Synthesis of zinc bacteriochlorophyll-d analogues with various 17-substituents and their chlorosomal self-aggregates in non-polar organic solvents

Zinc 3(1)-demethyl-bacteriopheophorbides-d possessing various 17-propionate residue (17(2)-COOCH2-) were prepared as models of light-harvesting pigments in major photosynthetic antennae of green bacteria. The synthetic compounds were monomeric in polar organic solvents to give the same visible absorption spectra, but self-aggregated in non-polar organic solvents to afford large oligomers with broadened and red-shifted absorption bands. The peak positions were identical for all the self-aggregates but the widths of the redmost (Qy) bands were dependent upon the substituents. More hydrophobic and less sterically hindered hydrocarbon moieties as the esterifying group sharpened oligomeric Qy bands, giving less inhomogeneous supramolecular structures due to stabilization of the self-aggregates. In the reverse esters (17(2)-CH2OCO-) structurally isomeric to the above compounds, the same optical behavior was observed. Movement of the oxo group from the 17(3)- to 17(5)-position (17(2)-COOCH2--->17(2)-CH2OCO-) slightly increased the widths of self-aggregated Qy bands without change of the peak positions, where minor less self-aggregated components would enhance in the solution due to a little more stabilization of such species by the 17(4)-carbonyl group.