Ratiometric sensing of lysine through the formation of the pyrene excimer: Experimental and computational studies

Article abstract of DOI:10.1039/c5cc01359c

Several pyrene based fluorescent probes undergo lysine assisted monomer to excimer conversion in a ratiometric manner. Intracellular lysine is detected using fluorescence microscopy. This journal is

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Garcia and D. Das, Chem. Commun., 2015, DOI: 10.1039/C5CC01359C.
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Ratiometric sensing of lysine through the formation of pyrene excimer:
Experimental and computational studies
Sisir Lohar,a Damir A. Safin,b Archya Sengupta,c Ansuman Chattopadhyay,c Jesús Sanmartín
Matalobos,d Maria G. Babashkina,*b Koen Robeyns,b Mariusz P. Mitoraj,*e Piotr Kubisiak,e Yann
Garcia,b and Debasis Das*a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
carboxaldehyde (BAACA) and naphthalene-1-carboxaldehyde
50 (BANA) (Scheme S2 in ESI†). The crystal structures of
BAPA and BAACA were elucidated by X-ray diffraction
(Fig. S1–S4 and Tables S1 and S2 in ESI†). All compounds
have been characterized by ESI-TOF mass spectrometry and
1H NMR spectroscopy (Fig. S5–S10 in ESI†). BAPA exhibits
55 a strong monomer emission at 400 nm along with a weak
excimer emission at 505 nm, while addition of lysine induces
a drastic change of its emission spectrum where the monomer
emission is diminished significantly at the cost of excimer
emission enhancement (Fig. 1) with a clear isoemissive point
60 at 455 nm (Fig. S11 in ESI†).
Several pyrene based fluorescence probes undergo lysine
assisted monomer to excimer conversion in a ratiometric
10 manner. Intracellular lysine is detected under fluorescence
microscope.
Lysine, an essential amino acid in the human body for the
protein synthesis, undergoes catabolism in liver of mammals1
via saccharopine pathway resulting in irreversible glutamate
15 and α-aminoadipate formation, which subsequently faces
deamination and oxidation. Lysine,
a
precursor for
biosynthesis of carnitine, also plays an important role in β-
oxidation of fatty acids.2 High lysine concentration in plasma
and urine indicates congenital metabolic disorders such as
20 cystinuria or hyperlysinemia.3 Thus, the trace level
determination of lysine in biological systems is of great
importance. Among available techniques such as HILIC
column liquid chromatography,4 liquid chromatography,5
amperometric enzyme electrode6 and fluorescence probes7, the
25 latter one being user friendly for biological studies. Despite
existence of few lysine selective fluorescence probes,8 its
detection in biological medium is still challenging. Moreover,
existing probes function at a single wavelength, susceptible to
environmental parameters with a possibility of erroneous
30 results for the quantitative determination. In contrast,
ratiometric fluorescence sensing is more reliable.9 The only
reported pyrene based lysine sensor10 suffers from number of
the mentioned limitations. Pyrene exhibits a well-defined
monomer emission at 370–430 nm along with an efficient
35 excimer emission at about 480 nm.11 The ratio of the excimer
to monomer emission (IE/IM), being highly sensitive to the
conformational change of the pyrene-appended receptors, is
useful for the analytes sensing.12 This inspired us to design
several pyrene derivatives to engineer ratiometric lysine
40 sensing via shifting of the monomer emission to the excimer
form which will be useful for intracellular detection under a
fluorescence microscope.
300
BAPA + Other amino acids
250
BAPA + Lysine
BAPA
200
150
100
50
0
400
450
500
550
600
650
700
 (nm)
Fig. 1 Fluorescent spectra and UV light exposed colour of BAPA (10
µM) in the presence of different amino acids (50 µM) in HEPES buffered
(0.1 M; MeOH:H2O, 1:1 v/v; pH 7.4; λexc = 360 nm) solution (solution:
65 blank (1), isoleucine (2), leucine (3), threonine (4), methionine (5), lysine
(6), phenylalanine (7), tryptophan (8), valine (9), alanine (10), arginine
(11), asparagines (12), aspartic acid (13), cysteine (14), glutamic acid
(15), glutamine (16), glycine (17), ornithine (18), proline (19), serine
(20), tyrosine (21) and cystine (22)).
Two series of probes have been synthesized. The first series
was obtained through the condensation of pyrene-1-
45 carboxaldehyde with benzylamine (BAPA), picolylamine
(PAPA), n-butylamine (NBAPA) and thiophene methyl amine
(PTA) (Scheme S1 in ESI†).13 The second series was
synthesized by reacting benzylamine with anthracene-9-
70
Other amino acids show no changes of the
monomer/excimer emission (Fig. 1). This might be due to the
efficient H-bonding of the ε-amino group of lysine with imine
N-centers of two BAPA molecules bringing two pyrene units
closer to form the excimer (Fig. 2). Although arginine bears
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also a free –NH2 at the far end, an adjacent =NH group
Changes in the UV-Vis spectra are similar as for BAPA (Fig.
restricts the excimer formation. Interestingly, only MeOH
and, less pronounced, EtOH favour the excimer formation
(Fig. 3). It is known that in a dynamic excimer, emission
arises from the pyrene dimer at the excited state whereas it
occurs from the ground state in case of a static excimer.11b In
BAPA, the excitation spectrum monitored at the monomer
wavelength is almost identical to that of the excimer
wavelength, indicating the latter emission from dynamic
S24–S26 in ESI†).
300
250
200
150
100
50
5
BAPA in MeOH
BAPA + Lysine in MeOH
BAPA in EtOH
BAPA + Lysine in EtOH
10 excimer (Fig. S12, left in ESI†).11 On the other hand, the
excitation spectrum of the [BAPA+lysine] adduct monitored
at 505 nm is 20 nm red-shifted in comparison to that measured
at 400 nm, indicating that the former emission originates from
the static excimer (Fig. S12, right in ESI†).11 Thus, chemical
15 species responsible for the 505 nm and 400 nm emissions are
different. The association constant of BAPA with lysine,
derived using the Benesi-Hildebrand equation,14 is 1100 M–1
(Fig. S13 in ESI†). BAPA offers the lowest detection limit for
lysine, 5 × 10–8 M.
BAPA in CH CN
3
BAPA + Lysine in CH CN
3
BAPA in DMF
BAPA + Lysine in DMF
BAPA in DMSO
BAPA + Lysine in DMSO
BAPA in THF
BAPA + Lysine in THF
0
400
450
500
550
600
650
700
 (nm)
Fig. 3 Fluorescent spectra and UV light exposed color changes of BAPA
50 (10 µM) in the absence and presence of lysine (50 µM) in different
solvents (A = CH3CN, B = MeOH, C = EtOH, D = DMF, E = DMSO and
F = THF; 1 = BAPA and 2 = BAPA + lysine).
Lysine causes small fluorescence enhancement for BAACA
and BANA. However no ratiometric sensing have been
55 observed (Fig. S27 and S28 in ESI†). Moreover, fluorescence
life time decay studies reveal that the presence of lysine
enhances the life time of BAPA from 9 ns to 13 ns (Fig. S29
in ESI†).
20
Fig. 2 Probable mechanism of the lysine sensing by BAPA, PAPA, PTA
and NBAPA.
Upon gradual addition of lysine to BAPA, strong
absorbance at 358 nm gradually decreases with concomitant
25 increase of the band at 393 nm (Fig. S14 in ESI†) generating
an isosbestic point at 367 nm. In the presence of lysine, the
red-shift and band broadening of the UV spectrum of BAPA
is attributed to the intermolecular π–π stacking of two pyrene
units to form dimer in the ground state.12b–e This fact is also
30 reflected in the 1H NMR titration of BAPA by lysine (Fig.
S15 in ESI†). Other amino acids do not interfere the
[BAPA+lysine] adduct (Fig. S16 in ESI†). Additionally, the
IE/IM ratio of free BAPA and its lysine adduct slightly
increases under acidic condition. No appreciable lysine
35 sensing is observed below pH 3. This might be due to the
protonation of BAPA (Fig. S17 in ESI†). However, lysine
sensing efficiency increases at higher pH, ranging from 3 to 8.
Close to physiological pH 7.4, the IE/IM ratio reaches
maximum, allowing to chose this pH for the entire studies.
M
D
D-Lys (neutral)
D-Lys (cationic)
40
Although PAPA, PTA and NBAPA show similar lysine
selectivity (Fig. S18–S20 in ESI†), corresponding change in
the IE/IM ratios of their lysine adducts are 3.8, 3.5 and 2.4
compared to 6.9 for BAPA. The association constants of
PAPA, PTA and NBAPA with lysine are 800 M–1/2, 1200 M–
Fig. 4 DFT/BLYP-D3/TZ2P optimized structures of the monomer (M)
60 and dimer (D) of BAPA, and the complex D–Lys. For D–Lys we have
considered both the neutral and cationic lysine molecules. The hydrogen
bonds of the type N∙∙∙H–N in D–Lys are marked by dotted lines.
For deeper insight into the binding interaction between
1/2
45
and 2000 M–1/2, respectively (Fig. S21–S23 in ESI†).
2
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BAPA and lysine, static DFT calculations, based on the ADF
indicate that, considering the binding of the cationic lysine
package15 have been performed using the BLYP-D3/TZ2P 40 with D, the stabilization becomes very high with the
protocol, which nicely describes non-covalent interactions
(including the stacking ones).16 Binding patterns between the
BAPA units as well as between BAPA and lysine have been
elucidated using ETS-NOCV17 as implemented in the ADF
interaction energy being –63.65 kcal/mol (Table S3 in ESI†).
Interestingly, in this case, not the electrostatic (Eelstat = –
51.62 kcal/mol), but the orbital (Eorb = –56.04 kcal/mol)
interaction term appears to be the leading stabilizing term
5
program.15 The lowest ground state energy structures were 45 (Table S3 in ESI†).
obtained for the monomer (M) and dimer (D) of BAPA as
well as BAPA–lysine adducts (D-Lys) (Fig. 4).
The S1 state of M was optimized using the Gaussian 09
program (revision D01).18 The energy difference between S1
The NOCV deformation density contributions (1, 2,
3, 4), along with the corresponding orbital interaction
terms (Eorb(1), Eorb(2), Eorb(3), Eorb(4)) are shown in Fig.
S32 in ESI†. Two major terms (1 and 2) with
10
and S0 is 408 nm (Fig. S30 in ESI†) that nicely fits to the 50 corresponding Eorb(1) = –18.26 kcal/mol and Eorb(1) = –
strong monomer emission band at 400 nm observed
experimentally (Fig. 1).
9.73 kcal/mol, characterize the formation of two hydrogen
bonding bridges of the type N∙∙∙H–N, which connect the NH3
+
fragment of lysine with two stacking layers of BAPA. They
originate from the charge donation from the lone pair of
55 nitrogen centers to the empty *(N–H) of lysine. The third
and the fourth terms (3 and 4), corresponding to the
stabilizations Eorb(3) = –3.17 kcal/mol and Eorb(4) = –2.97
kcal/mol, respectively, exhibit the interaction of NH3 of
lysine with both the o-phenyl Cortho–H bonds and the CH=N
60 fragment of the BAPA unit.
0.8
43.6%, HOMO
39%, HOMO-1
4%, HOMO-1
LUMO+1
LUMO
 = 372.8 nm
f = 0.7102 a. u.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
}
LUMO+2
+
Calculation of the 1H NMR proton shifts upon interaction
of lysine with D, using the GIAO approach as implemented in
the ADF program,15 reveals the qualitative agreement with
experiment (Fig. S15 in ESI†). The o-phenyl and CH=N
65 protons, engaged in the interaction with D, are high-field
shifted by 0.715 and 0.24 ppm, respectively. Moreover, it is
also consistent with the presence of weak Cortho–H∙∙∙H3N+ and
CH=N∙∙∙H3N+ interactions, identified by 3 and 4.
225
250
275
300
325
350
375
400
 (nm)
15
Fig. 5 Simulated TD-DFT/B3LYP/TZP spectrum of D in the gas phase
(top) together with the contours of molecular orbitals (0.03 a. u.) involved
in the dominant transitions (bottom).
70 Fig.
6 Fluorescence microscope images of the MCF7 cells after
incubation for 2 h with lysine (50 µM) (A), followed by addition of
BAPA (20 µM) (B). Cells after incubation for 2 h with BAPA (20 µM)
followed by addition of lysine (50 µM) (C).
20
Results of the ETS-NOCV calculations show that D is
formed due to strong π–π stacking interactions. The calculated
total interaction energy (Etotal) is –17.79 kcal/mol while the
main stabilizing factor is the dispersion contribution
(Edispersion) –29.57 kcal/mol followed by significantly less
In order to gain further insight into the changes of the
75 absorption spectra of BAPA upon interaction with lysine, TD-
DFT calculations have been performed using the ADF
25 important parameters like electrostatic (Eelstat
)
–9.50
program15 for
D and D-Lys (cationic). The simulated
kcal/mol and the orbital interaction (Eorb) –4.46 kcal/mol
(Table S3 in ESI†). Such a significant dimerization energy
suggests that BAPA units in solution can indeed aggregate via
stacking interactions. We have further considered the binding
30 of neutral and cationic forms of lysine with D (Fig. 4). The
binding of cationic lysine is predominantly studied as this
form exists at physiological pH ~7. It was found that the
spectrum for the gas phase structure of D indicates that the
dominant absorption band, having an oscillator strength f =
80 0.7102 a.u., is observed at 372.8 nm. Decomposition of this
transition into the molecular orbitals shows that the absorption
is of the π→π* type and it engages both stacked BAPA rings
(Fig. 5). More importantly, complexation of D with lysine
leads to both decrease in the oscillator strength from f =
85 0.7102 a. u. to f = 0.6534 a. u. as well as to the red-shift of the
dominant π→π* band from 372.8 nm to 402.5 nm (Fig. S33 in
ESI†). These observations are qualitatively in line with the
experimental spectra (Fig. S14 in ESI†). It is interesting to
note that the TD-DFT absorption spectra for D-Lys (neutral)
neutral lysine is rather weakly bonded to
D with the
interaction energy being –9.41 kcal/mol and the electrostatic
35 stabilization is a dominant factor (Table S3 in ESI†). It should
be noted that this binding pattern involves single hydrogen
bonding bridge of the type N∙∙∙H–N between D and lysine
(Fig. S31 in ESI†). More importantly, ETS-NOCV studies
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DOI: 10.1039/C5CC01P35a9gCe 4 of 4
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BAPA was used to image lysine in human breast cancer
cells, MCF7, under fluorescence microscope. BAPA can stain
lysine in living cells without any harm, making it useful to
monitor lysine accumulation in biological systems (Fig. 6).
S. Lohar thanks CSIR for fellowship. Help from CAS
(UGC) is gratefully acknowledged.
70
75
80
85
5
Notes and references
10 aDepartment of Chemistry, The University of Burdwan, 713104, Burdwan,
West Bengal, India. Fax: (+91) 342 2530452; Tel: (+91) 342 2533913;
E-mail: ddas100in@yahoo.com
bInstitute of Condensed Matter and Nanosciences, Molecules, Solids and
Reactivity (IMCN/MOST), Université Catholique de Louvain, Place L.
15 Pasteur 1, 1348 Louvain-la-Neuve, Belgium. Fax: +32(0) 1047 2330;
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† Electronic Supplementary Information (ESI) available: Additional data,
25 Schemes S1 and S2, Fig. S1–S34 and Tables S1–S3. CCDC numbers
931269 (BAPA) and 938292 (BAACA). For ESI and crystallographic
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