Visser Hendrik, Dubé Christopher E, Armstrong William H, Sauer Kenneth, Yachandra Vittal K
Contribution from the Melvin Calvin Laboratory, Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
J Am Chem Soc. 2002 Sep 18;124(37):11008-17. doi: 10.1021/ja020409j.
The IR spectra and normal-mode analysis of the adamantane-like compound Mn(4)O(6)(bpea)(4) (bpea = N,N-bis(2-pyridylmethyl)ethylamine) in two oxidation states, Mn(IV)(4) and Mn(III)Mn(IV)(3), that are relevant to the oxygen-evolving complex of photosystem II are presented. Mn-O vibrational modes are identified with isotopic exchange, (16)O-->(18)O, of the mono-micro-oxo bridging atoms in the complex. IR spectra of the Mn(III)Mn(IV)(3) species are obtained by electrochemical reduction of the Mn(IV)(4) species using a spectroelectrochemical cell, based on attenuated total reflection [Visser, H.; et al. Anal. Chem. 2001, 73, 4374-4378]. A novel method of subtraction is used to reduce background contributions from solvent and ligand modes, and the difference and double-difference spectra are used in identifying Mn-O bridging modes that are sensitive to oxidation state change. Two strong IR bands are observed for the Mn(IV)(4) species at 745 and 707 cm(-1), and a weaker band is observed at 510 cm(-1). Upon reduction, the Mn(III)Mn(IV)(3) species exhibits two strong IR bands at 745 and 680 cm(-1), and several weaker bands are observed in the 510-425 cm(-1) range. A normal-mode analysis is performed to assign all the relevant bridging modes in the oxidized Mn(IV)(4) and reduced Mn(III)Mn(IV)(3) species. The calculated force constants for the Mn(IV)(4) species are f(r)(IV)= 3.15 mdyn/A, f(rOr) = 0.55 mdyn/A, and f(rMnr) = 0.20 mdyn/A. The force constants for the Mn(III)Mn(IV)(3) species are f(r)(IV)= 3.10 mdyn/A, f(r)(III)= 2.45 mdyn/A, f(rOr) = 0.40 mdyn/A, and f(rMnr) = 0.15 mdyn/A. This study provides insights for the identification of Mn-O modes in the IR spectra of the photosynthetic oxygen-evolving complex during its catalytic cycle.
本文展示了与光系统II析氧复合物相关的两种氧化态（Mn(IV)₄和Mn(III)Mn(IV)₃）的类金刚烷化合物[Mn₄O₆(bpea)₄]ⁿ⁺（bpea = N,N-双(2-吡啶甲基)乙胺）的红外光谱和简正模式分析。通过复合物中单微氧桥连原子的同位素交换（¹⁶O→¹⁸O）来识别Mn-O振动模式。基于衰减全反射[维瑟，H.；等人。分析化学。2001年，73卷，4374 - 4378页]，使用光谱电化学池通过对Mn(IV)₄物种进行电化学还原获得Mn(III)Mn(IV)₃物种的红外光谱。采用一种新颖的减法方法来减少溶剂和配体模式的背景贡献，并使用差谱和双差谱来识别对氧化态变化敏感的Mn-O桥连模式。对于Mn(IV)₄物种，在745和707 cm⁻¹处观察到两个强红外带，在510 cm⁻¹处观察到一个较弱的带。还原后，Mn(III)Mn(IV)₃物种在745和680 cm⁻¹处呈现两个强红外带，在510 - 425 cm⁻¹范围内观察到几个较弱的带。进行简正模式分析以确定氧化态的Mn(IV)₄和还原态的Mn(III)Mn(IV)₃物种中的所有相关桥连模式。Mn(IV)₄物种计算得到的力常数为f(r)(IV)= 3.15 mdyn/Å，f(rOr)= 0.55 mdyn/Å，f(rMnr)= 0.20 mdyn/Å。Mn(III)Mn(IV)₃物种的力常数为f(r)(IV)= 3.10 mdyn/Å，f(r)(III)= 2.45 mdyn/Å，f(rOr)= 0.40 mdyn/Å，f(rMnr)= 0.15 mdyn/Å。该研究为在光合析氧复合物催化循环过程中其红外光谱中Mn-O模式的识别提供了见解。
