Crystal Structure of an Immunoglobulin Molecule by X-Ray Diffraction and Electron Microscopy

We have previously reported an X-ray diffrac-tion investigation of the three-dimensional struc-ture at 6A resolution of a human ~G1 immunoglobulin molecule, Dob, (Sarma et al., 1971). We present here the results of a new electron-density map calculated using phases obtained with two additional derivatives. We also present the results of an electron microscopic investigation (Labaw and Davies, 1971) of the immunoglobulin crystals which are in complete accord with the X-ray results. EXPERIMENTAL METHODS X-ray d~ffraction. The details of the crys-tallization of the Dob immunoglobulin, prepara-tion of heavy atom derivatives and the method of data collection have been described earlier (Sarma et al., 1971). The data for the K~PtC1 e derivative which was used for phasing the previous map were not entirely satisfactory and, in addition, we later had difficulty in reproducing this deriva-tive. Consequently we have rejected these data from the present phase calculations. Two new derivatives, K2PtC14 and Baker dimercurial