The spin-polarized EPR spectra at 95 GHz (W-band), 24 GHz
(K-band), and 9 GHz (X-band) of the radical
pair
in highly purified photosystem I particles are presented. The
spectra are analyzed to obtain both
the magnetic parameters of the radical pair as well as the relative
orientation of the two species. From the
analysis, the g-tensor of
is
found to be g
xx
= 2.0062,
g
yy
= 2.0051, and
g
zz
= 2.0022, and it is shown
that
A1 is oriented such that the carbonyl bonds are parallel to
the vector joining the centers of
and
.
The
anisotropy of the g-tensor is considerably larger than that
obtained for chemically reduced phylloquinone in
frozen 2-propanol solution. Possible reasons for this difference
and their implications for the A1 binding site
are discussed. The relative orientation of
and
is
compared with earlier estimates obtained using less
accurate g-values for
.
A comparison with the spectra of
in bacterial reaction centers (bRCs) of
Rhodobacter sphaeroides R-26 in which the nonheme iron has
been replaced by zinc (Zn-bRCs) allows the
structural and magnetic properties of the charge-separated state in the
two systems to be compared. From
the similarity of the two W-band spectra in the region around the free
electron g-value it is clear that the
dipolar vector, z
d, between
P•+ and
/
has a similar orientation relative to P700 in PS I and
P865 in bRCs.
This is compatible with the similar overall structural arrangement
of P700 and P865. In contrast, the
low-field
parts of the two spectra are very different as a result of differences
in the orientation of A1 and QA
with
respect to z
d.
The electron transfer in photosystem I (PS I) from the secondary acceptor A1 to the iron-sulfur centers is studied by X-band transient EPR with a time resolution of approximately 50 ns. Results are presented for a series of different PS I preparations from the cyanobacterium Synechococcus 6301 ranging from whole cells to core particles in which the iron-sulfur centers have been successively removed. In addition, results from PS I preparations from spinach and the cyanobacterium Synechocystis 6803 are presented. In all samples containing iron-sulfur centers, two consecutive spin-polarized EPR spectra are observed. The two signals have previously been assigned to the charge-separated states P700+.A1-. and P700+.(FeS)-, where (FeS) is one of the three iron-sulfur centers, FX, FA, or FB [Bock, C., van der Est, A., Brettel, K., & Stehlik, D. (1989) FEBS Lett. 247, 91-96]. In agreement with this, the second spectrum is not observed in the sample in which the iron-sulfur centers have been removed. For (P700-FX), core particles which do not contain FA and FB, the second spectrum can unambiguously be assigned to the pair P700+.FX-. In all samples containing FX, the transition from the first to the second spectrum occurs with t1/e approximately 280 ns (t1/2 approximately 190 ns) both in the presence and absence of FA and FB, which strongly suggests that this phase reflects electron transfer from A1-. to FX in intact PS I.(ABSTRACT TRUNCATED AT 250 WORDS)
The use of light-induced spin polarization to study the structure and function of type I reaction centres is reviewed. The absorption of light by these systems generates a series of sequential radical pairs, which exhibit spin polarization as a result of the correlation of the unpaired electron spins. A description of how the polarization patterns can be used to deduce the relative orientation of the radicals is given and the most important structural results from such studies on photosystem I (PS I) are summarized. Quinone exchange experiments which demonstrate the influence of protein-cofactor interactions on the polarization patterns are discussed. The results show that there are significant differences between the binding sites of the primary quinone acceptors in PS I and purple bacterial reaction centres and suggest that pi-pi interactions probably play a more important role in PS I. Studies using spin-polarized EPR transients and spectra to investigate the electron transfer pathway and kinetics are also reviewed. The results from PS I, green-sulphur bacteria and Heliobacteria are compared and the controversy surrounding the role of a quinone in the electron transfer in the latter two systems is discussed.
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