Decreasing the chlorophyll a/b ratio in reconstituted LHCII: Structural and functional consequences

Trimeric (bT) and monomeric (bM) light-harvesting complex II (LHCII) with a chlorophyll nib ratio of 0.03 were reconstituted from the apoprotein overe xpressed in Escherichia coli. Chlorophyll/xanthophyll and chlorophyll/prote in ratios of bT complexes and 'native' LHCII are rather similar, namely, 0. 28 vs 0.27 and 10.5 +/- 1.5 vs 12, respectively, indicating the replacement of most chlorophyll a molecules with chlorophyll b, leaving one chlorophyl l a per trimeric complex. The LD spectrum of the bT complexes strongly sugg ests that the chlorophyll b molecules adopt orientations similar to those o f the chlorophylls a that they replace. The circular dichroism (CD) spectra of bM and bT complexes indicate structural arrangements resembling those o f 'native' LHCII. Thermolysin digestion patterns demonstrate that bT comple xes are folded and organized like 'native' trimeric LHCII. Surprisingly, in the bT complexes at 77 K, half of the excitations that are created on eith er chlorophyll b or xanthophyll are transferred to chlorophyll a. No or ver y limited triplet transfer from chlorophyll b to xanthophyll appears to tak e place. However, the efficiency of triplet transfer from chlorophyll a to xanthophyll is close to 100%, even higher than in 'native' LHCII at 77 K. I t is concluded from the triplet-minus-singlet and CD results that the singl e chlorophyll a molecule that on the average is present in each bT complex binds preferably next to a xanthophyll molecule at the interface between th e monomers.