Homogeneous catalytic hydrogenation of the polar lipids of pea chloroplasts in situ and the effects on lipid polymorphism

The pattern of hydrogenation of polar lipids of pea chloroplasts incubated in the presence of the homogeneous catalyst Pd(QS)₂, a sulphonated alizarine complex of Pd(II) has been examined. Analysis of the fatty acyl residues of the major lipid classes from chloroplast suspensions at intervals during incubation under hydrogenating conditions showed that susceptibility to hydrogenation increased in the order monogalactosyldiacylglycerol > digalactosyldiacylglycerol > sulphoquinovosyldiacylglycerol > phosphatidylglycerol. Almost 80% of the total number of double bonds in the polar lipids were removed after 2-h incubation under the conditions employed. The consequence of hydrogenation on the phase behaviour of total polar lipid extracts in aqueous dispersions were examined by freeze-fracture electron microscopy, X-ray diffraction and differential scanning calorimetry. These data indicate that progressive hydrogenation of tne lipids in situ produce a change in the organisation of the lipid when dispersed in water. Single bilayer vesicles are converted to large aggregates of planar bilayer stacks in which the hydrocarbon chains are predominantly in the gel phase configuration. Studies of lipids dispersed in 20 mM MgCl₂ suggest that cohesion between the hydrocarbon chains gradually ameliorates the repulsive effects of the charged lipids, sulphoquinovosyldiacylglycerol and phosphatidylglycerol. This results in the formation of a sheet-like lamellar phase characteristic of dispersions of saturated monogalactosyldiacylglycerols which dominates the total polar lipid extracts of pea chloroplasts.