Electron spin echo envelope modulation spectroscopy in photosystem I

The applications of electron spin echo envelope modulation (ESEEM) spectros copy to study paramagnetic centers in photosystem. I (PSI) are reviewed wit h special attention to the novel spectroscopic techniques applied and the s tructural information obtained. We briefly summarize the physical principle s and experimental techniques of ESEEM, the spectral shapes and the methods for their analysis. In PSI, ESEEM spectroscopy has been used to the study of the cation radical form of the primary electron donor chlorophyll specie s, P-700(+) and the phyllosemiquinone anion radical, A(1)(-), that acts as a low I potential electron carrier. For P-700(+) ESEEM has contributed to a debate concerning whether the cation is localized on a one or two chloroph yll molecules. This debate is treated in detail and relevant data from othe r methods, particularly electron nuclear double resonance (ENDOR), are also discussed. It is concluded that the ESEEM and ENDOR data can be explained in terms of five distinct nitrogen couplings, four from the tetrapyrrole ri ng and a fifth from an axial ligand. Thus the ENDOR and ESEEM data can be f ully accounted for based on the spin density being localized on a single ch lorophyll molecule. This does not eliminate the possibility that some of th e unpaired spin is shared with the other chlorophyll of P-700(+); so far, h owever, no unambiguous evidence has been obtained from these electron param agnetic resonance methods, The ESEEM of the phyllosemiquinone radical A(1)( -) provided the first evidence for a tryptophan molecule pi -stacked over t he semiquinone and for a weaker interaction from an additional nitrogen nuc leus. Recent site-directed mutagenesis studies verified the presence of the tryptophan close to A(1), while the recent crystal structure showed that t he tryptophan was indeed pi -stacked and that a weak potential H-bond from an amide backbone to one of the (semi)quinone carbonyls is probably the ori gin of the to the second nitrogen coupling seen in the ESEEM. ESEEM has alr eady played an important role in the structural charaterization on PSI and since it specifically probes the radical forms of the chromophores and thei r protein environment, the information obtained is complimentary to the cry stallography. ESEEM then will continue to provide structural information th at is often unavailable using other methods. (C) 2001 Published by Elsevier Science B.V.