[phenixbb] restraints for dna planarity

Hi, I'm refining a low resolution structure of a dna-protein complex (3.7A) & wondering if someone could help me with setting restraints in Phenix to maintain the base-pair planarity of the dna? With the restraints I have, even with a reference model, I'm getting 'puckering' of the base-pairs. Although the h-bonding distances seem to hold over time, I can't seem to find a way to keep the planarity intact. Thanks in advance for your help. -John