The goal of the project is to answer the questions about localization, physical nature and role of low-energy states, as well as to explain the nature and role of excitonic coupling features observed in photosynthetic Photosystem I (PSI). The low-energy states were hypothesized to extend the spectral range of light that can be utilized in the process of photosynthesis. However, there are also alternative hypotheses about the role of long-wavelength absorbing pigments (chlorophylls). Similarly, their nature is not clear. Their unusual spectral properties may be result of dimerization, trimerization, interaction with protein environment and/or interaction with neighboring carotenoid molecules. These questions will be studied with a variety of state-of -art optical techniques available in the host lab. Femtosecond optical spectroscopy, including polarized pump-probe experiments, will be applied to study the nature of excitonic coupling features observed recently by the applicant in PSI and ascribed to six strongly interacting reaction center chlorophylls (Gibasiewicz et al.J.Pys.Chem. B (2001) 105, 11498). The importance of the excitonic interactions for the plant reaction center of Photo system II has previously been pointed out (Durrant et al., PNAS USA(1995) 92, 4798). Finally, a physical model analogous to that described in van Grondelle and Novoderezhkin (Biochemistry (2001) 40, 15057) will be applied to the PSI reaction center.In this project, several state-of -art optical techniques will be applied. Some of them are new for the applicant and he will benefit from learning them as well as from interacting with people in the excellent scientific group of prof. van Grondelle. Studies on low-energy states in the LHCI complex will extend the applicant' s scientific competence into another antenna system complementary to the PSI core that has been subject of his research for the last two years. Experience, skills and knowledge acquired in Amsterdam will contribute to his habilitation work after returning to Poland. The host lab will benefit from the applicant's experience in energy transfer measurements in light-harvesting systems. He will support the group of dr. van Amerongen in realization of a broader 4-year project on low-energy states in photosynthesis. The applicant will introduce the new problem of excitonic coupling in the PSI reaction center.