Photosynthesis Research 70: 129–153, 2001. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 129 Review Excitation energy transfer in Photosystem I from oxygenic organisms Alexander N. Melkozernov Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, AZ 85287-1604, USA (e-mail: melkozer@imap2.asu.edu; fax: +1-480-965-2747) Received 10 October 2000; accepted in revised form 1 June 2001 Key words: excitation energy equilibration, excitonic interactions, LHC I, photochemical trapping, PS I, red pigments, transient hole-burning Abstract This Review discusses energy transfer pathways in Photosystem I (PS I) from oxygenic organisms. In the trimeric PS I core from cyanobacteria, the efficiency of solar energy conversion is largely determined by ultrafast excitation transfer processes in the core chlorophyll a (Chl a) antenna network and efficient photochemical trapping in the reaction center (RC). The role of clusters of Chl a in energy equilibration and photochemical trapping in the PS I core is discussed. Dimers of the longest-wavelength absorbing (red) pigments with strongest excitonic interactions localize the excitation in the PS I core antenna. Those dimers that are located closer to the RC participate in a fast energy equilibration with coupled pigments of the RC. This suggests that the function of the red pigments is to concentrate the excitation near the RC. In the PS I holocomplex from algae and higher plants, in addition to the red pigments of the core antenna, spectrally distinct red pigments are bound to the peripheral Chl a/b-binding light- harvesting antenna (LHC I), specifically to the Lhca4 subunit of the LHC I-730 complex. Intramonomeric energy equilibration between pools of Chl b and Chl a in Lhca1 and Lhca4 monomers of the LHC I-730 heterodimer are as fast as the energy equilibration processes within the PS I core. In contrast to the structural stability of the PS I core, the flexible subunit structure of the LHC I would probably determine the observed slow excitation energy equilibration processes in the range of tens of picoseconds. The red pigments in the LHC I are suggested to function largely as photoprotective excitation sinks in the peripheral antenna of PS I. Abbreviations: A 0 – primary electron acceptor in Photosystem I; Chl – chlorophyll; DAS – decay associated spectrum; LHC I – light-harvesting complex I; LHCI-730 – subpopulation of LHC I; Lhca1 and Lhca4 – subunits of the LHC I-730 heterodimer; ND – nondecaying component; PS I – Photosystem I; P 700 – primary electron donor in Photosystem I Introduction Photosynthesis is a fundamental biological process that supplies the Earth’s biosphere with oxygen and highly energetic reducing equivalents for CO 2 assim- ilation (Blankenship 2002). Photosystems I and II in chloroplasts of cyanobacteria, algae and higher plants are the main parts of the molecular photosynthetic ma- chinery. In thylakoid membranes of higher plants and green algae each photosystem (PS) has two structur- ally distinct parts, the core complex and the peripheral (or distal) light-harvesting complex (LHC) (Green 1998). In cyanobacterial PS I and PS II core com- plexes, a lack of membrane-bound peripheral light- harvesting antennas is compensated by an extrinsic phycobilisome complex (MacColl 1998). The function of the chlorophyll (Chl) a-containing core complexes in thylakoid membranes is the capture of solar energy and the efficient delivery of the excitation to the reac- tion center (RC), where this energy is converted into the electrochemical energy of separated charges in a series of transmembrane electron transfer reactions.