Formation of bacteriochlorophyll a coordination states under external high-pressure

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The pressure-dependencies up to 1100 MPa of the Q y , Q x, and Soret absorption bands for five- and six-coordinate monomeric complexes of bacteriochlorophyll a (Bchl a) in the solutions have been measured and quantitatively analysed. Here we show that the dependencies of the six-coordinate complexes can be accounted for within semiclassical theories, and the additional pressure-shifts in the dependencies of the five-coordinate complexes are caused by the Mg–O sixth axial ligand formation. Thereby the decrease of the Q x and Soret transitions energies, induced by the Mg–O bond, have been estimated.