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The orientations of the transition dipole moments in chlorophyll (Chl) are among the most useful spectroscopic properties for determining macromolecular architecture in photosynthetic complexes; however, the relationships between these orientations and the Chl molecular geometry are unknown. In order to solve this problem, we have prepared single crystals of the synthetic 1:1 complex between pyrochlorophyllide a and apomyoglobin. The protein crystallizes readily in the orthorhombic (B) form, space group P2(1)2(1)2(1), and the unit cell dimensions are determined to be within 0.5% of those for native MetMb crystals of the same type. These green crystals are highly dichroic, and the strong absorption along the crystallographic a axis in the Q(y) band is red-shifted by about 9 nm, relative to the corresponding feature in a solution of the protein. Although the crystal structure for native Mb in this space group has not been determined, the direction cosines of the heme normal relative to the crystal axes have been measured. By using these values, an appropriate trigonometric analysis, and the measured polarized single-crystal spectra, the orientation of the Chl transition dipole moment for the Q(y) transition can be specified relative to the crystal axes. With the completion of the protein crystal structure, this result will lead directly to the orientations of the optical transition dipole moments relative to the molecular geometry. The effects of vibronic coupling and the protein environment on the absorption properties of Chl are discussed in detail.

Oriented properties of the chlorophylls: Electronic absorption spectroscopy of orthorhombic pyrochlorophyllide a -apomyoglobin single crystals