Evidence for two spectroscopically different dimers of light-harvesting complex I from green plants

A preparation consisting of isolated dimeric peripheral antenna complexes f rom green plant photosystem I(light-harvesting complex I or LHCI) has been characterized by means of (polarized) steady-state absorption and fluoresce nce spectroscopy at low temperatures. We show that this preparation can be described reasonably well by a mixture of two types of dimers. In the first dimer about 10% of all Q(y) absorption of the chlorophylls arises from two chlorophylls with absorption and emission maxima at about 711 and 733 nm, respectively, whereas in the second about 10% of the absorption arises from two chlorophylls with absorption and emission maxima at about 693 and 702 nm, respectively. The remaining chlorophylls show spectroscopic properties comparable to those of the related peripheral antenna complexes of photosys tem II. We attribute the first dimer to a heterodimer of the Lhca1 and Lhca 4 proteins and the second to a hetero- or homodimer of the Lhca2 and/or Lhc a3 proteins. We suggest that the chlorophylls responsible for the 733 nm em ission (F-730) and 702 nm emission (F-702) are excitonically coupled dimers and that F-730 originates from one of the strongest coupled pair of chloro phylls observed in nature.