Contrasting packing modes for tubular assemblies in chlorosomes

Author:

Miloslavina Yuliya A.,Thomas Brijith,Reus MichaelORCID,Gupta Karthick Babu Sai Sankar,Oostergetel Gert T.,Andreas Loren B.,Holzwarth Alfred R.,de Groot Huub J. M.

Abstract

AbstractThe largest light-harvesting antenna in nature, the chlorosome, is a heterogeneous helical BChl self-assembly that has evolved in green bacteria to harvest light for performing photosynthesis in low-light environments. Guided by NMR chemical shifts and distance constraints for Chlorobaculum tepidum wild-type chlorosomes, the two contrasting packing modes for syn-anti parallel stacks of BChl c to form polar 2D arrays, with dipole moments adding up, are explored. Layered assemblies were optimized using local orbital density functional and plane wave pseudopotential methods. The packing mode with the lowest energy contains syn-anti and anti-syn H-bonding between stacks. It can accommodate R and S epimers, and side chain variability. For this packing, a match with the available EM data on the subunit axial repeat and optical data is obtained with multiple concentric cylinders for a rolling vector with the stacks running at an angle of 21° to the cylinder axis and with the BChl dipole moments running at an angle ß ∼ 55° to the tube axis, in accordance with optical data. A packing mode involving alternating syn and anti parallel stacks that is at variance with EM appears higher in energy. A weak cross-peak at -6 ppm in the MAS NMR with 50 kHz spinning, assigned to C-181, matches the shift of antiparallel dimers, which possibly reflects a minor impurity-type fraction in the self-assembled BChl c.

Funder

EuroSolarFuels Eurocores research program of the Foundation for Fundamental Research on Matter (FOM), which is part of the Netherlands Organisation for Scientific Research

Deutsche Forschungsgemeinschaft

Dutch BioSolar Cells research program, sponsored by the Ministry of Economy, Agriculture, and Innovation

Publisher

Springer Science and Business Media LLC

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