Cell adhesion, biomembrane fission and fusion, lipidꢀpro-
tein binding, etc. depend critically on intracellular pH and
the protonation state of the phospholipid headgroup.19,20
The photoactivatable phospholipid probe 3 developed here
could constitute a useful means to study these processes by
19F NMR in addition to the photolabeling approach.
Moreover, 19F NMR has the unique advantage of being
highly sensitive and informative for the study of biological
systems using fluorinated probes21 because of the absence of
natural fluorinated compounds in most biological systems,
the high natural abundance of fluorine, and the wide range
of chemical shifts sensitive to the chemical environment.
In conclusion, we have synthesized novel photoactiva-
table phospholipidic probes containing tetrafluoropheny-
lazido groups either at the polar head via an amine bridge
or in the fatty acid chain via an ether linkage. The lipid-like
amphiphilic characteristics and excellent photochemical
properties of these probes forecast their potential applica-
tion in photolabeling studies of biomembranes. Further-
more, when linked to the amine function at the phospho-
lipid headgroup, the tetrafluorophenylazide chromophore
exhibited protonation state-dependent 19F NMR signals,
which could prove useful in investigating pH-dependent
membrane processes. These are often difficult to study but
are nevertheless involved in important phenomena includ-
ing lipidꢀprotein interactions, membrane fission/fusion,
drug delivery, etc.
Figure 4. 19F NMR spectral recording of 8, 80, and their mixture
at a ratio of 1/1 were recorded in CDCl3.
ppm in free amine 8 and protonated amine 80, respectively
(Figure 4). By mixing 8 with 80 in a 1:1 ratio,16 the 19F
NMR signals at ꢀ145 ppm (8) and ꢀ139 ppm (80) dis-
appeared and coalesced into a single, exchange broadened
NMR signal at ꢀ142 ppm (Figure 4). This clearly indicates
that there is chemical exchange between the two amine
forms 8 and 80, with an exchange rate in the order of the
chemical shift separation (expressed in Hz). Indeed, slow
chemical exchange on the NMR time scale would have led
to the observation of two distinct 19F signals, one due to
the free (ꢀ145 ppm) and the other to the protonated amine
(ꢀ139 ppm). In contrast, fast chemical exchange would
lead to the observation of a single line, the chemical shift of
which would be the average of the chemical shifts of the
two forms (i.e., around ꢀ142 ppm for a 1:1 molar ratio).
Only intermediate chemical exchange rates can yield ex-
change broadened signals similar to those depicted in
Figures 3 and 4.
Acknowledgment. We are grateful for the financial
support from National Mega Project on Major Drug
Development (2009ZX09301-014), ANR “ProKrebs”pro-
ject (07-PCVI-0028), Wuhan University, CNRS, and
French Embassy in China for the short-term fellowship
ꢀ
to support Y.X. We thank Drs. Stephane Viel (Aix-
Marseille University), Fabio Ziarelli (Spectropole,
^
Marseille), Han Chen (Harvard Medical School), and
Mr. Yang Wang (CINaM UPR 3118) for helpful discussions
and critical reading of the manuscript.
It is known that the electron-withdrawing character of
the fluorine atom can affect the pKa values of the corre-
sponding benzylic amines, as shown by the pH back-
titration of the compound 8 and benzyl amine (Figure
S4). This can be interesting as the fluorinated benzylic
amines may also serve as 19F NMR pH indicators for
an investigation on pH dependent biological events.17,18
Supporting Information Available. Experimental pro-
cedures and spectroscopic data for all new compounds
as well as Schemes S1ꢀS3 and Figures S1ꢀS4. This
material is available free of charge via the Internet at
(16) Due to the poor solubility of probe 3 and compound 8 in water,
we could not undertake a reliable pH titration process of these probes
using 19F NMR.
(19) Young, B. P.; Shin, J. J. H.; Orij, R.; Chao, J. T.; Li, S. C.; Guan,
X. L.; Khong, A.; Jan, E.; Wenk, M. R.; Prinz, W. A.; Smits, G. J.;
Loewen, C. J. R. Science 2010, 329, 1085–1088.
(17) He, S.; Mason, R. P.; Hunjan, S.; Mehta, V. D.; Arora, V.;
Katipally, R.; Kulkarni, P. V.; Antich, P. P. Bioorg. Med. Chem. 1998, 6,
1631–1639.
(20) Lahdesmaki, K.; Ollila, O. H.; Koivuniemi, A.; Kovanen, P. T.;
Hyvonen, M. T. Biochim. Biophys. Acta 2010, 1798, 938–46.
ꢀ
(21) Laurent, S.; Chen, H.; Bedu, S.; Ziarelli, F.; Peng, L.; Zhang, C.-
(18) Deutsch, C. J.; Taylort, J. S. Biophys. J. 1989, 55, 799–804.
C. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9907–9912.
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