Description of energy migration and trapping in photosystem I by a model with two distance scaling parameters.

The energy transfer and trapping kinetics in the core antenna of Photosystem I are described in a new model in which the distance between the core antenna chlorophylls and P700 is proposed to be considerably longer than the distance between the chlorophylls within the antenna. Structurally, the model describes the Photosystem I core antenna as a regular sphere around P700, while energetically it consists of three levels representing the bulk antenna, P700 and the red-shifted antenna pigments absorbing at longer wavelength than P700, respectively. It is shown that the model explains experimental results obtained from the Photosystem I complex of the cyanobacterium Synechococcus sp. (A.R. Holzwarth, G. Schatz, H Brock, and E. Bittersman (1993) Biophys. J. 64: 1813-1826) quite well, and that no unrealistic charge separation rate and organization of the long-wavelength pigments has to be assumed. We suggest that excitation energy transfer and trapping in Photosystem I should be described as a 'transfer-to-the-trap'-limited process.