Biology Reference
In-Depth Information
Collectively, these disparate modes of modulation enable the construction of
fluorogenic molecules from almost any fluorophore scaffold.
4. COUMARINS
4.1. Overview
Figure 1.2
shows examples of coumarin-based fluorophores and fluorogenic
compounds. The coumarin framework is a privileged structure in organic
chemistry and is found in many natural products and pharmacological
agents. Molecules bearing a heteroatom at the 7-position of coumarins
are widely used fluorophores. 7-Hydroxycoumarin (i.e., umbelliferone) is
a natural product found in the
Umbelliferae
family (e.g., carrots). To
improve chemical stability, a methyl group is often added to yield
4-methylumbelliferone (4-MU,
10
). The phenolate form of
10
is highly
fluorescent, exhibiting strong absorption in the UV (
l
¼
360 nm,
abs
10
4
M
1
cm
1
), emission in the blue (
e
¼
1.7
l
¼
450 nm), and good
em
0.63).
24
The p
K
a
of 4-MU is around 7.8, making it
largely protonated (and less fluorescent) at neutral pH. Thus, the p
K
a
of
7-hydroxycoumarins is often modified by attachment of halogen substi-
tuents to improve performance at physiologically relevant pH values.
24
quantum yields (
F
¼
HO
OO
H
2
N
OO
N
OO
N
OO
10
11
12
13
Enzyme substrates
Photoactivatable fluorophores
O
OO
2-
O
3
PO
OO
-
O
3
SO
OO
O
OO
O
NO
2
14
15
16
CO
2
H
24
HO
OH
N
O
OO
S
O
OO
OEt
O
N
6
O
O
17
18
CO
2
H
Indicators
OH
HO
F
O
OH
O
OO
TIPSO
OO
O
O
OO
HO
O
F
N
19
25
20
CF
3
Peptide
OO
O
O
N
OO
O
N
OO
21
N
OO
P
CO
2
CH
3
O
23
O
26
22
Figure 1.2 Fluorogenic molecules based on the coumarin scaffold.
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