C O M M U N I C A T I O N S
cellular autofluorescence. Relative to the free chelates 8 and 11,
coumarin fluorescence (510 nm) is attenuated in 8Nd and 11Nd
by 85 and 90%, respectively. Phosphorescence measurement of the
ketone/alcohol pair 5 and 11 furnished the emission maxima of
542 and 540 nm. This suggests that the luminescence switching
phenomenon is not likely founded on differences in triplet energies
between the ketone and alcohol chromophores, but rather on the
availability of their emissive excited states.
In summary, this study describes the rational design of NIR
luminescent lanthanide redox reporters. It also introduces a general
platform for transposing a visible fluorescent switch to longer
wavelengths by appending a luminescent lanthanide complex to a
small fluorogenic substrate and demonstrated the viability of this
concept in preserving enzyme substrate fidelity. Complex 5Nd
represents a promising NIR imaging probe for the cellular stress
marker AKR1C2. Coupled with two-photon excitation,12 this probe
will operate in the 800/1060 nm regime, which is ideal for in vivo
optical imaging.
Figure 3. The ketone/alcohol pairs constitute excellent redox switches (off/
on switch in the reduction direction). (A) Luminescence spectra for ketone
2Nd (blue) and alcohol 8Nd (red); it is representative of the trend found
for all examined Nd3+ complexes. (B) Relative intensities of complexes
7-12Nd (integrated intensity from 1020 to 1110 nm, normalized to 8Nd).
5 µM solution in 100 mM phosphate buffer, pH 7, 25 °C, excitation at 395
nm.
Table 1. Kinetic Parameters for AKR1C2
Km
M)
kcat
(min-
kcat/Km
1
1
(
µ
)
(min-1 µM-
)
1Nd
2Nd
3Nd
4Nd
5Nd
6Nd
27.7 ( 2.0
12.2 ( 1.1
4.8 ( 0.6
11.3 ( 1.4
3.8 ( 0.5
3.5 ( 0.4
4.1
2.3
8.4
2.5
5.7
3.0
0.15
0.19
1.75
0.22
1.50
0.86
Acknowledgment. We thank Professor Trevor M. Penning
(University of Pennsylvania School of Medicine) for supplying
human AKR1C2. M.H. is the recipient of the Novartis Graduate
Fellowship in Organic Chemistry. This work was supported by The
G. Harold & Leila Y. Mathers Charitable Foundation (D.S.) and
the NSF through grant CHE-04-15516 (N.J.T.).
Scheme 1. Enzymatic Reduction of Reporter Substrates
Supporting Information Available: Synthesis, characterization,
and emission profiles for complexes 1Nd-12Nd and enzymology
assays. This material is available free of charge via the Internet at http://
pubs.acs.org.
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