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The reaction of the irreversible chemical reduction of the 13(1)-keto C=O group of pheophytin a (Pheo a) with sodium borohydride in reaction centers (RCs) of functionally active spinach photosystem II (PS II) core complexes was studied. Stable, chromatographically purified PS II core complex preparations with altered chromophore composition are obtained in which ~25% of Pheo a molecules are modified to 13(1)-deoxo-13(1)-hydroxy-Pheo a. Some of the chlorophyll a molecules in the complexes were also irreversibly reduced with borohydride to 13(1)-deoxo-13(1)-hydroxy-chlorophyll a. Based on the results of comparative study of spectral, biochemical, and photochemical properties of NaBH4-treated and control preparations, it was concluded that: (i) the borohydride treatment did not result in significant dissociation of the PS II core complex protein ensemble; (ii) the modified complexes retained the ability to photoaccumulate the radical anion of the pheophytin electron acceptor in the presence of exogenous electron donor; (iii) only the photochemically inactive pheophytin PheoD2 is subjected to the borohydride treatment; (iv) the Qx optical transition of the PheoD2 molecule in the RC of PS II core complexes is located at 543 nm; (v) in the Qy spectral region, PheoD2 probably absorbs at ~680 nm.