Effect of Mg 2+ on excitation energy transfer between LHC II and LHC I in a chlorophyll-protein complex

Abstract

A chlorophyll-protein preparation has been isolated from the thylakoid membranes of Pisum sativum L. by the use of Triton X-100 in the presence of a Tris-HCl buffer. Analysis of this preparation by means of SDS-PAGE and freeze-fracture electron microscopy indicates that it contains the reaction centre complex of photosystem I (PS I) and its associated peripheral light-harvesting complex (LHC I), together with the peripheral light-harvesting complex of PS II (LHC II). The reaction centre complex of PS II is absent from this preparation. The polypeptide composition of the complex is similar to that of one previously isolated from barley [(1987) Eur. J. Biochem. 163, 221–2301. Fluorescence spectroscopy of the preparation at 77 K indicates that excitation of chlorophyll b at 472 nm (LHC II) gives fluorescence emission at 735 nm from LHC I, thereby indicating excitation energy transfer between the two complexes. The extent of excitation energy transfer from LHC II to LHC I is increased by the absence of Mg 2+ from the medium, a phenomenon similar to effects of Mg 2+ depletion promoting ‘spillover’ of excitation energy from PS II to PS I in chloroplast thylakoid membranes.