Synthesis and photostability of methoxycinnamic acid modified cyclodextrins

Article abstract of DOI:10.1016/j.jphotochem.2010.03.016

Various cyclodextrins, alpha, beta and gamma, were esterified with 4-methoxy-, 2,4,5- and 2,4,6-trimethoxycinnamic acids. Upon esterification with β-cyclodextrin, the photostability of 2,4,5-trimethoxycinnamate increased while no improvement was observed

Full text of DOI:10.1016/j.jphotochem.2010.03.016

Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 56–61
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and photostability of methoxycinnamic acid modified cyclodextrins
Thitinun Karpkirda,∗, Supason Wanichweacharungruangb
a Functional Materials Special Research Unit, Department of Chemistry, Faculty of Science, Kasetsart University, 50 Phahonyothin Rd, 10900 Bangkok, Thailand
b Sensor Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Various cyclodextrins, alpha, beta and gamma, were esterified with 4-methoxy-, 2,4,5- and 2,4,6-
trimethoxycinnamic acids. Upon esterification with ␤-cyclodextrin, the photostability of 2,4,5-
trimethoxycinnamate increased while no improvement was observed for 4-methoxycinnamate and
2,4,6-trimethoxycinnamate. However, increase in the photostability of 4-methoxycinnamoyl moiety
could be observed when esterified with ␣-CD and that of 2,4,6-trimethoxycinnamoyl moiety could be
observed after being esterified with ␥-CD. These photostability data together with the 2D NMR analyses
indicated that the 4-methoxycinnamoyl, 2,4,5-trimethoxycinnamoyl and 2,4,6-trimethoxy cinnamoyl
moieties could enter the ␣-CD, the ␤-CD, and the ␥-CD cavities, respectively.
Received 3 January 2010
Received in revised form 15 March 2010
Accepted 28 March 2010
Available online 3 April 2010
Keywords:
Cinnamic acids
Cyclodextrins
UV-filters
© 2010 Elsevier B.V. All rights reserved.
Photoisomerization
1. Introduction
An inclusion complex of 4-hydroxy-3-methoxycinnamic acid
(ferulic acid) in ␣-cyclodextrin not only significantly improved the
Skin damage by effects of sunlight has been known for many
years and it has been well accepted that skin cancer, photoimmun-
suppresion and aging of the skin are caused by UV radiation. UVB is
directly absorbed by DNA in the skin resulting in DNA damage by
dimeric photoproducts between pyrimidine bases [1] while UVA
can trigger the generation of reactive oxygen species which are
harmful to many biomolecules including DNA [2].
The most popular organic UV absorber, 2-ethylhexyl-4-
methoxycinnamate (EHMC), is commonly used as a UVB filter in
commercial sunscreens and many cosmetic formulations. The pho-
toisomerization of trans-EHMC to cis-EHMC causing a decrease
of absorption efficiency was studied both in solutions and for-
mulations [3]. Several studies suggested ways to improve the
photostability of cinnamates [4]. Encapsulation of EHMC into
nanoparticles consisting of polyvinylalcohol was studied for exam-
ple [5–8]. Other cinnamic acid derivatives such as 2,4,5- and
2,4,6-trimethoxycinnamate have also been reported as potential
UVA/B filters [9].
stability of ferulic acid against UVB but also slowed down the skin
penetration of the material [14,15]. However, the inclusion com-
plexes described above, are only held by several intermolecular
noncovalent forces, thus, may easily be dissociated under appropri-
ate conditions. The designing and synthesis of modified CDs have
been shown to improve the original binding ability and increase the
molecular selectivity of parent CDs [16,17]. Coulston et al. reported
a molecular machine, ␤-cyclodextrin modified with trans- and
cis-cinnamide, in which only one isomer exhibited the molecular
recognition and performed work, so the photoisomerization turns
the machine on and off [18]. In our previous work, we reported the
synthesis and photostability of a novel UVB/A filter, 2-ethylhexyl-
2,4,5-trimethoxycinnamate [9].
In this paper, we report the syntheses of modified CDs
by functionalization of their primary hydroxyl rims with vari-
ous methoxycinnamoyl moieties including 4-methoxycinnamoyl,
2,4,5-trimethoxycinnamoyl and 2,4,6-trimethoxycinnamoyl moi-
eties. The behavior of these methoxycinnamoyl moieties was
observed by 2D NMR (ROESY experiment). The effect of CD cav-
ity size to the photostability of methoxy cinnamoyl moiety was
investigated.
Cyclodextrins (CDs) are cyclic oligosaccharides consisting of
6–8 glucopyranoside units (Fig. 1). Many literatures reported the
increased photostability of organic UV-filters by forming inclusion
complexes with CDs [10–12]. Scalia et al. reported that the stability
of EHMC was increased to 26.1% after forming an inclusion complex
with ␤-CD (compared to free EHMC which was 35.8%) [13].
2. Experimental
2.1. Instruments
∗
UV–vis spectra were recorded in a quartz cell (light path 10 mm)
on a UV–vis spectrophotometer (PerkinElmer, CT, USA). All 1H
Corresponding author. Tel.: +66 2 5625555.
E-mail address: fscitnm@ku.ac.th (T. Karpkird).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.03.016

T. Karpkird, S. Wanichweacharungruang / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 56–61
57
1H), 5.10–5.09 (d, J = 4.0 Hz, 7H), 3.96–3.82 (m, 28H), 3.70–3.60 (m,
14H); EIMS calculated for [3−Na+] 1378.20 found: 1377.677.
␤-Cyclodextrin-2,4,6-trimethoxycinnamate (4): (16% yield)
white solid; 1H NMR (400 MHz, D2O) ı 7.04–7.02 (d, J = 10.8 Hz, 1H),
6.33–6.32 (d, J = 4.0 Hz, 2H), 6.12–6.09 (d, J = 12.4 Hz, 1H), 4.01–3.39
(m, 28H), 3.70–3.60 (m, 14H); EIMS calculated for [4−Na+] 1378.20
found: 1378.199.
␥-Cyclodextrin-2,4,6-trimethoxycinnamate (5): (30% yield)
white solid; 1H NMR (400 MHz, D2O) ı 7.82–7.77 (d, J = 16.0 Hz, 1H),
6.56–6.52 (d, J = 16.0 Hz, 1H), 6.22 (s, 2H), 5.15–5.14 (d, J = 4.0 Hz,
8H), 3.99–3.88 (m, 41H), 3.71–3.63 (m, 16H); EIMS calculated for
[5−Na+] 1540.34 found: 1539.617.
Fig. 1. General structure of cyclodextrins.
2.4. Photostability test
NMR spectra were obtained in D2O or DMSO-d6 on a Varian Mer-
cury NMR spectrometer, which operated at 400.00 MHz for 1H and
100.00 MHz for 13C nuclei (Varian Company, CA).
The photostability of each compound was tested in 10%
DMSO–H2O. The fresh sample solution was irradiated by using a
broadband UVB lamp (Daavlin, OH) at room temperature. UV inten-
sity was measured by using a UVB power meter (Optix Tech, Ltd.,
DC). UV absorption profiles of the irradiated samples were ana-
lyzed on a UV/vis spectrometer. Photostability of each compound
was expressed as percent relative absorbance at the maxi-
mum absorption wavelength of the compound. All experiments
were done in triplicate. Percent photostability was calculated as
2.2. Materials
␣-Cyclodextrin was purchased from Sigma–Aldrich (Steinheim,
Germany). ␤-Cyclodextrin and ␥-cyclodextrin were purchased
from Thai Isekyi Co. Ltd. All benzaldehydes, pyridine and mal-
onic acid were purchased from Fluka Chemical Company (Buchs,
Switzerland). Piperidine and 4-(dimethylamino) pyridine (DMAP)
were purchased from Sigma (Sigma Chemical Co., Steinheirg,
Germany). Solvents used in syntheses and spectroscopic tech-
niques were reagent or analytical grades purchased from Labscan
(Bangkok, Thailand).
ꢀ
ꢁ
absorbance of irradiated sample
absorbance of unirradiated sample
follows:% Photostability = 100 ×
3. Results and discussion
3.1. Synthesis
Five cinnamoyl modified CDs, including 4-methoxycinnamoyl-
␣-cyclodextrin (1), 4-methoxycinnamoyl-␤-cyclodextrin (2),
2,4,5-trimethoxycinnamoyl-␤-cyclodextrin (3), 2,4,6-trimethoxy-
cinnamoyl-␤-cyclodextrin (4) and 2,4,6-trimethoxycinnamoyl-
␥-cyclodextrin (5) were successfully obtained by esterification
reaction between cinnamoyl chloride and cyclodextrin (Scheme 1)
[16]. The crude products were easily purified by recrystallization in
acetone (see the characterization details in Section 2). It has been
known that the CDs’ hydroxyl group at the 6-position is a primary
hydroxyl group and the most basic (and often most nucleophilic)
while the hydroxyl group at the 2-position is the most acidic, and
that at the 3-position is the most inaccessible and under normal
circumstances, an electrophilic reagent should be attacked from
the 6-position of the hydroxyl group [16,17,21]. In this work, it
was, thus, assumed that all esterification were localized at the
6-hydroxyl groups of the CDs [20–22].
2.3. Synthesis
All five cyclodextrin-methoxy cinnamates were carried
out using the modified method of Chan et al. [16]. Trans-
methoxycinnamic acid (75 mM) was prepared as previously
described [19] and the obtained acid was dissolved in
dichloromethane (25 mL) under
a nitrogen atmosphere. The
suspension was cooled in an ice bath before oxalylchloride
(80 mM) was added. The reaction mixture was heated up to room
temperature and stirred for 1 h. Then the solvent was removed
under reduced pressure. Crude cinnamoyl chloride was added
dropwise to a 25 mL anhydrous DMF solution containing CD
(5 mM), 4-(dimethylamino)-pyridine (1 mM) and distilled pyridine
(7.5 mL) at 0 ◦C under a nitrogen atmosphere and stirred overnight.
Then the reaction mixture was added dropwise to cold acetone
(300 mL), while stirring. A white precipitate was collected by
filtration. The white solid was then dissolved in water (5 mL) and
added dropwise to MeOH (300 mL). This procedure was repeated
one more time and a white precipitate was again collected by
filtration and dried under vacuum.
3.2. UV absorption of cinnamoyl modified cyclodextrins
The absorption properties of all cinnamoyl modified CDs were
similar to their parent cinnamic acids. The UVB (280–320 nm)
absorption was observed in 1, 2, 4 and 5 while both UVA
(320–400 nm) and UVB absorptions were detected in 3 (350 and
290 nm) (Fig. 2). The orientation of each cinnamoyl moiety could be
interpreted from how the solvents affected its light absorption pro-
file. The UV absorption spectra of 4 in 10% aqueous DMSO showed a
slight red shift compared to that in the DMSO solution, suggesting
that 2,4,6-trimethoxycinnamoyl moiety was outside the ␤-CD cav-
ity, thus was stabilized by a more polar solvent (water). In contrast,
the absorption of 1, 2, 3 and 5 in 10% aqueous DMSO showed a blue
shift compared to those in the DMSO solution. As a result, it was
speculated that these methoxycinnamoyl moieties were located
inside the CD cavities which were of more hydrophobicity envi-
ronment. In the case of 2, the similarity between the absorption
spectrum in DMSO and that in 10%DMSO suggested that the 4-
␣-Cyclodextrin-4-methoxycinnamate (1): (15% yield) white
solid; 1H NMR (400 MHz, D2O) ı 7.80–7.77 (d, J = 8.8 Hz, 2H),
7.79–7.75 (d, J = 16.0 Hz, 1H), 7.21–7.19 (d, J = 8.0 Hz, 2H), 6.46–6.42
(d, J = 16.0 Hz, 1H), 5.06–5.05 (d, J = 4.0 Hz, 1H), 3.97–3.87 (t, 24H),
3.68–3.64 (t, 6H), 3.58–3.57 and 3.56–3.55 (d, J = 4.0 MHz, 12H);
EIMS calculated for [1−Na+] 1156.00 found: 1155.524.
␤-Cyclodextrin-4-methoxycinnamate (2): (55% yield) white
solid; 1H NMR (400 MHz, 50% DMSO-d6:D2O) ı 7.91–7.86 (d,
J = 16.8 Hz, 1H), 7.87–7.85 (d, J = 8.0 Hz, 2H), 7.30–7.28 (d, J = 8.0 Hz,
2H), 6.71–6.67 (d, J = 16.0 Hz, 1H), 5.21–5.20 (d, J = 4.0 Hz, 7H), 4.11
(s, 3H), 4.03–3.91 (m, 28H), 3.79–3.72 (m, 14H); EIMS calculated
for [2−Na+] 1318.14 found: 1317.50.
␤-Cyclodextrin-2,4,5-trimethoxycinnamate (3): (73% yield)
pale yellow solid; 1H NMR (400 MHz, D2O) ı 7.84–7.80 (d,
J = 16.0 Hz, 1H), 7.06 (s, 1H), 6.67 (s, 1H), 6.36–6.32 (d, J = 16.0 Hz,

58
T. Karpkird, S. Wanichweacharungruang / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 56–61
Scheme 1. Synthesis pathway of 1–5.
methoxycinnamoyl moiety probably kept moving in and out the
␤-CD cavity. This was because a large size of ␤-CD cavity did not fit
to 4-methoxycinnamoyl moiety.
1–5, ROESY experiments were performed. The ROESY spectrum
of 4-methoxy-cinnamoyl-␣-CD (1), in a D2O solution, displayed
clear NOE correlations between the H3 protons of ␣-CD and the
ꢀ
Hb/Hb proton of 4-methoxycinnamoyl moiety (cross-peaks A), as
ꢀ
3.3. Photostability studies
well as between the H5 protons of ␣-CD and both Ha/Ha and
ꢀ
Hb/Hb protons of the 4-methoxycinnamoyl moiety (cross-peaks
Photostability of cinnamoyl moieties of the inclusion complexes
was conducted in a 10% aqueous DMSO solution (Fig. 3). Com-
pound 3 was more photostable than free 2,4,5-trimethoxycinnamic
acid, thus, implying that the 2,4,5-trimethoxycinnamoyl moiety
was inside the ␤-CD cavity. Compounds 1 and 5 were also more
photostable than their corresponding free 4-methoxycinnamic
acid and 2,4,6-trimethoxycinnamic acid, thus suggesting that both
4-methoxycinnamoyl moiety and 2,4,6-trimethoxycinnamoyl moi-
ety in 1 and 5 were inside the ␣-CD cavity and ␥-CD cavity, respec-
tively. In contrast, the facts that no improvement was observed in
2 and 4 comparing to their corresponding free 4-methoxycinnamic
acid and free 2,4,5-trimethoxycinnamic acid, implied that both 4-
methoxycinnamoyl and 2,4,5-trimethoxycinnamoyl moieties in 2
and 4 were not in the ␤-CD cavity.
C and B, Fig. 4a). These correlations indicated that the ring of 4-
methoxycinnamoyl moiety entered the CD cavity (Fig. 4b), thus
confirming the above conclusion which was drawn from the pho-
tostability information.
Because of the poor solubility in water of 4-methoxycinnamoyl-
␤-CD (2), the conformation could not be investigated by using the
ROESY spectrum even in the presence of 10% DMSO.
The ROESY spectrum of 2 in DMSO-d6 showed no appreciable
NOE correlations between interior protons of the ␤-CD cavity and
the protons of the 4-methoxycinnamoyl moiety, indicating that the
4-methoxy-cinnamoyl moiety was entirely located outside of the
␤-CD cavity. Thus the interaction in water could not be concluded
with the present information. In fact, DMSO has been shown to act
as a competing guest molecule and could decrease the formation
of inclusion complexes through non-specific solvent effects [23].
The ROESY spectrum of 2,4,5-trimethoxy-cinnamoyl-␤-CD (3)
in a D2O solution (Fig. 5a) showed a NOE correlation between the
H3 proton of ␤-CD and the Hb proton of 2,4,5-trimethoxycinnamoyl
moiety (cross-peak B). In the same way, cross-peak A was assigned
In addition to the use of photostability information to spec-
ulate the configuration of cinnamoyl moieties in compounds
Fig. 3. Photostability of 1 × 10−5
M 1–5 in a 10% DMSO solution compared
to their parent cinnamic acids; pCA = 4-methoxycinnamic acid, 245CA = 2,4,5-
trimethoxycinnamic acid and 246CA = 2,4,6-trimethoxycinnamic acid.
Fig. 2. Absorption spectrum of 1–5 (1 × 10−5 mM) in 10% DMSO (solid line) and
DMSO (dashed line).

T. Karpkird, S. Wanichweacharungruang / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 56–61
59
Fig. 5. (a) 1H ROESY spectrum of 3 (0.03 mM) in a D2O solution at 27 ◦C with a mixing
time of 800 ms. (b) Possible structure of 3.
Fig. 4. (a) 1H ROESY spectrum of 1 (0.03 mM) in a 10% DMSO-d6-D2O solution at
27 ◦C with a mixing time of 800 ms. (b) Possible structure of 1.
over the range of the concentrations tested. This result indicated
that an intermolecular complexation via the cinnamoyl group could
be formed at higher concentrations [24,25]. The concentration
dependence NMR experiments of 1 and 3 were also investigated
in DMSO-d6 at even higher concentrations (due to a better solu-
bility of the materials in DMSO, see supporting information, Figs.
S18–19). The upfield shift of all cinnamoyl protons and the broad-
to the intermolecular NOE correlation between the H5 proton of
␤-CD and the Ha proton of 2,4,5-trimethoxycinnamoylmoiety. This
indicated a good intramolecular inclusion complexation (Fig. 5b).
The concentration dependent of NMR was studied for 3 in D2O
(Fig. 6). The upfield shift of all cinnamoyl protons were detected
Fig. 6. 1H NMR spectra of 3: (a) 0.07 (b) 0.15 and (c) 0.2 mM in 10% DMSO-d6-D2O 27 ◦C.

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T. Karpkird, S. Wanichweacharungruang / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 56–61
cinnamoyl moiety (8.30 Å) was larger than the cavity size of ␤-
CD. This explained why the whole cinnamoyl moiety could not fit
into the ␤-CD cavity. In the case of 2,4,6-trimethoxycinnamoyl-
␥-CD (5), 10% DMSO-d6 was added to the D2O suspension due to
its poor solubility. The ROESY of 5 in 10% DMSO-d6-D2O showed
no appreciable NOE correlation between the interior protons of the
␥-CD cavity and the protons of the 2,4,6-trimethoxycinnamoyl moi-
ety, suggesting poor inclusion complexation. However, it should be
kept in mind that the spectrum was taken in a 10% DMSO-d6-D2O
mixture. The concentration dependence 1H NMR of 5 in 10% DMSO-
d6-D2O showed no change over the range of the concentrations
tested, thus, indicating that 5 did not undergo any intermolecular
complexation [24].
Characterization of the complexes in a solid state by DSC
showed that upon forming the inclusion complexes with CDs, the
methoxycinnamic acids melting peak disappeared, suggesting no
free methoxycinnamic acid remained in the products. On the con-
trary, a broad peak near 250 ◦C was prominent in the thermograms
due to the cinnamoyl modified cyclodextrins melting.
4. Conclusion
Five modified CDs with different methoxycinnamoyl moieties
(1–5) were synthesized in moderate yield. Their conformations
were investigated by spectroscopic techniques. A photostability
study of methoxycinnamoyl moieties attached to the cyclodex-
trins indicated that size matching played an important role in the
molecular recognition process of the methoxy cinnamoyl modified
cyclodextrins. In other words, the smallest 4-methoxycinnamoyl
moiety was more photostable in the small ␣-CD cavity, while
the largest 2,4,6-trimethoxy cinnamoyl was more photostable in
the large ␥-CD cavity. The 2,4,5-trimethoxycinnamoyl moiety best
fitted into the moderate size of the ␤-CD cavity. Inclusion com-
plexation could improve photostability of the cinnamoyl moieties
in an aqueous solution. Moreover, these complexes have been eval-
uated for safety by using human cancer cells (see the result in
supporting information). The results indicated that methoxycin-
namoyl modified cyclodextrins are not cytotoxic compared to their
parent methoxycinnamic acids and shows promise to be applied in
cosmetic technology.
Fig. 7. Proposed structure of supramolecular structured of 1 and 3.
ening of resonance peaks of CD protons were clearly observed over
the range of the concentrations tested. This result indicated the
formation of some intermolecular complexes or polymers in DMSO
solution (Fig. 7). In fact, this intermolecular complexation has been
proposed for 6-aminocinnamoyl-␣-CD in aqueous solution [20].
However, the 2,4,6-trimethoxycinnamoyl-␤-cyclodextrin (4)
showed a particular NOE correlation (cross-peak A) between the
ꢀ
Ha and Ha protons of 2,4,6-trimethoxycinnamoyl moiety and
the H5 proton of ␤-CD in an aqueous solution (Fig. 8a). This
result suggested that only half of the aromatic ring of 2,4,6-
trimethoxycinnamoyl moiety was inside the ␤-CD cavity (Fig. 8b).
A simple calculation by semi-empirical AM1 indicated that the dis-
tance between the two methoxy substituents of 2,4,6-trimethoxy-
Acknowledgements
This project was supported by Faculty of Science Kaset-
sart University, Thailand Research Fund (TRF-MRG5180279) and
Kasetsart University Research and Development Institute (KURDI-
5110507000/2551). The authors also thank Assist. Prof. Dr. Tanapat
Palaka and Sakulna Wangtong, Department of microbiology, Fac-
ulty of Science, Chulalongkorn University for the cytotoxic test
experiment.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.03.016.
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