The Photosynthetic Mn Complex in Its Reaction Cycle: An Attempt to Obtain Pure FTIR Difference Spectra for the Four Transitions Between Semi-Stable S-States and for Q_B Redox Transitions

Abstract

In Oxygenic Photosynthesis, Water Is Oxidized At A Mn₄Ca Complex Bound To The Proteins Of Photosystem Ii (Psii). The Water-Oxidation Cycle Of The Mn Complex May Involve Nine Distinct States Of The Mn Complex (Dau And Haumann 2006), But Only Four Of These Are Stable For Several Seconds (S₂, S₃), Several Hours (₀), Or Fully Dark-Stable (S₁). The Four Transitions Between These Semi-Stable S-States Have Been Studied By A Variety Of Spectroscopic Techniques. Due To The Pioneering Work Of Several Groups, Today Also Ftir Spectroscopy Plays A Prominent Role Frequently In Fruitful Conjunction With Mutagenesis Studies (See, E.G., Studies Of Debus And Coworkers). After Application Of Ns Or μS Flashes Of Saturating Light And Subsequent Ftir Measurements, Difference Spectra Can Be Calculated Which Are Mostly Interpreted As S-State Difference Spectra. However, Potential Complications Come From (I) Acceptor Side Contributions To The Ftir Spectra, (Ii) Psii With A Partially Defect Donorside, And (Iii) Significant Mixing Of S-State Populations Caused By So-Called Miss Events. We Have Measured Ftir Spectra On Psii Membrane Particles In The Intervals Between Laser Flashes Of An Extended Flash Sequence. Taking Into Account Also Complementary Results Obtained By Time-Resolved Prompt And Delayed Fluorescence Measurements, We Attempt A Correction (Or Deconvolution) For (I) To (Iii). These Investigations Aim At A Rationale To Get The Pure S-State Difference Spectra In Ftir Investigations On The Reaction Cycle Of Psii Water Oxidation. First Results Are Presented; Problems And Perspectives Are Discussed.