Synthesis and photophysical properties of a new amino acid possessing a BODIPY moiety

Article abstract of DOI:10.1016/j.tetlet.2009.03.195

The synthesis of a new amino acid possessing a BODIPY fluorophore, which is of use in peptide synthesis, is described. The influence of the amino acid as well as of benzoxazole moieties on the BODIPY spectral and photophysical properties is discussed. The

Full text of DOI:10.1016/j.tetlet.2009.03.195

Tetrahedron Letters 50 (2009) 2908–2910
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Tetrahedron Letters
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Synthesis and photophysical properties of a new amino acid possessing
a BODIPY moiety
*
Katarzyna Guzow , Kinga Kornowska, Wiesław Wiczk
´
´
Faculty of Chemistry, University of Gdansk, Sobieskiego 18, 80-952 Gdansk, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a new amino acid possessing a BODIPY fluorophore, which is of use in peptide synthesis,
is described. The influence of the amino acid as well as of benzoxazole moieties on the BODIPY spectral
and photophysical properties is discussed. The photophysical properties of this fluorophore were modi-
fied only to a small extent compared to those of the parent compound.
Received 7 January 2009
Revised 12 March 2009
Accepted 27 March 2009
Available online 2 April 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
BODIPY
Unnatural amino acid
Benzoxazolylalanine
Fluorescence probe
4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) deriva-
tives have become popular fluorophores as a result of their valuable
spectroscopic and photophysical properties. These include elevated
photostability, narrow absorption and emission bands in the visible
In this Letter we describe the synthesis and photophysical prop-
erties of an alanine derivative, with a side chain containing a 2-(4-
BODIPY-phenyl)-benzoxazole moiety (Scheme 1). The presence of
an amino acid moiety allows this derivative to be incorporated di-
rectly into a peptide chain enabling a broadening of the range of
its possible application, especially taking into account that a benz-
oxazole moiety may serve as a type of extension arm for BODIPY.
Moreover, the presence of an additional chromophore absorbing
at a shorter wavelength than BODIPY results in the excitation of
the new compound also being possible in the UV range (the absorp-
tion spectrum covers the range from 220 nm to 550 nm). Also, the
introduction of such a large substituent as the benzoxazole moiety
region, a high absorption coefficient (e
> 50,000 MÀ1 cmÀ1) and high
fluorescence quantum yields (often approaching 1.0). Recently, a re-
view on the synthesis and spectroscopic properties of BODIPY and its
derivatives was published by Loudet and Burgess.1 Both the absorp-
tion and emission spectra of BODIPY derivatives are relatively insen-
sitive to the polarity and pH of their environment (except those of
derivatives carrying an amino2a,b or hydroxy substituent2c). Also,
these compounds are reasonably stable under physiological condi-
tions. As a result, they have found many applications in numerous
fields of science and medicine. They are used as sensitizers for solar
cells,2,3 as molecular photonic wires,4 fluorescence sensing materi-
als,2,5 probes for biolabelling6 as well as for laser applications7 and
labelling of nanoparticles.1
BODIPY derivatives can be modified easily by introducing suit-
able substituents at the 3- and 5-positions.2b,5b,6a,8 Alkylation or
arylation at C-8 does not affect the positions of the absorption
and emission maxima significantly,9 except for 4-amino2a or 4-
hydroxyphenyl derivatives.2c BODIPY derivatives with substitu-
ent(s) at C-3 and/or C-5, carrying an amino or carboxy group are
used to label biomolecules.6b Also, it is possible to attach them to
the side chain of an amino acid6c which allows them to be incorpo-
rated into a peptide chain.
extends the p-electron system of this compound significantly and
as a result should modify the spectral and photophysical properties
of the BODIPY fluorophore. Hence, its influence on these properties
was studied.
The synthesis of the BODIPY-containing 2-phenyl-benzoxazol-5-
yl-alanine derivative is outlined in Scheme 1. First, N-Boc-3-[2-
(4-formylphenyl)benzoxazol-5-yl]alanine methyl ester ([(4-CHO)
Ph]Box-Ala) (1) was synthesized according to the procedure pub-
lished previously.10,11 Product 1 was isolated in 10% yield following
silica gel column chromatography. N-Boc-3-[2-(4-(4,4-difluoro-
1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)phenyl)benzo-
xazol-5-yl]alanine methyl ester ((BODIPY)Box-Ala) (2) was
synthesized according to the published procedure for such deriva-
tives2,6a,8b by condensing 1 with 2 equiv of 2,4-dimethyl-1H-pyrrole
in CH2Cl2 under an argon atmosphere at room temperature in the
presence of one drop of TFA as the catalyst, followed by oxidation
with p-chloranil. The product was further reacted with BF3ÁOEt2 in
the presence of triethylamine to afford 2. In addition, the synthesis
* Corresponding author. Tel.: +48 58 5235413/74; fax: +48 58 5235472.
E-mail address: kasiag@chem.univ.gda.pl (K. Guzow).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.195

K. Guzow et al. / Tetrahedron Letters 50 (2009) 2908–2910
2909
Figure 1. Normalized absorption (top) and emission (bottom) spectra of N-Boc-3-
[2-(4-(4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene)phenyl)ben-
zoxazol-5-yl]alanine methyl ester ((BODIPY)Box-Ala) 2 and 8-phenyl-4,4-difluoro-
1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY) 3 in methanol.
The molar absorption coefficients for the new BODIPY analogues
are relatively high [in the range 46,000–73,000 dm3 molÀ1 cmÀ1
(Table 1)]. Also, the emission features of the studied compounds
are typical for BODIPY fluorophores.1 The bands are narrow and
slightly Stokes-shifted with a mirror image shape. The fluores-
cence quantum yields (uf) are high and solvent polarity indepen-
dent. The fluorescence band of 3 is hypsochromically shifted in
contrast to that of 2 which is shifted slightly bathochromically
with increased solvent polarity. The features that differentiate
(BODIPY)Box-Ala 2 from the model compound 3 are the smaller
solvent polarity dependence of the absorption and emission bands
and its higher values of full width at half maximum as well as the
twofold lower fluorescence quantum yield (2 uf = 0.26, 3 uf = 0.52
being comparable to the published value5h). In addition, more dis-
tinct differences between these two compounds are observed for
their fluorescence intensity decays. For compound 3, the fluores-
cence intensity decay in all three solvents studied is described
by a mono-exponential function. The fluorescence lifetime is about
3.2 ns in methanol and acetonitrile whereas in cyclohexane it is
slightly lower (2.42 ns). The presence of the Box-Ala moiety
strongly modifies the fluorescence intensity decay of the BODIPY
core. Only in cyclohexane is the fluorescence intensity decay
mono-exponential with a short lifetime (1.36 ns) whereas in polar
solvents, a bi-exponential function is necessary to describe the
fluorescence intensity decay correctly (Table 1) and the fluores-
cence lifetimes depend on the solvent polarity. In methanol, the
fluorescence lifetimes are 1.57 ns with a contribution in the decay
Scheme 1. Synthesis of N-Boc-3-[2-(4-(4,4-difluoro-1,3,5,7-tetramethyl-4-bora-
3a,4a-diaza-s-indacene)phenyl)benzoxazol-5-yl]alanine methyl ester ((BODIPY)-
Box-Ala 2).
of 8-phenyl-4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-dia-
za-s-indacene (3) was performed using the same procedure as in
the case of compound 2 with 2,4-dimethyl-1H-pyrrole and benzal-
dehyde as substrates. The purified and fully characterized (1H and
13C NMR, IR and mass spectra) BODIPY derivatives were dissolved
in various solvents in order to study their spectral properties.
Figure 1 shows the absorption and steady-state emission spec-
tra of compounds 2 and 3 dissolved in methanol.
The shape of the absorption spectra is similar to those of previ-
ously described BODIPY dyes with a strong absorption band
around 500 nm and a shoulder at shorter wavelength. However,
the absorption spectrum of 2 is bathochromically shifted slightly
(ꢀ2 nm) compared to that of 3. Also, it is a little wider [fwhm (full
width at half maximum) of 2 = 867 cmÀ1, 3 = 791 cmÀ1] and
possesses an additional band around 310 nm as a result of the
benzoxazole moiety absorption.10b The absorption spectra are only
affected slightly by the solvent polarity; the absorption maximum
is only marginally hypsochromically shifted on increasing the
polarity (Table 1). These results are consistent with the general
behaviour of the BODIPY chromophore.1
of
a = 0.940 and 3.08 ns with a = 0.060, whereas in acetonitrile, the

2910
K. Guzow et al. / Tetrahedron Letters 50 (2009) 2908–2910
Table 1
Photophysical properties of BODIPY compounds 2 and 3 in cyclohexane, methanol and acetonitrile
eb
kem
fwhmabs
fwhmem
D
mf
ufg
sh (ns)
ai
v2
R
a
c
d
e
j
BODIPY
Solvent
kabs
(nm)
(dm3 molÀ1 cmÀ1
)
(nm)
(cmÀ1
)
(cmÀ1
)
(cmÀ1
)
k
2
Cyclohexane
Methanol
—
—
515
516
—
867
1155
1343
—
600
0.25 0.02
0.26 0.01
1.36 0.055
1.57 0.043
3.08 0.277
1.66 0.056
3.36 0.253
1.000 0.080
0.940 0.022
0.060 0.026
0.894 0.023
0.106 0.032
1.02
1.08
501
72,600 700
Acetonitrile
500
49,500 400
517
899
1432
680
0.25 0.01
1.01
3
Cyclohexane
Methanol
Acetonitrile
503
498
497
50,600 900
46,300 900
47,600 900
515
511
511
716
791
818
908
932
980
464
492
520
0.49 0.02
0.52 0.03
0.52 0.03
2.42 0.004
3.21 0.004
3.27 0.004
1.000 0.003
1.000 0.003
1.000 0.003
1.08
1.12
1.04
a
Absorption maximum.
b
c
d
e
f
Molar absorption coefficient.
Emission maximum.
Full width at half maximum of the absorption band.
Full width at half maximum of the emission band.
Stokes shift.
g
h
i
Fluorescence quantum yield.
Fluorescence lifetime.
Pre-exponential factor obtained from the fluorescence intensity decay analysis.
The statistical parameter defining the adequacy of the exponential function fitting to the fluorescence intensity decay.
Solubility was too low for correct measurement of the absorption spectrum.
j
k
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lifetimes are 1.66 ns and 3.36 ns and
a = 0.894 and a = 0.106,
respectively. The wider fluorescence band of 2 than that of 3, and
the lower fluorescence quantum yield and dependence on solvent
polarity indicated that for 2, an interaction between the two parts
of the molecule (Box-Ala and BODIPY) exists in spite of the nearly
perpendicular position of both moieties. Also, it seems that this
interaction has a charge transfer (CT) nature,2,5b,5c,8 however, the
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and ꢀ4.5 nm in acetonitrile, Fig. 4 in Supplementary data) and also
the contributions of the fluorescence lifetimes change according to
analysis (the contribution of the shorter lifetime increases whereas
the contribution of the longer lifetime decreases with solution dilu-
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ing interactions were observed for BODIPY derivatives by
Bergström et al.12
In conclusion, the described new compound 2 has spectral and
photophysical properties similar to BODIPY derivatives, however,
its advantage over the types of compound previously described
in the literature is the presence of an amino acid moiety allowing
its direct incorporation into a peptide chain.13–15
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Acknowledgement
8. (a) Rohand, T.; Baruah, M.; Qin, W.; Boens, N.; Dehaen, W. Chem. Commun.
2006, 266–268; (b) Rohand, T.; Qin, W.; Boens, N.; Dehaen, W. Eur. J. Org. Chem.
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K.; Beljonne, D.; Van Averbeke, B.; Van der Auweraer, M.; Boens, N. J. Phys.
This work was supported by the Polish Ministry of Science and
Higher Education under grants BW-8000-5-0141-8 and DS-8351-
4-0132-8.
Chem.
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Supplementary data
9. Chen, Y.; Thakar, R.; Snee, P. T. J. Am. Chem. Soc. 2008, 130, 3744–3745.
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Supplementary data (Experimental details of the synthesis [pro-
cedures, characterization of the compounds] and spectroscopic
measurements [absorption and fluorescence spectra]) associated
with this Letter can be found, in the online version, at doi:10.1016/
j.tetlet.2009.03.195.
}
}
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K.; Wiczk, W.; Rolka, K. Abstracts of the 30th European Peptide Symposium,
Helsinki, Finland, Aug 31–Sep 5, 2008; J. Pept. Sci. 2008, 14, 88 (P20215-030).
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References and notes
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