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concentrations of the OCP. They observed a small fluorescence quenching
of the ApcE chromophore that increased with OCP concentration. ApcE
is very hydrophobic and nearly insoluble in most buffers. It can be kept in
solution only in the presence of 2M urea and formic acid at pH 2-3. Under
these conditions, ApcE and OCP are both somewhat denatured and this
denaturation could induce an interaction between the OCP and the ApcE
chromophore that does not exist when ApcE is in the phycobilisome.
Tian et al. (2011
,
2012)
used spectrally resolved picosecond fluorescence
measured by a streak-camera to elucidate the site of quenching in the phy-
cobilisome. They studied
Synechocystis
cells (WT, ΔOCP and overexpress-
ing OCP) and isolated phycobilisomes (WT, CK (only the core) and CB
(the core plus rods containing only one PC hexamer)) in quenched and
unquenched states. A compartmental model was constructed to fit the data
and describe excitation energy transfer and trapping using target analysis. All
spectra and most of the transfer rates are nearly identical between cells and
isolated phycobilisomes (
Tian, Gwizdala, et al., 2012
;
Tian, van Stokkum,
et al., 2011
). The fitting of the quenched state used an additional decay rate,
kq, to various compartments and keeping all other rates as for unquenched
samples. Only quenching at the APC
660
compartment led to a satisfac-
tory fit to the data in cells and isolated phycobilisomes (
Tian, Gwizdala,
et al., 2012
;
Tian, van Stokkum, et al., 2011
). In cells, the overall quench-
ing rate was (16 ± 4 ps)
−1
(
Tian, van Stokkum, et al., 2011
). In isolated
WT and CB phycobilisomes, it is (only) somewhat slower (33 ± 3 ps)
−1
and (39 ± 4 ps)
−1
respectively (
Tian, Gwizdala, et al., 2012
). Since only
one of the 66 APC
660
pigments present in the core is directly quenched
by the OCP, in cells, the molecular quenching rate of APC
660
Q
is at most
(240 ± 60 fs)
−1
, which is extremely fast and leads to efficient quenching
(80%). In isolated WT phycobilisomes, the molecular quenching is also
very fast, (500 ± 50 fs)
−1
or faster. Tian et al. proposed that this fast quench-
ing is most probably caused by a charge transfer between APC
660
Q
and the
hECN of OCP in its activated form; however, they did not discard the
possibility that excitation energy transfer (EET) from APC
660
Q
to hECN
could be responsible for quenching (
Tian, Gwizdala, et al., 2012
;
Tian, van
Stokkum, et al., 2011
).
Berera, van Stokkum, et al. (2012)
, based on the
characteristics of the excited states of OCP
r
, prefer the latter hypothesis.
Ultrafast transient absorption measurements of OCP-phycobilisome com-
plexes will be needed to distinguish between these two possibilities.
Kuzminov, Karapetyan, et al. (2012)
used nonlinear laser fluorimetry
to elucidate the site of quenching in the phycobilisome. They measured
