FTIR Difference Spectroscopy for the Study of Photosystem I A$_{1 }$Acceper

Photosystem I (PS I) is a protein complex which carries out light-induced charge separation in oxygenic photosynthesis. Phylloquinone acts as the secondary electron acceptor in PS I. The A$_{1}$accepter is of interest because it has the lowest reduction potential of any quinone found in nature. In \textit{Men}B mutant PS I particles from \textit{Synechocystis} sp. 6803, a plastoquinone-9 molecule occupies the A$_{1}$ binding site instead of phylloquinone. Using \textit{men}B PS I particles, it has been shown that it is possible to replace plastoquinone-9 in the A$_{1}$ site with phylloquinone. To probe the molecular properties of phylloquinone and its environment in both the neutral and reduced state, we have used time-resolved step-scan FTIR difference spectroscopy (TRSS FTIR DS) to supply dynamic structural information concerning the electron-transfer cofactor. We have produced time-resolved A$_{1}^{-}$/A$_{1}$ FTIR DS using \textit{men}B mutant PS I particles in which phylloquinone has been reintroduced into the A$_{1}$ binding site. We also have obtained time-resolved A$_{1}^{-}$/A$_{1}$ FTIR difference spectra for \textit{men}B PS I particles that are globally $^{13}$C labeled where $^{12}$C labeled phylloquinone was incorporated into the A$_{1}$ binding site. By incorporating $^{12}$C labeled phylloquinone into $^{13}$C labeled PS I, we are able to identify carbonyl (C=O)-sensitive bands of A$_{1}^{-}$ and A$_{1}$.