| Abstract
| - Absorption, fluorescence excitation, emission, and hole-burning (HB) spectra were measured at liquid heliumtemperatures for the PS I−CP43‘ supercomplexes of Synechocystis PCC 6803 grown under iron stress conditionsand for respective trimeric PS I cores. Results are compared with those of room temperature, time-domainexperiments (Biochemistry 2003, 42, 3893) as well as with the low-temperature steady-state experiments onPS I−CP43‘ supercomplexes of Synechococcus PCC 7942 (Biochim. Biophys. Acta2002, 1556, 265). Incontrast to the CP43‘ of Synechococcus PCC 7942, CP43‘ of Synechocystis PCC 6803 possesses two low-energy states analogous to the quasidegenerate states A and B of CP43 of photosystem II (J. Phys. Chem. B2000, 104, 11805). Energy transfer between the CP43‘ and the PS I core occurs, to a significant degree,through the state A, characterized with a broader site distribution function (SDF). It is demonstrated that thelow temperature (T = 5 K) excitation energy transfer (EET) time between the state A of CP43‘ (IsiA) and thePS I core in PS I−CP43‘ supercomplexes from Synechocystis PCC 6803 is about 60 ps, which is significantlyslower than the EET observed at room temperature. Our results are consistent with fast (≤10 ps) energytransfer from state B to state A in CP43‘. Energy absorbed by the CP43‘ manifold has, on average, a greaterchance of being transferred to the reaction center (RC) and utilized for charge separation than energy absorbedby the PS I core antenna. This indicates that energy is likely transferred from the CP43‘ to the RC along awell-defined path and that the “red antenna states” of the PS I core are localized far away from that path,most likely on the B7−A32 and B37−B38 dimers in the vicinity of the PS I trimerization domain (near PsaLsubunit). We argue that the A38-A39 dimer does not contribute to the red antenna region.
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