²⁵Mg solid-state NMR in an ultra-high magnetic field:Structures of bacteriochlorophyll c complex, chlorosome

Mg atoms play an important role in biological materials, but ²⁵Mg NMR measure-  ments are difficult since ²⁵Mg is low-γ nucleus and exhibits quadrupolar broaden-  ing in solids. An ultra-high magnetic field may, however, dramatically enhance  sensitivity.
This Note introduces a study on stacking structures of bacteriochlorophyll c in chlorosome clarified using  ²⁵Mg solid-state NMR at 21.8 T.
* This is collaborative research with Prof. Y. Koyama of Kwansei Gakuin Univ., Prof. H. Nagae of  Kobe City Univ. of Foreign Studies, and Prof. T. Shimizu of  National Institute of Materials Science.

Chlorosome is a light-harvesting organ of green  photosynthesis bacteria, and bacteriochlorophyll c  (BChl c) aggregate in it. Stacking structures of BChl c  have attracted interest for a long time.

Using a 21.8 T ultra-high field, ²⁵Mg MAS NMR  spectrum of chlorosome was obtained within 80 hours,unprecedentedly short time.

²⁵Mg spectrum of chlorosome suggests two Mg sites:Large quadrupolar or low symmetry Mg (Site 1) and  small quadrupolar or high symmetry Mg (Site 2). Fig. 3 shows a plausible model:  Mg-coordinated OH deviates Mg atom of Site 1 from the ring plane, whereas Mg atom of Site 2 stays within the plane  because of a ligand water molecule.

 

Thus, BChl c molecules in chlorosome prove to have stacking structures depicted in Fig. 4.

This study demonstrates that ultra-high field NMR system is useful for low-γ nuclei in real samples, such as biological materials.

Reference

Y. Kakitani, Y. Koyama, Y. Shimoikeda, T. Nakai, H. Utsumi, T. Shimizu, and H.  Nagae, Biochemistry 2009, 48, 74-86.