Changes of Low-Frequency Vibrational Modes Induced by Universal15N- and13C-Isotope Labeling in S2/S1FTIR Difference Spectrum of Oxygen-Evolving Complex

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Abstract
The effects of universal 15N- and 13C-isotope labeling on the low- (650−350 cm-1) and mid-frequency (1800−1200 cm-1) S2/S1 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 15N-labeling, whereas, in addition to the amide II, the amide I and carboxylate modes were markedly affected by 13C-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 S2/S1 spectrum obtained by subtracting the QA-/QA from the S2QA-/S1QA spectrum, considerable numbers of bands found in the light-induced spectrum were assigned to the S2/S1 vibrational modes in the unlabeled PS II core particles. Upon 13C-labeling, changes were observed for most of the prominent bands in the S2/S1 spectrum. Although 15N-labeling also induced changes similar to those by 13C-labeling, the bands at 616(−), 605(+), 561(+), 555(−), and 544(−) cm-1 were scarcely affected by 15N-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 15N- and 13C-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.

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