Oxidized primary electron donor P740, a special pair of,chlorophyll (Chl) d in photosystem (PS) I reaction center of a newly identified cyanobacterium Acaryochloris marina was studied by EPR and proton ENDOR spectroscopy. EPR and ENDOR spectra of P740(+) were compared with those of P700(+) in PS I reaction center of spinach. The g-factors of the purified Chl a(+) and Chl d(+) in CH2Cl2/THF, P700(+) and P740(+) in PS I reaction centers were determined to be 2.0025, 2.0032, 2.0027 and 2.0028, respectively. Hyperfine coupling constants of 1.9, 2.8 and 3.8 MHz that were detected in the ENDOR spectrum of P700(+) were absent in the ENDOR spectrum of P740(+). These features of P740(+) were mainly ascribed to the difference between the chemical structures of Chl a and Chl d. (c) 2005 Elsevier B.V. All rights reserved.