Conformational heterogeneity observed in simulations of a pyrene-substituted DNA

NMR studies previously carried out for a DNA system with a pyrene-substituted base did not observe NOEs involving the adenine located 5' to the pyrene, and thus the conformation of the adenine was poorly defined in the resulting family of refined structures. However, chemical shift data suggested that an AT base pair may be present. We have carried out fully unrestrained molecular dynamics simulations starting from several members of the family of structures, and these simulations support the existence of an AT base pair for this region. Simulations in both explicit and implicit solvent were carried out, with each converging to either anti or syn conformation for adenine and base pairing in all cases. During these simulations, large and dramatic conformational changes are observed that suggest pathways for complex conformational changes in the highly packed DNA interior. Our analysis reveals little difference in the energies of these syn and anti conformations, in contrast to control calculations carried out for standard DNA (in the absence of a neighboring pyrene). While no interconversion between the conformations was observed in standard simulations, reversible anti/syn exchange was directly simulated using the locally enhanced sampling approach. No exchange was seen in the non-pyrene control sequence. Together, these results suggest that an increased flexibility is introduced as a consequence of the pyrene substitution, offering an explanation that is consistent with the available experimental data. These results increase our optimism that simulations in atomic detail may provide accurate models for experimental observations in complex systems.