Hi. I'm one of the Richardsons' people. I have more experience with RNA, but have looked at DNA some as well. 1. On C2'-endo and C3'-exo being equivalent: no, not quite equivalent, but related. If you do have C1', C4', and O4' in the plane, then "popping" C2' out on one side or C3' out on the other is relatively similar (sort of) and there's a recognized "twist" conformation where you've got them both out of plane a bit. However, you can really get C3' mostly in-plane with the other three and have a conformation that's distinctly a C2' endo pucker rather than looking like it could be either one. If you're building ideal B-form DNA, it would be better to use a real C2'-endo pucker than the twist, and certainly better than using the C3'-exo. 2. I'm afraid I don't know the appropriate format for use with PHENIX, but there are pucker-specific parameters available on the Nucleic Acid Database site: http://ndbserver.rutgers.edu -- specifically, here: http://ndbserver.rutgers.edu/standards/parameter.html -- these are set up for XPLOR, but the values should help. At any rate, these files distinguish the dihedrals for C2'-endo and C3'-endo sugar puckers. (The set for higher resolution also provides slightly different bond lengths and angles for them, since the ring strains are a bit different.) And I don't think they make a habit of turning up C3'-exo. Laura Murray