Disclosed are the atomic coordinates of compositions comprising Fc heterodimer proteins in crystalline form derived from high resolution x-ray diffraction. Further disclosed are systems and methods for using all or a portion of these atomic coordinates to identify and design improved Fc heterodimer proteins. Further disclosed are compositions comprising a mixture of (i) a solubilized Fc heterodimer protein and (ii) a mother liquor solution. The mother liquor solution comprises between 2% and 10% (v/v) ethylene glycol, between 10% and 25% (w/v) polyethylene glycol having an average molecular weight of between 2000 Daltons and 10000 Daltons, and between 0.05 M and 0.40 M ammonium iodide. Further disclosed are systems and methods of identifying a mutation which promotes heterodimeric Fc chain pair formation in which structure based modeling is performed to identify a candidate mutation to an Fc chain using all or a portion of the disclosed three-dimensional atomic coordinates.
