We present a molecular-scale model of Bacteriochlorophyll a (BChl a) binding to the chlorosome protein A (CsmA) of Chlorobaculum tepidum, and the aggregated pigment–protein dimer, as determined from protein–ligand docking and quantum chemistry calculations. Our calculations provide strong evidence that the BChl a molecule is coordinated to the His25 residue of CsmA, with the magnesium center of the bacteriochlorin ring situated\3 A° from the imidazole nitrogen atom of the histidine sidechain, and the phytyl tail aligned along the nonpolar residues of the a-helix of CsmA. We also confirm that the Qy band in the absorption spectra of BChl a experiences a large (?16 to ?43 nm) redshift when aggregated with another BChl a molecule in the CsmA dimer, compared to the BChl a in solvent; this redshift has been previously established by experimental researchers. We propose that our model of the BChl a–CsmA binding motif, where the dimer contains parallel aligned N-terminal regions, serves as the smallest repeating unit in a larger model of the para-crystalline chlorosome baseplate protein.

Electronic structure calculations on atomistic models of magnetite (Fe 3 O 4 ) provide valuable insight into the structure and properties that dictate the behaviour of magnetite under environmental conditions. The charge ordering in the bulk oxide controls the reactivity of the exposed surfaces, but it has been difficult to measure experimentally or predict theoretically. We use spin-polarised density functional theory to calculate the structure of bulk Fe 3 O 4 and its (001) and (111) surface terminations. We then present an ab initio thermodynamics approach to determine the most energetically favourable clean and hydroxylated surface terminations of Fe 3 O 4 (001). We present results on molecular water adsorption and heterolytic dissociation at both quarter-and half-monolayer coverage for Fe 3 O 4 (001). Our calculations suggest that the tetrahedral (001) surface termination is the most energetically stable across all oxygen chemical potentials, and that water molecules preferentially dissociate at the surface. The calculated hydroxylated and hydrated surface terminations, as obtained from careful consideration of the charge ordering in the bulk, serve as a strong basis for future studies of the electrical double layer at the mineral -water interface.

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