An improved reaction coordinate for nucleic acid base flipping studies

Base flipping is a common strategy utilized by many enzymes to gain access to the functional groups of nucleic acid bases in duplex DNA which are otherwise protected by the DNA backbone and hydrogen bonding with their partner bases. Several X-ray crystallography studies have revealed flipped conformations of nucleotides bound to enzymes. However, little is known about the base-flipping process itself, even less about the role of the enzymes. Computational studies have used umbrella sampling to elicit the free energy profile of the baseflipping process using a pseudodihedral angle to represent the reaction coordinate. In this study, we have used an unrestrained trajectory in which a flipped base spontaneously reinserted into the helix in order to evaluate and improve the previously defined pseudodihedral angle. Our modified pseudodihedral angles use a new atom selection to improve the numerical stability of the restraints and also provide better correlation to the extent of flipping observed in simulations. Furthermore, on the basis of the comparison of potential of mean force (PMF) generated using different reaction coordinates, we observed that the shape of a flipping PMF profile is strongly dependent on the definition of the reaction coordinate, even for the same data set.