Synthesis, photophysical, photochemical, and computational studies of coumarin-labeled nicotinamide derivatives

Article abstract of DOI:10.1021/jo2025527

The syntheses and photophysical/photochemical properties of two amide-tethered coumarin-labeled nicotinamides are described. Photochemical studies of 6-bromo-7-hydroxycoumarin-4-ylmethylnicotinamide (BHC-nicotinamide) revealed an unexpected solvent effect. This result is rationalized by computational studies of the different protonation states using TD-DFT with the M06L/6-311+G** method with implicit and explicit solvation models. Molecular orbital energies responsible for the λ_max excitation show that the functionalization of the coumarin ring results in a strong red-shift from 330 to 370 nm when the pH of solution is increased from 3.06 to 8.07. From this MO analysis, a model for solvent interactions has been proposed. The BHC-nicotinamide proved to be photochemically stable, which is also interpreted in terms of NBO calculations. The results provide a set of principles for the rational design of either photostable labeling reagents or photolabile cage compounds.

Full text of DOI:10.1021/jo2025527

Article
pubs.acs.org/joc
Synthesis, Photophysical, Photochemical, and Computational
Studies of Coumarin-Labeled Nicotinamide Derivatives
Pauline Bourbon,† Qian Peng,† Guillermo Ferraudi,†,‡ Cynthia Stauffacher,§ Olaf Wiest,*,†
and Paul Helquist*,†
†Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556,
United States
‡Notre Dame Radiation Research Laboratory, Notre Dame, Indiana 46556, United States
§Department of Biological Sciences, Purdue University, 915 West State Street, West Lafayette, Indiana 47907, United States
S
* Supporting Information
ABSTRACT: The syntheses and photophysical/photochemical
properties of two amide-tethered coumarin-labeled nicotinamides
are described. Photochemical studies of 6-bromo-7-hydroxycoumar-
in-4-ylmethylnicotinamide (BHC-nicotinamide) revealed an un-
expected solvent effect. This result is rationalized by computational
studies of the different protonation states using TD-DFT with the
M06L/6-311+G** method with implicit and explicit solvation
models. Molecular orbital energies responsible for the λmax
excitation show that the functionalization of the coumarin ring
results in a strong red-shift from 330 to 370 nm when the pH of
solution is increased from 3.06 to 8.07. From this MO analysis, a
model for solvent interactions has been proposed. The BHC-nicotinamide proved to be photochemically stable, which is also
interpreted in terms of NBO calculations. The results provide a set of principles for the rational design of either photostable
labeling reagents or photolabile cage compounds.
Among the reasons for gaining an understanding of the
photobehavior of such compounds has been their application to
light activatable cage compounds as a basis for triggering
reactions in cells.12−15 One of the commonly employed
photoremovable protecting groups at cellular transparent
wavelengths higher than 330 nm is the coumarin moiety. 6-
Bromo-7-hydroxycoumarin-4-ylmethyl and 7-(diethylamino)-
coumarin-4-ylmethyl are examples that have been used to
release an amine as a glutamate,7 phosphate as RNA,16 diols,17
carboxylic acids18 and alcohols.19
The bulk of these previous photochemical and photophysical
studies have focused on compounds having C−O bonds such
as ester derivatives as the sites of reactions of interest, whereas
corresponding C−N derivatives such as amides have not
received comparable attention. We have therefore begun a
study of nicotinamide linked by amide-based C−N bonds to
different coumarins. The choice of nicotinamide in this study
has been selected as a model substrate for NAD+. In
comparison with the photocleavable esters, the amides may
be expected to be more stable and may therefore function as
useful photolabeled derivatives for cellular studies of NAD+-
dependent enzymatic reactions. In particular, we have begun
these studies with an investigation of the effects of pH and
INTRODUCTION
■
The photochemistry and photophysics of coumarins have been
well studied.1−5 Substitution on the coumarin ring can have
drastic effects on its photochemical properties. By changing the
substituent on the coumarin ring, one can tune its longest
absorption wavelength λmax.
6,7 Substitution at either the 6- or 7-
position with electron donating groups or heavy atoms induces
a bathochromic shift. For example, conversion of a non-
substituted coumarin to the 6-bromo-7-hydroxycoumarin
derivative shifts the wavelength from 310 to 370 nm.
Furthermore, the effect of the polarity of solvents on the λmax
has been subjected to numerous investigations.8−11 For
example, the solvent effect on the absorption spectra of 7-
(diethylamino)-, 7-(dimethylamino)-, or 7-amino-4-methyl-2H-
chromen-2-one is dependent on the nature of the amino group
(Figure 1). In 4-methylumbelliferone, the λmax in acetonitrile is
lower than in methanol. However, the reasons for these
significant shifts have not been fully understood.
Received: December 17, 2011
Published: February 23, 2012
Figure 1. Structure of different coumarin.
© 2012 American Chemical Society
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a
Scheme 1. Synthesis of Coumarin Derivatives
a
(i) Ethyl 4-chloroacetoacetate, methanesulfonic acid; (ii) NH4OH; (iii) nicotinoyl chloride, DIEA; (iv) SeO2; (v) NaBH4; (vi) pTsCl, py.; (vii)
NaN3; (viii) Pd/C, H2; (ix) nicotinoyl chloride, DIEA.
Figure 2. (A) UV−vis spectra of 4 at different pH. (B) pH profile of 4.
solvent polarity on the photophysics and photochemistry of the
newly synthesized 6-bromo-7-hydroxycoumarin-4-ylmethylni-
cotinamide (BHC-nicotinamide) and N-((7-(diethylamino)-2-
oxo-2H-chromen-4-yl)methyl)nicotinamide (DEACM-nicoti-
namide). We report here the unexpected behavior patterns
that were observed for the BHC-nicotinamide, and we provide
a rationalization for the experimental observations through use
of time dependent density functional theory (TD-DFT)
calculations to predict the λmax with the M06L/6-311+G**
method using two different implicit solvation models, the
conductor-like polarizable continuum model (CPCM) and the
more recent SMD model. We also propose a rationalization for
the experimentally observed solvent effects using explicit
solvent molecules as well as molecular orbital (MO) analysis.
Furthermore, with the use of natural bond orbital (NBO)
calculations, we are able to propose nicotinamide carbamate
derivatives that may be useful compounds for future studies of
new photocleavable agents.
The synthesis of the DEACM-nicotinamide, commenced by
benzylic oxidation of the commercially available 7-dimethyla-
mino-4-methylcoumarin (5) with selenium dioxide, and the
resulting aldehyde was reduced to the alcohol 6 in 50% yield.6
Conversion of the hydroxy derivative to the chloride 7 was
effected in 25% yield using p-toluenesulfonyl chloride.20
Treatment with sodium azide followed by hydrogenation
using palladium on carbon furnished the corresponding
amine derivative, which was subsequently coupled to nicotinoyl
chloride to provided DEACM-nicotinamide (8) in 50% yield.
In principle, this route could be made much shorter if a
reductive amination of the aldehyde derived from the SeO2
oxidation of 5 (step iv) could be effected, but this trans-
formation failed in our hands under several conditions.
Photophysical Characterization. 7-Hydroxy-4-methyl-
coumarin and 3-carboxy-7-hydroxycoumarin are known to
have photophysical properties that are dependent on pH.21,22
To determine the influence of the pH on the newly synthesized
BHC-nicotinamide (4), its pH profile was determined in a
citrate-phosphate buffer and revealed that the absorption
spectrum is strongly dependent on pH (Figure 2A). The
protonated phenol form exhibits a λmax at 330 nm and the
deprotonated phenolate at 370 nm. From this measurement,
the pKa of BHC-nicotinamide (4) was determined to be 5.9
(Figure 2B).
RESULTS AND DISCUSSION
■
Synthesis. The labeled BHC-nicotinamide derivative 4 was
prepared as shown in Scheme 1 by conversion of the
commercially available 4-bromoresorcinol (1) to coumarin 2
via a literature procedure in quantitative yield.18 Conversion to
the desired amine 3 was achieved using aqueous ammonium
hydroxide in 64% yield. Coupling with nicotinoyl chloride was
performed in DMF to give BHC-nicotinamide (4) in 58% yield.
Absorption properties were measured, and extinction
coefficients (ε) were calculated according to the Beer−Lambert
Law. The λmax of 8 in methanol is 380 nm and has an extinction
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coefficient of 24375 M−1 cm−1. Table 1 and Figure 3 show
some unexpected results. Increasing the polarity of the solvent
prediction of the UV−vis spectrum values of these three
possible protonation states of 4. DEACM-nicotinamide 8 was
also investigated to compare the predicted data for the two
systems as shown in Table 2. There are no significant
differences for the predicted spectra using the CPCM and
Table 1. Solvent Effect on 4
a
b
c
solvent
λmax
ε
polarity index P′
Methanol
Acetonitrile
DMSO
372
392
408
330
370
8744
11000
19491
8764
5.1
5.8
Table 2. Calculated UV−Vis Absorption Wavelength λmax
a
(nm)
7.2
d
CPCM/M06L/6-311+G**
pH 3.06
10.2
10.2
d
pH 8.07
13642
entry
structures
H2O
MeOH
MeCN
DMSO
a
b
Maximum absorbance in nm. Extinction coefficient in M−1 cm−1.
Reference 20. Phosphate-citrate buffer.
1
2
3
4
4−
385
329
332
388
386
329
331
387
386
329
331
387
387
330
333
389
c
d
4
4+
8
a
.All single point TD-DFT calculations employed in gas-phase
optimized structures.
SMD solvation models (for the results using the SMD model,
see the Supporting Information). The wavelengths predicted by
the CPCM model were slightly higher than that by the SMD
model. The UV−vis λmax shown in this paper were calculated
using the CPCM model.
As shown in Table 2, the predicted λmax of the cation 4+ and
neutral 4 states were similar to each other in all solvents
studied. These values all fell within a narrow range of 3 nm
centered at about 330 nm. Interestingly, the anion 4− and
neutral 8 have nearly the same λmax values in the range of 385−
389 nm without significant solvent effects. These predictions
indicated that going from the neutral to the anionic state of
BHC-nicotinamide 4 has a much greater impact on the UV−vis
spectra than going from the neutral to the cationic state. These
predicted results provide a good explanation of the
experimentally observed spectra of BHC-nicotinamide 4,
which show a strong red shift from 330 to 370 nm when the
pH of solution is increased from 3.06 to 8.07 as shown in
Figure 2. The anionic 4‑ state dominates in nearly neutral pH
conditions. However, further calculations were needed to
understand the experimentally observed solvent effect that is
not apparent in Table 2.
Despite the difficulties associated with predicting the
unexpected solvent effect, the MO pairs responsible for the
λmax excitation inspired us to seek an understanding of the
predicted spectra. Table 3 summarizes the MO contributions to
the UV−vis λmax for each protonation state of complex 4 (for
similar MO figures for 8, see the Supporting Information). All
of the MOs shown are localized on the coumarin ring, and the
major contributions to the λmax is from the π to the π* orbital of
this ring. The change in protonation state from 4 to 4+ occurs
in a different moiety that has no appreciable contribution to the
HOMO or LUMO, which is the underlying reason for the
smaller change in the HOMO−LUMO gap compared to the
change from 4 to 4− This MO analysis can explain the small
solvent effect seen for the protonation of the nicotinamide ring
from the neutral 4 to the cationic from 4+. Although the
visualized MOs are qualitatively only slightly different for the
four structures, their orbital energies are considerably different
upon going to the anionic state. For example, the π and π*
orbital energies are −0.218 and −0.0979 (Hartree) for neutral
4 and −0.172 and −0.0759 for anion 4−, respectively. These
results show that both the HOMO (π) and LUMO (π*)
energies are increased in the process of the neutral 4
undergoing deprotonation. The HOMO energy (−0.218 to
Figure 3. UV−vis spectra of 4 in different solvents. Blue, citrate-
phosphate buffer pH = 3.06; purple, citrate-phosphate buffer pH =
8.07, red, methanol; orange, acetonitrile; green, DMSO.
red-shifted the absorption maxima of BHC-nicotinamide (4),
which can be explained by the stabilization effect of a polar
solvent on the excited state. Interestingly, in the presence of a
weakly basic buffer, a blue-shift is observed. Normally, we
should have expected to observe a bathochromic shift upon
increasing the polarity of the solvent.
Understanding the Unexpected UV−Vis λmax in
Various Solvents. The results summarized in Table 1 and
Figures 2−3 do not provide direct experimental information on
the protonation states of BHC-nicotinamide 4. As shown in
Scheme 2, neutral BHC-nicotinamide 4 can either lose one
proton from the phenol ring acting as a Brønsted acid to form
an anion 4− or accept a proton on the pyridine ring as a
Brønsted base to form a cation 4+. Theoretical calculations can
be performed to probe these states and assign their absorption
wavelengths. Calculations were initially applied to the
Scheme 2. Possible Protonation States of 4 and Structure of
8
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a
Table 3. MO Pairs Responsible for the λmax Excitation for Each Charge State (Hartree)
a
All orbitals are taken from CPCM/M06L/6-311+G** single point calculations in water solvent. Isovalue for surface = 0.02.
Figure 4. Proposed solvent interactions (bond lengths in Å). All models are based on single-point TD-DFT calculations on each solvent-phase
optimized structure.
−0.172 hartree) increased more than the LUMO energy
(−0.0979 to −0.0759 hartree), which decreased the gap
between this MO pair (0.120 to 0.0961 hartree). These results
provide a rational explanation for the greater influence of the
transformation of the coumarin rings from the neutral form 4
to the anionic form 4−.
explains the predicted λmax red-shift (Figure 4). All predicted
spectra of these models are closely aligned with the
experimental observations and provide a rational explanation
for the solvent effects observed for BHC-nicotinamide 4.
Photochemical Characterization. We next investigated
the photochemical behavior of the C−N bonded coumarin-
labeled nicotinamide derivatives 4 and compared it to the C−O
bonded ester derivative 9, which undergoes cleavage upon
irradiation at 369 nm with a quantum yield of 0.036.23 In stark
contrast, the BHC-nicotinamide 4 is stable upon continuous
irradiation at 406 nm in DMSO. A possible reason is that the
amide bond of BHC-nicotinamide 4 is too stable to undergo
cleavage under the photolysis conditions, while BHC-acetate 9
with a relatively weak ester bond is a more suitable cleavage
substrate. This conclusion is supported by the energies of the
appropriate C−N and C−O σ* bonds obtained by NBO
calculations summarized in Figure 5a. The energy of the
antibonding σ* orbital is higher in 4 (0.323) than in 9 (0.230),
which indicates that the C−N bond in compound 4 is less
susceptible to cleavage than the C−O bond in compound 9.
These results show that the amide derivatives developed in this
study may serve as photostable labeling agents for cellular
studies, whereas the more labile ester derivatives functions are
already established as useful photocage agents for controlled
release of biochemical intermediates.
On the basis of our MO analysis above, we are able to
propose molecular models for possible solvent interactions.
Special attention was obviously given to hydrogen bond
interactions of substituents on the coumarin ring in the protic
solvents that were employed in this study. Anion 4− can form
hydrogen bonds with water and methanol as shown in Figure 4.
The predicted λmax values tend to undergo a blue shift with an
increasing number of hydrogen bonds, because the hydrogen
bonds stabilize the HOMO and increase the gap between the
HOMO and LUMO. This result can also be inferred from
Table 3 for which the HOMO of 4− has greater charge located
on the oxygen compared to the LUMO. Consequently, the H
bonding should stabilize the HOMO to a greater extent than
the LUMO, thus increasing the energy gap. In the aprotic
solvents acetonitrile and DMSO, partial proton dissociation
from neutral 4 may occur, which may be an explanation of an
extra peak observed at ca. 330 nm in the experimental UV−vis
spectrum (Figure 3). Although, there is a weaker hydrogen
bond between anion 4− and DMSO, a large change in the
conformation of anion 4− is induced by this interaction, which
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reaction was quenched with 6 N aq HCl, and the precipitate was
filtered. The solid was triturated in water to yield 1.1927 g (64% yield)
of 3 as an off white solid. IR (KBr) νmax 3230, 3024, 2940, 2669, 1721,
1
1681, 1591, 1353 cm−1. H NMR (300 MHz, DMSO-d6) δ 8.58 (brs,
2H), 8.00 (s, 1H), 6.98 (s, 1H), 6.35 (s, 1H), 4.35 (s, 2H). 13C NMR
(75 MHz, DMSO-d6) δ 159.2, 157.7, 153.5, 148.0, 128.7, 110.5, 110.3,
106.3, 103.1, 37.9. HRMS calcd for C10H9BrNO3 269.9613, found
269.9760.
N-[(6-Bromo-7-hydroxy-2-oxo-2H-chromen-4-yl)methyl]-
nicotinamide, BHC-nicotinamide (4). In a flame-dried flask, 3 (200
mg, 0.65 mmol), nicotinoyl chloride hydrochloride (174 mg, 0.975
mmol), and N,N-diisopropylethylamine (0.62 mL, 2.92 mmol) were
added to anhydrous dimethylformamide (6 mL). After the mixture was
stirred for 2 h, a satd aq sodium bicarbonate was added. The solution
was then evaporated under vacuum, and the resulting solid was
triturated with water. The suspension was filtered to obtain 183 mg
(50% yield) of 4 as beige solid. IR (KBr) νmax 3545, 3315, 3095, 1693,
1658, 1587, 1408 cm−1. 1H NMR (600 MHz, DMSO-d6) δ 9.23 (t, J =
5.4 Hz, 2H), 9.07 (dd, J = 0.6 Hz, J = 2.4 Hz, 1H), 8.73 (dd, J = 1.8
Hz, J = 4.8 Hz, 1H), 8.26 (m, 1H), 7.64 (s, 1H), 7.54 (m, 1H), 5.94 (s,
1H), 5.45 (s, 1H), 4.51 (d, J = 6.0 Hz, 2H). 13C NMR (150 MHz,
DMSO-d6) δ 170.7, 165.4, 162.3, 157.2, 153.1, 152.5, 148.9, 135.5,
129.9, 126.2, 124.0, 115.8, 103.6, 102.3, 99.3, 39.7. HRMS calcd for
C16H12BrN2O4 418.9614, found 418.9619. Mp > 230 °C
Figure 5. Energies of natural antibond orbital (Hartree).
CONCLUSION AND OUTLOOK
■
7-(Diethylamino)-4-(hydroxymethyl)-2H-chromen-2-one (6). In a
flame-dried flask, 7-dimethylamino-4-methylcoumarin (5) (570 mg,
2.46 mmol) and selenium dioxide (414 mg, 3.73 mmol) were placed in
freshly distilled xylene (100 mL). The solution was heated at reflux for
24 h, during which the solution turned dark red. The solution was
cooled to 25 °C, filtered through a pad of Celite, and evaporated.
Ethanol (100 mL) and sodium borohydride (46 mg, 1.23 mmol) were
added to the flask, and the mixture stirred for 4 h. After the solution
was neutralized with 1 N HCl, the solution was extracted three times
with DCM, dried over magnesium sulfate, and evaporated under
vacuum. The residue was purified by flash chromatography (DCM/
acetone 5/1) affording 240 mg (39% yield) of 6 as an orange solid. 1H
NMR (300 MHz, CDCl3). δ 7.32 (d, J = 9.2 Hz, 1H), 6.57 (d, J = 9.2
Hz, 1H), 6.50 (d, J = 2.4 Hz, 1H), 6.27 (s, 1H), 4.84 (s, 2H), 3.40 (q, J
= 7.2 Hz, 4H), 2.39 (s, 1H), 1.20 (t, J = 6.8 Hz, 6H) (lit. 1H NMR).20
4-(Chloromethyl)-7-(diethylamino)-2H-chromen-2-one (7). 6
(100 mg, 0.404 mmol), p-toluenesulfonyl chloride (100 mg, 0.526
mmol), pyridine (97 μL, 1.21 mmol) and dry DCM (5 mL) were
placed in a round-bottom flask and stirred at 25 °C for 48 h under
argon atmosphere. Water was added, and the mixture was extracted
three times with DCM, and the combined organic extracts were dried
over magnesium sulfate and evaporated. The residue was purified by
flash chromatography (pet ether/acetone 15/1) affording 25 mg (24%
yield) of 7 as an orange solid. 1H NMR (300 MHz, CDCl3) δ 7.45 (d,
J = 9.2 Hz, 1H), 6.63 (dd, J = 2.4 Hz, 9.2 Hz, 1H), 6.53 (d, J = 2.4 Hz,
1H), 6.19 (s, 1H), 4.58 (s, 2H), 3.42 (q, J = 7.2 Hz, 4H), 1.21 (t, J =
BHC-nicotinamide (4) and DEACM-nicotinamide (8) have
been synthesized, and upon study of solvent effects on the
UV−vis absorption of BHC-nicotinamide, we found an unusual
photophysical behavior. As the polarity of the solvent increases,
a hypsochromic shift was observed in the case for 4, which is
opposite from the usual bathochromic shift. The experimental
results were rationalized by predicting the λmax using TD-DFT
calculations with the CPCM solvation model. Based on MO
analysis, different molecular models were also proposed for
possible solvent interactions. Based on NBO analysis, we can
suggest future studies of compounds such as BHC-
nicotinoylcarbamate 1024 (Figure 5B), which may be expected
to have photochemical properties that are intermediate
between those of photolabile esters such as 9 and the
photostable amides such as BHC-nicotinamide 4 in this
paper. In preliminary studies, the carbamate C−O σ* bond
energy of compound 10 is calculated to be 0.215 hartree, which
is close to the value of 0.230 hartree for compound 9. Using
NBO and TD-DFT calculations, we anticipate the possibility of
designing related compounds with appropriately tuned wave-
lengths that may function as desired as either photostable
labeling agents or as photolabile cage compounds for cellular
studies.
20
1
7.2 Hz, 6H) (lit. H NMR).
EXPERIMENTAL SECTION
General Methods. All experiments were done in the air unless
N-((7-(Diethylamino)-2-oxo-2H-chromen-4-yl)methyl)-
nicotinamide, DEACM-nicotinamide (8). 7 (50 mg, 0.188 mmol) and
sodium azide (14 mg, 0.216 mmol) were placed in a flask with dry
DMF (1 mL) and stirred at 25 °C for 1 h. Water was added, the
mixture was extracted three times with Et2O, and the combined
organic extracts were dried over magnesium sulfate and evaporated.
The residue was dissolved in methanol (1 mL) and palladium on
carbon (5 mg) was added. The mixture was stirred overnight under
hydrogen and filtered through Celite. The solvent was evaporated to
give 38 mg of 4-(aminomethyl)-7-(diethylamino)-2H-chromen-2-one
as an orange solid. This solid was suspended in dry DMF (1 mL) with
nicotinoyl chloride (42 mg, 0.23 mmol) and DIEA (170 μL, 1.03
mmol), and the mixture was stirred for 3 h. Satd aq sodium
bicarbonate was added, and the mixture was extracted three times with
ethyl acetate, and the combined organic extracts were dried over
magnesium sulfate and evaporated. The residue was triturated with a
mixture of ethyl acetate and hexane and filtered to give 48 mg (60%
yield) of 8 as a pale-yellow solid. IR (KBr) νmax 2972, 1712, 1602,
1526, 1420 cm−1. 1H NMR (600 MHz, CDCl3) δ 9.12 (d, J = 2.4 Hz,
■
otherwise stated. NMR chemical shifts are given in ppm relative to
1
residual solvent peaks: H (7.27 for CDCl3 and 2.5 ppm for DMSO-
d6) and 13C (77.23 for CDCl3 and 39.51 ppm for DMSO-d6). Flash
chromatography was performed using 60 Å silica gel.
Synthesis. 6-Bromo-4-(chloromethyl)-7-hydroxy-2H-chromen-
2-one (2). 4-Bromoresorcinol (1) (5 g, 26.4 mmol) and ethyl-4-
chloroacetoacetate (5.36 mL, 39.7 mmol) were added to
methanesulfonic acid (40 mL) and stirred at 25 °C for 2 h.
Then ice and water were added, the suspension was filtered,
and the solid was dried to obtain 7.63 g (quantitative) of 2 as a
beige solid. 1H NMR (300 MHz, DMSO-d6) δ 11.59 (brs, 1H),
1
7.99 (s, 1H), 6.91 (s, 1H), 6.47 (s, 1H), 4.99 (s, 2H) (lit. H
NMR).18
4-(Aminomethyl)-6-bromo-7-hydroxy-2H-chromen-2-one (3). 6-
Bromo-4-(chloromethyl)-7-hydroxy-2H-chromen-2-one (2) (2 g, 6.9
mmol) was suspended in aq ammonium hydroxide (70 mL). The
solution was degassed, and stirred under nitrogen at 50 °C for 1 h. The
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1H), 8.73 (dd, J = 1.8 Hz, J = 4.5 Hz, 1H), 8.22 (dt, J = 1.8 Hz, J = 7.8
Hz, 1H), 7.47 (d, J = 9 Hz, 1H), 7.40 (dd, J = 4.8 Hz, J = 7.8 Hz, 1H),
6.58 (dd, J = 2.4 Hz, J = 9 Hz, 1H), 6.42 (d, J = 2.4 Hz, 1H), 6.01 (s,
1H), 4.77 (d, J = 5.4 Hz, 2H), 3.39 (q, J = 7.2 Hz, 4H), 1.20 (t, J = 7.2
Hz, 6H). 13C NMR (150 MHz, CDCl3) δ 165.6, 162.4, 156.3, 152.4,
152.2, 150.8, 148.3, 135.3, 125.0, 123.5, 108.9, 106.7, 97.6, 44.7, 40.1,
12.4. HRMS calcd for C20H22N3O3 352.1656, found 352.1643. Mp: 95
°C (dec.).
UV−Vis Spectra. UV−vis spectra were recorded in different
solvents at a concentration of 50 nM of BHC-nicotinamide or
DEACM-nicotinamide. Stock solutions of BHC-nicotinamide at
different pH’s (3.06−8.07) were made in a 100 mM citrate-phosphate
buffer, and UV−vis spectra were recorded immediately. Absorption
spectra were corrected for baseline and dilution.
Computational Methods. All structures were optimized in the
gas phase with the 6-311+G(d,p) basis set25,26 under the M06-L27
level theory of M06 suite functional. No constraints were imposed in
the geometry optimizations. To obtain the calculated λmax values,
single-point calculations with time dependent density functional
theory (TD-DFT)28−32 were performed on the optimized structures
with the same basis set. The TD-DFT calculations were carried out in
both the gas-phase and the solvent-phase to evaluate the description of
the solvent effect on the calculated spectra. Both the conductor-like
polarizable continuum model (CPCM)33,34 and SMD35 solvent
models were employed within the TD-DFT calculations. The explicit
models in Figure 4 are based on single-point TD-DFT calculations on
each solvent-phase optimized structure. Natural bond orbital (NBO)
analysis was employed. The molecular orbitals (MO) were visualized
by Gaussview 5.08. All calculations were done within the Gaussian 09
program.36
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ASSOCIATED CONTENT
■
S
* Supporting Information
Copies of 1H and 13C NMR spectra for all the new compounds,
geometries and energies from the CPCM and SMD solvation
model calculations, structural and photophysical character-
ization of compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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(23) Fedoryak, O. D.; Dore, T. M. Org. Lett. 2002, 4, 3419−3422.
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photolysis at 379 nm.
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Corresponding Author
*E-mail: phelquis@nd.edu; owiest@nd.edu.
Author Contributions
Pauline Bourbon and Qian Peng contributed equally to this
paper.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the Dubois Laboratory in the Department of
Chemistry and Biochemistry at the University Notre Dame for
the use of their UV−vis spectrometer and Notre Dame Center
for Research Computing for the use of computing resources.
P.B. gratefully acknowledges support from the Notre Dame
Center for Rare and Neglected Diseases. C.V.S. gratefully
acknowledges support of this research through NSF grant
MCB-0444247. P.H. is grateful to the National Research
Council of Sweden for the award of the 2011 Tage Erlander
Guest Professorship at the University of Gothenburg and the
University of Stockholm.
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