Hello all, I am currently trying to refine a structure (with phenix.refine) that contains a number of disulfide bonds. Many of these disulfides appear to be partly broken such that the default restraints given to these disulfides by phenix.refine cause the cysteine sulfur atoms to be placed too close to one another. This results in difference density in the fo-fc map. I want to try to model these partially broken states a bit more accurately, either by modelling two alternative conformations (i.e. one cysteine pair in a disulfide and the other pair as free thiols) or by allowing the disulfide bond distance to stretch. Currently refinement drives the S-S bond length close to dictionary values regardless of how far apart I model the cysteines in my input pdb file and whether the density implies a bridge or not. My two questions are: 1) How do I specifically prevent a disulfide bond from being defined between two sulfurs that are close together (I tried manipulating the "disulfide_distance_cutoff" variable but this doesn't seem to stop a disulfide being formed). 2) How can I soften the restraint on the disulfide bond length for specific cysteine residues. Thanks for your time. Dr. Allister Crow