Computational study of short-range interactions in bacteriochlorophyll aggregates

Abstract

Chlorosomes are light harvesting complexes, interior of which is composed of bacteriochlorophylls that are assembled into lamellar aggregates. In this work, short-range structural parameters of aggregated bacteriochlorophyll-c were explored. The knowledge of bonds involved in aggregation together with other experimental results provides constraints which are satisfied by eight possible structural motifs. Each motif is based on a dimer unit with esterifying alcohols extending to opposite sides of the lamellar layer. The possible models include two of the previously proposed structural models, so-called antiparallel piggy-back dimer [1,2] and a parallel syn-anti model [3]. Structural models of aggregates were built as a single layer consisting of 8×10 monomers and were optimized by Amber program using modified General Amber Force Field. From the optimized models the central tetramer unit was selected for single-point energy analyses. Three electronic methods: RI-MP2/SVP, B97-D/TZVP, and PM6-DH2 were chosen for revealing energy relations in the structural models. Calculations disclose that the most preferable structure is the antiparallel chain model corresponding to the antiparallel piggy-back dimer, followed by one of antiparallel sheet models and one of parallel sheet arrangements. All simulations showed that due to the crystal packing a large deformation of monomeric building block occurred. The deformation energies (the difference between the energy of optimized isolated monomer and that of the monomer in assembled structures) are about 15kcal/mol for all considered computational methods. From energy decomposition, contributions of individual interactions to the overall stabilization energy were estimated. In addition, powder diffraction and optical spectra were calculated for all models and compared with experimental results.