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The very first step in photosynthesis of green plants is the excitation of the accessory pigments in the antenna system. Then a chain of energy transfer steps funnels the excitation energy to the reaction center where its capture leads to creation of chemical energy. Of special interest is the antenna system of photosystem II, LHC II, which contains chlorophyll cM, and chlorophyll cM. In LHC II cM and chl are equally abundant and thus, this system constitutes a suitable candidate for studying the cM to cM energy transfer in detail. Furthermore, some investigations have already been carried out on this system. In an early study by Gillbro et a! [1] the energy transfer time between chl and cM was estimated to be in the range 240 ps. Later studies have, however, indicated that this was an underestimate of the transfer rate between cM and chl . From a study on a bacterial system, Eads et al [2] concluded from time resolved fluorescence experiments that the transfer time between chi and cM is in the order of 0.5 ps. Returning to the antenna systems of higher plants, Kwa et a! [3] in a recent work reported that this energy transfer step must be completed within 1 ps. LHC II does also belong to an exclusive group of membrane bound photosynthetic protein complexes that have been crystalized and whose structure have been partially determined (Kuhlbrandt et a! [4]). The outcome of that study was that the chiorophylls in each monomer are organized in two different levels, with each lve1 close to the bilayer leaflets. The 6 A resolution of this crystal structure did not, however, permit Kuhlbrandt et al [4] to distinguish between the chlorophyll residues of a or b type. In the same study Kuhlbrandt et at [4] also estimated the rates of energy transfer within one monomeric unit. This estimate is, however, rather crude since all the interacting chiorophylls were of only one type, namely cM and were interacting according to the exciton mechanism. With this assumption the transfer time was calculated to be as short as 10- 100 fs within the monomer unit, even with the Qy transition dipoles randomly orientated. With regard to the energy transfer step chi k to chl , thisis most likely an overestimate of the transfer rate. Furthermore, a more appropriate interaction mechanism could in fact be the Förster mechanism especially when chi and chl are interacting. We have recently shown that the Förster mechanism is valid for as short distances as ca 20 A in the PEC and other trimers of phycobilisomes [5]. The aim of this study was accordingly to re-examine the energy transfer dynamics within the LHC II complex. Previous studies showed that the need for better time resolution is obvious [1,3]. With a more accurate mesurement of the transfer time it should also be possible to see how well the distances calculated with the Förster model fit those obtained from crystal data
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