Biology Reference
In-Depth Information
Fig. 15 Proposed mechanism of the reversible photoswitching mechanism of asFP595, repro-
duced with permission from Schafer et al. [
63
]. QM/MM simulations have led to the proposal that
protonation controls the photoswitching in asFP595. Legend from original figure: “The fluorescent
state Zcis is
highlighted
. The
green arrows
indicate ground state equilibria, whereas the
red
arrows
indicate excited state processes. The major protonation states are the zwitterionic and
the anionic chromophores in the
trans
conformation, and the neutral chromophore in the
cis
conformation, as indicated in the
square bracket
”
In addition, the timescale of the deactivation 320 fs [
98
] would be very fast for the
proposed ESPT reaction [
63
]. A number of theoretical calculations evaluated the
potential energy of excited states of GFP chromophore models in the different proton-
ation states [
96
,
99
]. These suggest that the zwitterions have a conical intersection in a
twisted geometry involving the rotation of only a single bond [
96
].
Li et al. [
90
] performed QM/MM calculations to study the photoswitching in the
related Dronpa protein [
90
]. In contrast to the results for asFP595 [
63
], these
calculations supported the anionic
cis
form for the “on” state and the neutral
trans
conformation for the “off” state of Dronpa [
90
]. The protonation is mentioned
to be of critical importance for the non-fluorescent (dark) state of the
trans
-oriented
chromophores in both asFP595 and Dronpa. It would appear that the general trend
is that
cis
chromophores are bright states and
trans
chromophores are dark states.
However, an exception is the eqFP611 fluorescent protein from
A. sulcata
which
has a
trans
-oriented Met-Tyr-Gly-derived chromophore with a relatively high
fluorescence quantum yield of 0.45 [
103
,
104
]. It is possible that in the
trans
orientation of the eqFP611, chromophore is not in the zwitter-ionic state, which
would support the conclusions reached for asFP595 [
63
,
64
]. Vibrational spectros-
copy information is not yet available to address this issue.
Search WWH ::
Custom Search