMBER force fields?

When DNA interacts with a protein, its structure often undergoes a significant conformational adaptation, usually involving a transition from B-DNA towards the A-DNA form. This is not a two-state, but rather a multistate transition. The A- and B- forms differ mainly in sugar pucker (north/south) and glycosidic torsion χ (anti/high-anti). The combination of A-like pucker and B-like χ (and vice versa) represents the nature of the intermediate states between the pure A- and B- forms. Here we study how the A/B equilibrium and the A/B intermediate states at protein-DNA interfaces are modeled by current AMBER force fields. Eight diverse protein-DNA complexes and their naked (unbound) DNAs were simulated with OL15 and bsc1 force fields and an experimental combination OL15χ_OL3. We found that while the geometries of the A-like intermediate states agree well with the native X-ray geometries, their populations (stabilities) are significantly underestimated. Different force fields predict different propensities for A-like states growing in the order OL15 < bsc1 < OL15_χOL3, yet all underestimate A-like form populations. Interestingly, the force fields seem to predict the correct sequence-dependent A-form propensity, as they predict larger populations of the A-like form in unbound DNA in those steps that acquire A-like conformations in protein-DNA complexes. The instability of A-like geometries in current force fields significantly alters the geometry of simulated protein-DNA complexes and destabilizes the binding motif, suggesting that refinement is required to improve description of protein-DNA interactions in AMBER force fields.