There are two distinct questions relevant to the roles of electrostatic forces in DNA conformation. Question #1 centers on positions and modes of binding of counter ions to DNA. In one possible mode, counter ions could be fully dynamic and delocalized. In another mode, counter ions could partition selectively into various territories defined by groove shape and sequence. In a third mode, counter ions could engage in long-lived site-binding. Question #2 centers on conformational consequences of counter ion interactions. Counter ions might play specific roles in groove width, axial curvature, A to B transitions, etc. Alternatively intrinsic forces such as stacking, hydrogen bonding, and steric repulsion among exocyclic groups might dominate. Here we describe x-ray structures of modified and unmodified DNA in the presence of anomalously scattering counter ions. The goal is to dissect modes of binding and roles of electrostatics in DNA conformation.
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