Linear dichroism in the XANES of partially oriented samples: theory and application to the photosynthetic manganese complex.

For molecular systems which are partially ordered with respect to one macroscopic axis, for example, the sample-surface normal, X-ray absorption spectroscopy (XAS) with linearly polarized synchrotron radiation can provide information on structure and orientation of the X-ray absorbing site (polarized or linear-dichroism XAS). Examples for such partially oriented systems are protein-carrying membrane particles deposited in the form of multilayers on surfaces, layered minerals, surface films or imperfect 2D crystals, planar electrodes or catalytic surfaces. For electric dipole transitions, equations are derived describing how partial unidirectional orientation determines the linear dichroism (LD). The approach presented facilitates description of 1) LD in multiple-scattering contributions of the extended X-ray absorption fine-structure (EXAFS) and 2) of LD in the X-ray absorption near-edge structure (LD-XANES). Structural and orientation information becomes accessible by combination with ab initio XANES calculations. The LD-XANES approach is applied to the water-oxidizing Mn complex of photosystem II. The results suggest that the (mu-O)-(mu-O) vectors of the Mn-(mu-O)(2)-Mn units are at an average angle to the membrane normal of 30-45 degrees. The best-fit structure in connection with crystallographic data suggests a specific oxidation-state assignment: Mn(1)(III)Mn(2)(III)Mn(3)(IV)Mn(4)(IV). However, currently this assignment remains uncertain. In future studies, the LD-XANES analysis should play an important role in construction of unequivocal atomic-resolution model of the photosynthetic Mn complex.