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Infrared Crystallography: Structural Refinement through Spectroscopy

A method for determining the orientation of individual bonds within complex 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 macromolecules in the crystalline state. However, resolution limitations and conformational disorder limit the accuracy of the resulting structural model. A quantitative understanding of protein function often requires 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 crystals of myoglobin are presented. A synthesis of such measurements on different crystal forms leads to an N−3 orientation consistent with IR measurements on both P21 and P212121 crystals, but significantly different from the orientation reported in X-ray crystal structures. Subtle structural changes between these two crystal forms of the CN− complex may account for small differences in the stretching frequency and orientation of the C–N bond.