THE TETRANUCLEAR MANGANESE CLUSTER IN PHOTOSYSTEM .2. LOCATION AND MAGNETIC-PROPERTIES OF THE S-2 STATE AS DETERMINED BY SATURATION-RECOVERY EPR SPECTROSCOPY
D. Koulougliotis et al., THE TETRANUCLEAR MANGANESE CLUSTER IN PHOTOSYSTEM .2. LOCATION AND MAGNETIC-PROPERTIES OF THE S-2 STATE AS DETERMINED BY SATURATION-RECOVERY EPR SPECTROSCOPY, Biochemistry, 36(32), 1997, pp. 9735-9746
The spin-lattice relaxation enhancement of the dark-stable tyrosine ra
dical, Y-D(.), by the S-2 state of the O-2-evolving complex (OEC) of p
hotosystem II (PSII) has been measured by using saturation-recovery EP
R spectroscopy. Two forms of the S-2 state have been compared: the mul
tiline EPR signal species in untreated PSII and the altered multiline
EPR signal species in NH3-treated PSII. Previous work has shown that t
he non-single-exponential spin-lattice relaxation kinetics of Y-D(.) i
n S-2-state PSII result from a dipole-dipole interaction with the Mn-4
cluster of the OEC. By taking into account the temperature variation
of the effective magnetic moment of the S-2-state multiline EPR signal
form of the OEC, we provide a quantitative analysis of its temperatur
e-dependent enhancement of the spin-lattice relaxation of Y-D(.). Diff
erent spin states of the Mn-4 cluster in the S-2 state are responsible
for the effect at different temperature regimes: for T less than or e
qual to 10 K, it is the ground spin state (S = 1/2); for T greater tha
n or equal to 30 KI it is the first excited spin state; and at interme
diate temperatures, the contributions of the two spin states are compa
rable. The relaxation enhancement of Y-D(.) is equivalent for both for
ms of the S-2-state multiline EPR signal examined, indicating that the
magnetic properties of the Mn-4 cluster are very similar in the S-2 s
tate for both untreated and NH3-treated PSII. EPR progressive microwav
e-power saturation has also been used to assess the spin-lattice relax
ation properties of the Mn-4 cluster giving the altered S-2-state mult
iline EPR signal in the NH3 derivative of PSII. The Orbach mechanism i
s shown to provide the dominant relaxation pathway; the energy differe
nce between the ground and first excited spin states is estimated to b
e 30 +/- 2 cm(-1), which is very similar to the value found for the S-
2-state multiline EPR signal species in untreated PSII. Below 4 K, the
effectiveness of the S-2-state multiline EPR signal species as a spin
relaxation enhancer of Y-D(.) drops dramatically. This is interpreted
to occur because of temperature-dependent Mn-55 nuclear spin-lattice
relaxation which causes averaging of the effective Larmor frequency of
the S-2-state multiline EPR signal species during the time scale for
spin-lattice relaxation of Y-D(.) because the line shape of the S-2-st
ate multiline EPR signal is dominated by isotropic Mn-55 nuclear hyper
fine splittings, such nuclear relaxation processes allow frequencies i
n near resonance with that of Y-D(.) to be accessed, thereby producing
a greater relaxation enhancement. By using a dipolar model that inclu
des the line shapes of both the Y-D(.) and S-2-state multiline EPR sig
nals, the spin-lattice relaxation enhancement of Y-D(.) is analyzed to
obtain a lower limit of 22 Angstrom for the distance between Y-D(.) a
nd the OEC. Together with recent studies showing a close proximity of
the Mn-4 cluster to Y-Z(.), these results provide further support for
an asymmetric location of the Mn-4 cluster with respect to the two red
ox-active tyrosines in PSII.