Red antenna states of photosystem I from cyanobacterium Synechococcus elongatus: a spectral hole burning study V. Zazubovich a , S. Matsuzaki a , T.W. Johnson b , J.M. Hayes a , P.R. Chitnis b , G.J. Small a, * a Department of Chemistry, Ames Laboratory – US Deportment of Energy, Iowa State University, Ames, IA 50011, USA b Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, IA 50011, USA Received 18 May 2001; in final form 18 September 2001 Abstract The existence of at least three low-energy chlorophyll a (Chl a) antenna states is shown for photosystem I (PS I) of Synechococcus elongatus by the use of nonphotochemical hole burning (NPHB) spectroscopy. In addition to the pre- viouslyreportedstatesat708and719nm,itisdemonstratedthatthereisathirdstateat715nm.TheresponsibleChl a moleculesarereferredtoasC-708,C-715andC-719.Forboth Synechococcus and Synechocystis,thelowestenergystate is shown to be characterized by strong electron–phonon coupling (large Huang-Rhys factor S), large permanent dipole moment change ðf  DlÞ, and large linear pressure shift rates attributable to electron exchange of dimeric Chl a. The lowest energy state of Synechocystis is at 714 nm. The properties of the 714 and 719 nm states are very similar, sug- gesting that their dimer structures are also similar. Although the other red antenna states of these cyanobacteria have smaller values for S, f  Dl, and linear pressure shift rate, these are still larger than typically measured for monomeric antenna Chl a. Possible assignments of red absorption bands to particular chlorophyll dimers or trimer are dis- cussed. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction An intriguing aspect of photosystem I (PS I) of green plants, algae and cyanobacteria is the presence of antenna chlorophyll a (Chl a) mole- cules whose S 0 ! S 1 ðQ y Þ absorptions are at lower energy than that of P700, the primary electron donor of the reaction center (RC). The ‘‘red’’ antenna states serve as traps for excitation energy from higher energy antenna states, efficiently transfer energy to the RC at biological tempera- tures [1] and extend absorption further to the red. Thus, they play an important role in the func- tioning of PS I. For this reason and because they absorb to lower energy than P700 which is known to be a strongly coupled Chl a dimer [2,3], the red antenna states have long been of much interest as reviewed in [4]. Important questions are the lo- cations and number of red antenna states in PS I of a given species and whether they are due to Chemical Physics 275 (2002) 47–59 www.elsevier.com/locate/chemphys * Corresponding author. Tel.: +1-515-294-3859; fax: +1-515- 294-1699. E-mail address: gsmall@ameslab.gov (G.J. Small). 0301-0104/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0301-0104(01)00535-3