FT IR SPECTROSCOPY AND MOLECULAR MODELLING OF
BENZOCAINE / β -CYCLODEXTRIN INCLUSION COMPLEX
I. Bratu1, A. Hernanz2, Alexandra Olaru3
1
National Institute for R&D of Isotopic and Molecular Technologies, P.O Box 700, R-400293
Cluj-Napoca, Romania
2
UNED, Depto de CC y TT Físico Quimicas Senda del Rey 9, 28040 Madrid, Spain
3
"Babes-Bolyai" University, Faculty of Physics, 1 Kogalniceanu st., Cluj-Napoca, Romania
Abstract:
Benzocaine is a local anesthetic used for different pain treatments. Including it in
various cyclodextrins one can increase the solubility of this drug. Several inclusion compounds
of β-cyclodextrin (β-CD) with benzocaine, prepared by kneading and coprecipitation methods,
were investigated. In order to understand the inclusion process mechanisms FT IR
spectroscopic investigation and molecular mechanics calculations were used. The intensity
changes and frequency shifts for several molecular vibrations indicate the molecular groups
implied in the complexation process. The aromatic ring enters inside cyclodextrin cone, its
motional freedom being restricted. The hydrogen bonds play an important role in the inclusion
process. MM+ molecular mechanics calculations established the spatial geometry of this
inclusion complex. The para-amino group is located near the primary O-H groups (the narrow
rim) of the cyclodextrin whereas the ester group is situated near the secondary O-H groups (the
larger rim) belonging to the CD truncated cone.
Keywords: IR spectroscopy, inclusion complexes, cyclodextrin, benzocaine
INTRODUCTION
Benzocaine (BZC, molecular formula C9H11NO2, CAS Registry
Number 94-09-7) (Synonyms: 4-aminobenzoic acid ethyl ester; ethyl 4aminobenzoate; ethyl p-aminobenzoate; ethyl p-aminophenylcarboxylate; paminobenzoic acid ethyl ester) is a topical anesthetic. It numbs wherever it is
applied. It is widely used in first aid creams and sunburn remedies. Benzocain
is an ester, a compound made from p-amino benzoic acid (PABA) and ethanol:
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BZC cross-reacts with other benzoic acid derived local anaesthetics
(both topical and injectable forms). It also has the potential to cross-react with
p-phenylenediamine found in permanent hair dyes, sulfonamides,
sulfonylureas, PABA-based sunscreens, and thiazide-related diuretics. It is used
in pharmaceutical preparations and rarely cosmetics. Doctors and dentists use
BZC preparations, especially on mucosal surfaces such as the mouth, to prepare
or 'numb' a site for injection. BZC eardrops are used for temporary relief of ear
pain. BZC will not treat an ear infection. Antibiotics are still needed if ear pain
is due to an infection. It is used also for topical anaesthesia in a wide variety of
clinical situations including mucous membrane anesthesia prior to endoscopic
examination or instrumentation, gag reflex suppression, and anorectal disorders
and various pain syndromes. It is available in many dosage forms including
gels, creams, ointments, lotions, aerosols, and lozenges. Many nonprescription
products are available for the temporary relief of dental or oral pain. BZC and
tetracaine are more likely to cause contact sensitization than are other local
anesthetics.
BZC interferes with the ability of certain nerves to conduct electrical
signals, which blocks the transmission of nerve impulses that carry pain
messages. BZC works by stopping the sodium entering the nerve ending at the
site of the pain. This prevents an electrical signal building up and passing along
the nerve fibres to the brain. In this way benzocaine causes numbness and
relieves pain at the area it is applied to.
In order to increase the water solubility, BZC was included [1] in
various cyclodextrins - cyclic oligosaccharides containing six (α-CD), seven
(β-CD) or eight (γ-CD) α-1,4-linked glucopiranose units. Crystallographic
studies on β -CD (cyclomaltoheptaose or cycloheptaamylose) inclusion complex
with BZC were published [2]. Dimeric arrangement of cyclodextrins in crystal
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state was demonstrated [3]. It is important to determine the structure and the
forces implied in the inclusion complex formed between cyclodextrin and BZC
molecules. Some theoretical investigations (free energy prediction) of several
organic molecules (among them BZC) with α and β -CD inclusion complexes
were published [4]. The aim of the present paper was to establish the molecular
groups implied in the inclusion complex of BZC with β-CD based on FT IR
spectral analysis and to compare these results with molecular mechanics
calculations.
EXPERIMENTAL
Inclusion complexes of BZC with β -CD by physical mixture (pm),
kneading (kn) and coprecipitation (co), respectively were prepared. The so
obtained complexes were recorded in the 4000-400 cm-1 spectral domains with
a JASCO FT IR-610 spectrometer using the KBr pellet technique.
Molecular mechanics
In order to optimize the geometry in vacuum, molecular mechanics
computations have been done with the HyperChem software [5]; the details of
the algorithms are given elsewhere [6]. The well-known MM+ (with the PolakRibière conjugate gradient) method was used to energy-minimize the structures
until a RMS gradient lower than 0.015 kcal mol-1 Å-1 was obtained.
RESULTS AND DISCUSSION
Infrared absorptions
The following IR bands, observed in the FT IR spectra of BZC and its β-CD
complex, see Fig. 1, were assigned: O-H stretching band (between 3200 and
3600 cm-1, broad band), N-H stretching band (3300-3500 cm-1), aliphatic C-H
stretching bands (2850-2960 cm-1), aromatic C-H stretching bands (3000-3100
cm-1).
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3363
2.5
2.0
Absorbance
3383
solid BZC
dash -pm
dot -kn
3341
dash dot -co
3222 short dash β-CD
1.5
3420
1.0
0.5
0.0
4000
3800
3600
3400
3200
3000
2800
-1
Wavenumber / cm
Fig. 1.- FT IR spectra, 4000-2800 cm-1
The absorption band at 1685 cm-1, see Fig. 2, is the characteristic for the ester
group.
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solid BZC
dash pm
dot kn
dash dot co
short dash β -CD
1685
1603
2,0
Absorbance
1,5
1597
1514
1635
1,0
1518
0,5
0,0
1750
1700
1650
1600
1550
W avenumber / cm
1500
1450
-1
Fig. 2.- FT IR spectra, 1750-1450 cm-1
1280
s o lid B Z C
dash pm
dot kn
dash dot co
s h o rt d a s h β -C D
2 .0
Absorbance
1 .5
1 .0
0 .5
0 .0
1400
Fig. 3.- FT IR spectra, 1400-900 cm-1
1200
1000
W avenum ber / cm
Fig.3
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-1
772
1,2
solid BZC
dash pm
dot kn
701 dash dot co
short dash β -C D
1,0
Absorbance
0,8
0,6
0,4
0,2
0,0
900
850
800
750
W avenum ber / cm
700
650
-1
Fig. 4.- FT IR spectra, 900-650 cm-1
The C-O stretching can be observed at 1280 cm-1, see Fig. 3. Other
amine vibrations, N-H bending and out-of-plane N-H bending are located at
1640~1560 cm-1 and 900~600 cm-1, see Figs.2 and 4, respectively.
By comparing the FT IR spectra of the pure BZC and β -CD with the
spectra of pm, kn and co samples, the following changes may be noticed:
- The band at 1685.2 cm-1 remains unchanged in complexes as well as the
band located at 1574 cm-1.
- The characteristic aromatic absorption of the BZC, 1597.7 cm-1, is
shifted to 1603.8 cm-1 for kn and co.
- The band at 1514 cm-1 for BZC pure compound is shifted to 1518 cm-1
for kn and co complexes. Possibly, the hydrogen bonds are broken out.
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-
The bands at 701 and 775 cm-1, characteristic for the aromatic out-ofplane bending, decreases in intensity, i.e. the motional freedom of the
aromatic ring protons is restricted.
In the high frequency region, between 4000 and 2500 cm-1 one can
identify the O-H stretching band at 3380 cm-1 for β-CD. This band is
shifted to 3366 cm-1 (co) and 3373 cm-1 (kn), respectively. Hydrogen
bonding provides an important contribution to the formation of the
inclusion complex and it can explain this frequency shift. The N-H
stretching bands (3300-3500 cm-1) may be identified perfectly in the pm
spectrum whereas for the really obtained inclusion complexes (kn and
co) these bands are smeared out, being observed as shoulders of the O-H
stretching envelope. Because the high frequency side of this band for co
spectrum has a base line of lower intensity than the corresponding ones
for pm and kn spectra, one cannot exclude the expelling of the inner
cyclodextrin water molecules during the inclusion process [7].
Molecular mechanics
In order to find a theoretical support for the previous interpretations of
the observed spectral changes, molecular mechanics calculations have been
done. The CDs act as complexing agents with different molecules, which enter
inside CD torus, and interact with the OH groups of CD by Van der Waals,
electrostatic forces and hydrogen bonding [8]. The models have been built
starting by locating the BZC molecules at the larger side of the β-CD cavity.
The results of the geometry optimisation in vacuum through MM+ molecular
mechanics of a β -CD molecule and BZC molecules is presented in Fig. 5.
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Fig. 5: Spatial geometry of BZC / β-CD system after MM+ molecular
mechanics computations.
X-ray diffraction studies confirm this supramolecular structure [2], i.e. the
para-amino group is located near the primary O-H groups (the narrow rim) of
the cyclodextrin whereas the ester group is situated near the secondary O-H
groups (the larger rim) belonging to the CD truncated cone.
CONCLUSIONS
True inclusion complexes of BZC with β-CD were obtained by using
kneading and coprecipitation methods. They were investigated by FT IR
spectroscopy showing the molecular groups involved in the inclusion process.
Hydrogen bonding is one of the important contributions in this process. The
intensity decrease of the aromatic vibrations suggests its motional hindering,
i.e. the aromatic ring is included inside cyclodextrin cone. The expelling of the
water molecules during the inclusion process cannot be neglected, also. The
spatial structure of this supramolecular assembly was obtained by MM+
molecular mechanics computations and confirmed by X-ray diffraction.
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REFERENCES
[1] J.P. Moldenhauer, Ger. Offen. (2002) CODEN GWXXBX DE 10033059
A1 20020131 Preparation of inclusion complex of benzocaine with γ-CD
and its use as local anesthetics.
[2] J.A. Hamilton, M.N Sabesan, Carbohydr. Res., 1982, 102 31-46.
[3] J.A. Hamilton, M.N. Sabesan, L.K. Steinrauf, Carbohydr. Res., 1981, 89 3353.
[4] T. Suzuki, J. Chem. Inf. Comput. Sci., 2001, 41(5) 1266-1273.
[5] HyperChemTM Release 4, Hypercube, Inc. (1994), Waterloo, Ontario N2l
3X2, Canada.
[6] “Computational Chemistry”, Hypercube, Inc. Waterloo, Ontario N2l 3X2,
Canada (1994).
[7] Jose M. Gavira, Antonio Hernanz, Ioan Bratu, Vibr. Spectrosc., 2003,. 32,
Issue 2, 137-146.
[8] Lei Liu, Qing-Xiang Guo, Journal of Inclusion Phenom. and Macrocyclic
Chem. 2002, 42, 1-14.
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