Imperial College London

Professor Bill Rutherford FRS

Faculty of Natural SciencesDepartment of Life Sciences

Chair in Biochemistry of Solar Energy



+44 (0)20 7594 5329a.rutherford Website




702Sir Ernst Chain BuildingSouth Kensington Campus






BibTex format

author = {Boussac, A and Ugur, I and Marion, A and Sugiura, M and Kaila, VRI and Rutherford, AW},
doi = {10.1016/j.bbabio.2018.02.010},
journal = {Biochim Biophys Acta},
pages = {342--356},
title = {The low spin - high spin equilibrium in the S2-state of the water oxidizing enzyme},
url = {},
volume = {1859},
year = {2018}

RIS format (EndNote, RefMan)

AB - In Photosystem II (PSII), the Mn4CaO5-cluster of the active site advances through five sequential oxidation states (S0to S4) before water is oxidized and O2is generated. Here, we have studied the transition between the low spin (LS) and high spin (HS) configurations of S2using EPR spectroscopy, quantum chemical calculations using Density Functional Theory (DFT), and time-resolved UV-visible absorption spectroscopy. The EPR experiments show that the equilibrium between S2LSand S2HSis pH dependent, with a pKa≈8.3 (n≈4) for the native Mn4CaO5and pKa≈7.5 (n≈1) for Mn4SrO5. The DFT results suggest that exchanging Ca with Sr modifies the electronic structure of several titratable groups within the active site, including groups that are not direct ligands to Ca/Sr, e.g., W1/W2, Asp61, His332 and His337. This is consistent with the complex modification of the pKaupon the Ca/Sr exchange. EPR also showed that NH3addition reversed the effect of high pH, NH3-S2LSbeing present at all pH values studied. Absorption spectroscopy indicates that NH3is no longer bound in the S3TyrZstate, consistent with EPR data showing minor or no NH3-induced modification of S3and S0. In both Ca-PSII and Sr-PSII, S2HSwas capable of advancing to S3at low temperature (198K). This is an experimental demonstration that the S2LSis formed first and advances to S3via the S2HSstate without detectable intermediates. We discuss the nature of the changes occurring in the S2LSto S2HStransition which allow the S2HSto S3transition to occur below 200K. This work also provides a protocol for generating S3in concentrated samples without the need for saturating flashes.
AU - Boussac,A
AU - Ugur,I
AU - Marion,A
AU - Sugiura,M
AU - Kaila,VRI
AU - Rutherford,AW
DO - 10.1016/j.bbabio.2018.02.010
EP - 356
PY - 2018///
SN - 0006-3002
SP - 342
TI - The low spin - high spin equilibrium in the S2-state of the water oxidizing enzyme
T2 - Biochim Biophys Acta
UR -
UR -
UR -
VL - 1859
ER -