Temperature Dependence of Light-Induced Absorbance Changes Associated with Chlorophyll Photooxidation in Manganese-Depleted Core Complexes of Photosystem II

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A. A. Zabelin^1*, V. A. Shkuropatova¹, A. Ya. Shkuropatov¹, and V. A. Shuvalov^1,2

¹Institute of Basic Biological Problems, Russian Academy of Sciences, 142290 Pushchino, Moscow Region, Russia; fax: +7 (4967) 33-0532; E-mail: zabelin.bio@gmail.com

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

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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 Q_A. 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 P_D1 and P_D2, 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 P_D1⁺ and P_D2⁺ 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 P_D1 and P_D2 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.

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KEY WORDS: core complex of photosystem II, FTIR spectroscopy, chlorophyll, photooxidation, radical cation

DOI: 10.1134/S0006297915100089

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