Chemistry Reference
In-Depth Information
the
membranes
of organelles, the
internal
aqueous phase, and the
exterior
cell
surface. Probes 14c, 15a, and 15b have absorption and emission profiles that closely
match the Cy5 chromophore in control compound Cy5-Zn-DPA and therefore, they
can be imaged using a standard Cy5 filter set.
The relatively nonpolar squaraine rotaxane 14c was found to interact with cells
in a very similar way to the well-known lipophilic dye Nile Red: this probe rapidly
accumulates at lipophilic sites inside a living cell, such as the endoplasmic reticu-
lum and intracellular lipid droplets [
55
]. The red emission band for probe 14c is
quite narrow and permits the acquisition of
multicolor images
. It displayed
high
chemical stability
and
low toxicity
.
The squaraine rotaxane tetracarboxylic acid 15a is soluble in aqueous solution at
physiological pH and acts as an excellent fluorescent marker with extremely high
photostability, which allows trafficking processes in cells to be monitored in real-
time, with constant sample illumination, over many minutes. This type of
real-time
monitoring
cannot be done with other available NIR fluorescent probes, such as the
amphiphilic styryl dye FM4-64 and water-soluble dextran-Alexa 647 conjugate,
because they are rapidly photobleached.
With the example of stained
E. coli
cells, the squaraine rotaxane 15b containing
a zinc(II)-dipicolylamine (Zn-DPA) ligand, which is known to selectively associate
with the anionic surfaces of bacterial cells, was found to be almost 100 times more
photostable as compared to Cy5-Zn-DPA [
55
]. This can be attributed to stronger
cell-surface affinity of 15b, leading to a slower off rate for the probe. The remark-
able stability of 15b permits fluorescence imaging experiments that are impossible
with probes based on conventional NIR cyanine dyes such as Cy5. Squaraine
rotaxanes are likely to be superior substitutes for conventional cyanine dyes for
biomedical imaging applications that require NIR fluorescent probes.
S
S
O
3
K
O
3
N
N
Zn
2+
(NO
3
−
)
4
N
N
N
O
O
Et
(CH
2
)
5
CONH(CH
2
)
4
O
N
N
N
Cy5-Zn-DPA
Zn
2+
N
NH
HN
O
R
1
R
1
N
R
2
N
N
N
R
2
Zn
2
+
(NO
3
−
)
4
O
N
NH
HN
: R
1
= R
2
=
15a
CH
2
COOH
N
O
O
N
N
Zn
2
+
: R
1
= Et; R
2
=
N
15b
(CH
2
)
2
OCH
2
CONH(CH
2
)
4
O
The
anthracene
-containing tetralactam macrocycles 16a and 16b were found to
have an extremely high affinity for aniline-based squaraine dyes in chloroform [
56
].
Search WWH ::
Custom Search