Changes of low-frequency vibrational modes induced by universal 15N- and 15C-isotope labeling in S2/S1 FTIR difference spectrum of oxygen-evolving complex

The effects of universal N-15- and C-13-isotope labeling on the low- (650-350 cm(-1)) and mid-frequency (1800-1200 cm(-1)) S-2/S-1 Fourier transform infrared (FTIR) difference spectrum of the photosynthetic oxygen-evolving complex (OEC) were investigated in histidine-tagged photosystem (PS) II core particles from Synechocystis sp. PCC 6803. In the mid-frequency region, the amide II modes were predominantly affected by N-15-labeling, whereas, in addition to the amide II, the amide I and carboxylate modes were markedly affected by C-13-labeling. In the low-frequency region, by comparing a light-induced spectrum in the presence of ferricyanide as the electron acceptor, with the double difference S-2/S-1 spectrum obtained by subtracting the QA(-)/QA from the S(2)Q(A)(-)/S(1)Q(A) spectrum, considerable numbers of bands found in the light-induced spectrum were assigned to the S-2/S-1 vibrational modes in the unlabeled PS II core particles. Upon C-13-labeling, changes were observed for most of the prominent bands in the S-2/S-1 spectrum. Although N-15-labeling also induced changes similar to those by C-13-labeling, the bands at 616(-), 605(+), 561(+), 555(-), and 544(-) cm(-1) were scarcely affected by N-15-labeling. These results indicated that most of the vibrational modes found in the low-frequency spectrum are derived from the coupling between the Mn-cluster and groups containing nitrogen and/or carbon atom(s) in a direct manner and/or through hydrogen bonding. Interestingly, an intensive band at 577(-) cm(-1) was not affected by N-15- and C-13-isotope labeling, indicating that this band arises from the mode that does not include either nitrogen or carbon atoms, such as the skeletal vibration of the Mn-cluster or stretching vibrational modes of the Mn-ligand.