The Transient Photosystem I EPR Spectra Correlated Spin Pair Concept and Structural Conclusions

For many years it has been known that primary charge separation in photosynthetic reaction centers is associated with spin polarized EPR-signals. Such signals can be detected directly in preparations containing PS I whereas their observation in bacterial reaction centers or PS II requires removal or decoupling of the non-heme iron, which acts as a strong relaxation center. However, their usefullness has been hampered for some time by a lack of understanding of their origin. With the present knowledge of the molecular arrangement of the chromophores in the bacterial reaction center it is obvious that transient EPR-de-tection occurs in coupled spin pairs. With this concept /1–5/ observed transient EPR spectra can be understood satisfactorily /6/. Here, we want to address some of the questions raised in the context of this interpretation and present confirming evidence for the major conclusions reached previously /6/.