Photosynthesis Research

, Volume 41, Issue 1, pp 225–233

On the structure of bacteriochlorophyll molecular aggregates in the chlorosomes of green bacteria. A molecular modelling study

  • Alfred R. Holzwarth
  • Kurt Schaffner
Group 5: Chlorosomes and Pigments Regular Papers
  • 265 Downloads

Abstract

The supramolecular structure of methyl (31R)-BChlided aggregation has been explored by molecular modelling in order to elucidate the unusual structure of the BChl rods in the chlorosomal antennae of green bacteria. The aggregate construction progressed from a BChlide monomer in 5c coordination which was stepwise combined to form trimeric, pentameric and decameric chlorin stacks, all incorporating Mg····O-H as a basic interaction element which links two chlorins between the 31-hydroxyl oxygen and the Mg. Up to the level of the trimer, the structures were optimized by both a semiempirical quantum chemical method (PM3) and a force field method, while larger structures were only modelled by the force field (MM+). Strong interactions were found by extended stacking of chlorins which are in van der Waals contact. Extended hydrogen bonding networks upon stack pairing brought about by OH····O=C bonds (bond length ca. 2.2Å, angle 139–153°) between appropriately situated chlorin pairs and by electrostatic interactions lead to very large energy stabilizations. The structural features of a modelled 40mer BChl aggregate are in full accord with all spectroscopic and low-resolution structural information on the in-vitro and chlorosomal BChl aggregates. Most important, from the rotation angle between stacks of ca. 16° and the stack-to-stack distance of 7.6 Å a tubular structure can be extrapolated to form on further extension of the aggregate. It has a predicted diameter of about 5.4 nm (Mg-Mg distance), i.e. very similar to that found for the rod elements in the chlorosomes ofChloroflexus.

Key words

molecular modelling force field calculation semiempirical calculation bacteriochlorophyllc bacteriochlorophylld aggregate chlorosome green bacterium 

Abbreviations

BChl

bacteriochlorophyll

BChlide

bacteriochlorophyllide

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Copyright information

© Kluwer Academic Publishers 1994

Authors and Affiliations

  • Alfred R. Holzwarth
    • 1
  • Kurt Schaffner
    • 1
  1. 1.Max-Planck-Institut für StrahlenchemieMülheim an der RuhrGermany

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