2Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russia; fax: +7 (495) 651-2191
* To whom correspondence should be addressed.
Received May 15, 2015; Revision received June 19, 2015
Mid-infrared (4500-1150 cm–1) absorbance changes induced by continuous illumination of Mn-depleted core complexes of photosystem II (PSII) from spinach in the presence of exogenous electron acceptors (potassium ferricyanide and silicomolybdate) were studied by FTIR difference spectroscopy in the temperature range 100-265 K. The FTIR difference spectrum for photooxidation of the chlorophyll dimer P680 was determined from the set of signals associated with oxidation of secondary electron donors (β-carotene, chlorophyll) and reduction of the primary quinone QA. On the basis of analysis of the temperature dependence of the P680+/P680 FTIR spectrum, it was concluded that frequencies of 131-keto-C=O stretching modes of neutral chlorophyll molecules PD1 and PD2, which constitute P680, are similar to each other, being located at ~1700 cm–1. This together with considerable difference between the stretching mode frequencies of keto groups of PD1+ and PD2+ cations (1724 and 1709 cm–1, respectively) is in agreement with a literature model (Okubo et al. (2007) Biochemistry, 46, 4390-4397) suggesting that the positive charge in the P680+ dimer is mainly localized on one of the two chlorophyll molecules. A partial delocalization of the charge between the PD1 and PD2 molecules in P680+ is supported by the presence of a characteristic electronic intervalence band at ~3000 cm–1. It is shown that a bleaching band at 1680 cm–1 in the P680+/P680 FTIR spectrum does not belong to P680. A possible origin of this band is discussed, taking into account the temperature dependence (100-265 K) of light-induced absorbance changes of PSII core complexes in the visible spectral region from 620 to 720 nm.
KEY WORDS: core complex of photosystem II, FTIR spectroscopy, chlorophyll, photooxidation, radical cation