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[
101
]. A particularly redshifted value (415 nm) is achieved in the S65T/H148D
av
GFP variant, where the D148 carboxyl oxygen forms a very tight H-bond (distance
2.4
˚
) with the chromophore hydroxyl group [
102
].
The emission in variants with a neutral chromophore state is generally achieved
via ESPT; so the emitting state is the anionic chromophore [
18
]. The fluorescence
of mKalama1, peaking at 456 nm, is, however, due to intrinsic emission of the
neutral chromophore [
89
].
5.2 Proteins with RFP Chromophore
Like the anionic
av
GFP chromophore, also the spectral characteristics of the
anionic RFP chromophore are strongly tunable by the protein matrix (Table
6
).
Particular care needs to be put in selecting the right variants for the comparison,
because in some DsRed-derived FPs, the chromophore undergoes covalent mod-
ifications that alter its structure. Another
caveat
concerns the presence of a
trans
(E)
chromophore instead of the more common
cis
(Z) conformation, like in eqFP611
Table 6 Proteins containing the RFP-like anionic chromophore
Name
Environment residues (
DsRed #
)
Ex.
max
(nm)
Em.
max
(nm)
References
dr
16
dr
44
dr
66
dr
70
dr
83
dr
146
dr
163
dr
197
Red
DsRed
V VQKKSKS 83[
6
]
DsRed
K83M
VVQKMSKS 42[
103
]
mStrawberry
(from
DsRed)
V
A
T
K
L
S
M
I
574
596
[
32
]
mCherry
(from
DsRed)
V
A
M
K
L
S
Q
I
587
610
[
32
]
DsRed
V VQKKSKS 83[
6
]
DsRed
K83M
VVQKMSKS 42[
103
]
Far red
mKate (from
eqFP578)
L
M
M
K
F
S
M
R
585
635
[
104
]
mGrape3
(from
DsRed)
T
A
M
K
L
S
M
Y
608
646
[
41
]
mPlum (from
DsRed)
E
A
M
K
L
S
M
I
590
649
[
40
]
mNeptune
(from
eqFP578)
L
G
M
K
F
S
M
R
600
650
[
41
]
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