A. Boussac et al., HIGH-SPIN STATES (S-GREATER-THAN-OR-EQUAL-TO-5 2) OF THE PHOTOSYSTEM-II MANGANESE COMPLEX/, Biochemistry, 37(12), 1998, pp. 4001-4007
The Mn-4 complex which is involved in water oxidation in photosystem I
I (PSII) is known to exhibit two types of EPR signals in the S-2 state
, one of the five redox states of the enzyme cycle: either a multiline
signal (S = 1/2) or a signal at g = 4.1 (S = 3/2 or S = 5/2), The S =
1/2 state can be converted to that responsible for the g = 4.1 signal
upon the absorption of near-infrared (IR) light [Boussac, A., Girerd,
J.-J., and Rutherford, A. W. (1996) Biochemistry 35, 6984-6989]. It i
s shown here that a third state gives rise to signals at g = 10 and 6.
This state is formed by LR illumination of the S = 1/2 state at 65 K,
a temperature where LR illumination leads to the loss of the S = 1/2
signal but to no formation of the g = 4.1 state. On the basis of the c
orresponding decrease of the S = 1/2 state, the new state can be trapp
ed in similar to 40% of the PSII centers. Wanning of the sample above
65 K, in the dark, leads to the loss of the g = 10 and 6 resonances wi
th the corresponding appearance of the g = 4.1 signal. It is suggested
that the IR-induced conversion of the S = 1/2 state into the g = 4.1
state at 150 K involves the transient formation of the new state, The
new state is attributed to a S = 5/2 State of the Mn-4 complex (althou
gh a S value > 5/2 is also a possibility). Spectral simulations indica
te an E/D ratio of -0.05 with \D\ less than or equal to 1 cm(-1). The
resonances at g = 10 and 6 correspond to the g(z) of the +/-5/2 and +/
-3/2 transition, respectively. The temperature-dependent conversion of
this S = 5/2 State into the g = 4.1 state is proposed to be due to re
laxation of the ligand environment around the Mn-4 cluster that leads
to a change in the zero field splitting parameters, assuming an S = 5/
2 value for the g = 4.1 state. The new form of the S-2 state reported
here may explain some earlier data where the S-2 State was present and
yet not detectable as either a S = 1/2 or a g = 4.1 EPR signal.