INFRARED CRYSTALLOGRAPHY - STRUCTURAL REFINEMENT THROUGH SPECTROSCOPY

A method for determining the orientation of individual bonds within co mplex macromolecules from polarized IR measurements on oriented single crystals is described. At present, X-ray diffraction is the principal technique used to define the global structure and orientation of macr omolecules in the crystalline state. However, resolution limitations a nd conformational disorder limit the accuracy of the resulting structu ral model. A quantitative understanding of protein function often requ ires a more precise description of structural features at a localized active site. Polarized IR measurements of internal stretching bands of N-3(-), CN-, OCN-, and SCN- bound at the ferric heme iron of single c rystals of myoglobin are presented, A synthesis of such measurements o n different crystal forms leads to an Ng orientation consistent with I R measurements on both P2(1), and P2(1)2(1)2(1) crystals, but signific antly different from the orientation reported in X-ray crystal structu res. Subtle structural changes between these tyro crystal forms of the CN- complex may account for small differences in the stretching frequ ency and orientation of the C-N bond.