Two sets of nuclear magnetic resonances in the CH2Cl2-treated (31R)-type bacteriochlorophyll c solid aggregates under ultra high magnetic field

Abstract

Chlorosome in green sulfur and filamentous bacteria is a special light harvesting entity with absorption maximum around 740 nm. Bacteriochlorophyll (BChl) c is well-known to form rod-like large aggregates in the chlorosome without influence of protein. BChl c is characterized by the hydroxyl group at 31-position, which brings about a unique self-aggregation of BChl c, and the difference in stereochemistry is considered to influence the behavior of self-aggregation. In this study, we are concerned with the (31R)-type BChl c ((31R)-[E,E]BChl cF) which is the most abundant component in the chlorosome of green bacteria Chlorobium tepidum. (31R)-[E,E]BChl cF forms two small aggregates with absorption maxima at 680 and 703-10 nm in CH2Cl2, and CCl4. For the small aggregates, we have demonstrated that (31R)-[E,E]BChl cF forms an unstable T-shaped dimer with absorption only at ~680 nm and a stable antiparallel dimer with two Qy(0-0) exiton components at ~680 nm and ~710 nm. For the solid aggregates, we could form the solid aggregates with absorption maximum at 734 nm by drying the BChl c-CH2Cl2 solution. The CH2Cl2-treated solid aggregates are formed through the antiparallel dimer, and further, give rise to X-ray diffraction, differently from the aggregates treated with other methods. In this study, we have measured the solid-state 13C- and 15N- NMR spectra for the CH2Cl2-treated solid aggregates of (31R)-[E,E]BChl cF at ultra high magnetic field in order to discuss the structure of solid aggregates in relation with the antiparallel dimer in solution.