ABSTRACT
Efficient charge separation occurring within membrane-bound reaction center proteins is the most important step of photosynthetic solar energy conversion. All reaction centers are classified into two types, I and II. X-ray crystal structures reveal that both types bind two symmetric membrane-spanning branches of potential electron-transfer cofactors. Determination of the functional roles of these pairs of branches is of fundamental importance. While it is established that in type II reaction centers only one branch functions in electron transfer, we present the first direct spectroscopic evidence that both cofactor branches are active in the type I reaction center, photosystem I.
Subject(s)
Electron Spin Resonance Spectroscopy/methods , Photosynthesis/physiology , Photosynthetic Reaction Center Complex Proteins/metabolism , Photosystem I Protein Complex/metabolism , Electron Transport , Iron/chemistry , Models, Chemical , Oxidation-Reduction , Photosynthetic Reaction Center Complex Proteins/chemistry , Photosystem I Protein Complex/chemistry , Photosystem II Protein Complex/chemistry , Photosystem II Protein Complex/metabolism , Sulfur/chemistryABSTRACT
EPR measurements have been carried out on a single crystal of Mn(2+)-doped NH(4)Cl(0.9)I(0.1) at 170-GHz in the temperature range of 312-4.2K. The spectra have been analyzed (i) to estimate the spin-Hamiltonian parameters; (ii) to study the temperature variation of the zero-field splitting (ZFS) parameter; (iii) to confirm the negative absolute sign of the ZFS parameter unequivocally from the temperature-dependent relative intensities of hyperfine sextets at temperatures below 10K; and (iv) to detect the occurrence of a structural phase transition at 4.35K from the change in the structure of the EPR lines with temperature below 10K.
