ENDOR studies of substituted chlorophyll cation radicals

Cation radicals of substituted chlorophyll (Chl) a molecules were characterized by EPR and ENDOR spectroscopy. The effect of substitutions of the vinyl group at position 3 and the carbomethoxy group at 13(2) as well as the replacement of the central magnesium atom by zinc were investigated. All major hyperfine coupling constants could be determined and assigned to specific molecular positions. Comparison of the experimental results with data from related bacteriochlorophyll (BChl) a cation radicals shows that introduction of the same substitution in Chl a and BChl a causes a similar change of the electron spin density distribution in both radicals. Semi-empirical calculations of the RHF-INDO/SP type were performed on all systems and yielded the isotropic and the anisotropic part of the proton hyperfine coupling tensors. The experimental and theoretical results are in good agreement. The data obtained for the Chl a species investigated are discussed with regard to the structure of the primary donor cation radicals in plant photosystem I and II. (C) 1998 Elsevier Science B.V. All rights reserved.