D. Yancheva, et al.
ChemicalPhysics535(2020)110763
Scheme 1. Capsaicin and model compounds
under study.
2. Materials and methods
spectroscopic cell to record the spectra.
4-hydroxy-3-methoxybenzylamide hydrochloride (99% purity,
Sigma–Aldrich Co), benzyl chloride (99% purity, Sigma–Aldrich Co),
chloroform (p.a., Sigma–Aldrich Co) were used as synthetic reagents.
CD3ONa was prepared prior the experiments by reacting spectral grade
CD3OD (99% at. enrichment, Deutero GmBh) with Na and drying the
solid product under vacuum. Spectral grade deuterated dimethylsulf-
oxide (DMSO-d6) and chloroform (CDCl3) were purchased from Deutero
GmBh.
2.3. Computational methods
The computational study of molecular geometry, reaction en-
thalpies and vibrational spectra was conducted at B3LYP/6-11+
+G(d,p) level of theory [24–26] implementing the Gaussian 09 suite of
done to confirm the optimized structures as minima on the potential
energy hypersurface.
Melting points (mp) were determined using an Büchi B-540 appa-
ratus and were uncorrected. The FTIR measurements were carried out
on a Bruker Tensor 27 FT spectrometer. The solid state spectra were
measured in attenuated total reflectance (ATR) mode. The spectra in
Dissociation enthalpy (BDE), ionization potential (IP), proton dis-
sociation enthalpy (PDE), proton affinity (PA), and electron transfer
enthalpy (ETE) of the most stable conformers were calculated at 298 K
according the equations provided in the literature [28]:
DMSO-d6 solution were measured in transmittance mode by
a
0.129 mm CaF2 sample cell. All spectra were recorded by accumulating
64 scans at 2 cm−1 resolution. 1H NMR spectra were recorded in CDCl3
solution on a Bruker Avance II+ 600 MHz NMR instrument. The
spectra were referred to the solvent signal. Standard Bruker pulse se-
quences and software were used to record and process the spectra. The
reactions were monitored by thin layer chromatography, which was
performed on Merck pre-coated plates (silica gel. 60 F254, 0.25 mm)
and was visualized by fluorescence quenching under UV light (254 nm).
H (ArOH)
H (ArOH)
H (ArOH)
H (ArO )
2.1. Synthesis of N-(4-hydroxy-3-methoxybenzyl)benzamide (3)
The enthalpy of hydrogen atom, H(H), in the respective solvents
were obtained using the same functional and basis set. Solvation en-
thalpies of proton, H(H+), and electron, H(e−), were applied in ac-
cordance with previously estimated values [29].
The synthesis of compound 3 was carried out by interfacial reaction
between vanillylamine hydrochloride and acyl chloride in biphase
H2O/CHCl3 system according to the synthetic procedure provided in
[20]. The chemical structure and purity of the compound were con-
firmed by comparison with the previously reported data [20].
drochloride were dissolved in 1.6 ml water and 0.218 g (2.6 × 10−3
mol) NaHCO3 were added to the solution. After stirring the mixture for
30 min at 20 °C, 2.26 ml of chloroform were added. The mixture was
stirred for another 15 min and then 0.1 ml CHCl3 solution of ben-
zoylchloride (0.79 × 10−3 mol in 0.62 ml CHCl3) was added dropwise.
The mixture was stirred for 30 min, heated to 40 °C, and the organic
layer was separated. The water layer was extracted by chloroform
(3 × 0.6 ml). The combined organic layers were washed with 2% HCl
solution and dried with anhydrous NaSO4. The solvent was removed
under vacuum and the crude product was recrystallized in absolute
ethanol.
3. Results and discussion
3.1. Computational study on the structural properties of capsaicin and its
analogues related to their radical scavenging activity
The most stable geometry of the neutral compounds and their
benzylic and phynoxyl radical, radical-cationic and anionic species was
determined by energy analysis. Having in mind the reports on radical
scavenging activity exerted by NeH compounds [29–33] and their easy
protonated at the amide N-atoms) were included in our study. Several
possible conformers resulting from the rotation around the single bonds
in the studied species were fully optimized at IEFPCM-B3LYP/6-311+
+G** level of theory in gas phase, benzene, water and DMSO.
The optimization showed that the geometry of vanillylamide moiety
would not be substantially affected by the replacement of the octanyl
fragment of capsaicin with methyl group or phenyl ring.
Yield: 87%; mp 140–142 °C; Rf = 0.676 (C6H6:CH3OH = 3:1); IR
(ATR-FTIR): ν(cm−1) 3213 (νN-H), 3071 (ν(C
H)Ar), 2960, 2917, 2834
e
(ν(C
e
H)Alk), 1685 (νC=O), 1523 (δ(N
e
H)), 1282, 1272 (ν(C O)), 1257
e
(ν(C N)
e
); 1H NMR (600 MHz, CDCl3, δ): 7.76 (d, 2H, Ar), 7.50 (t, 1H,
The total energies (Etot) and relative energies (ΔE) with respect to
the most stable forms of the benzylic C7-centered radicals, phenoxyl
and amide radicals of the three compounds are summarized in Table 1.
The relative energies of the studied radicals in gas phase, benzene and
polar solvents pointed out that the formation of benzylic radical is
thermodynamically favored not only for capsaicin 1, but also for the
two model compounds 2 and 3 in any kind of medium.
Ar), 7.40–7.44 (m, 2H, Ar), 6.90–6.82 (m, 3H, Ar), 6.30 (br s, 1H, OH),
5.60 (s, 1H, NH), 4.55 (d, 2H, CH2), 3.88 (s, 3H, OCH3); Analysis: Calc.
for C15H15NO3: C, 70.02; H, 5.88; N, 5.44; O, 18.66; Found: C, 70.00; H,
5.85; N, 5.42.
2.2. Conversion of N-(4-hydroxy-3-methoxybenzyl)benzamide into anion 4
However, as the calculated relative energies of the benzylic and
phenoxyl radicals are within the range 7–16 kJ.mol−1, the simulta-
neous formation of both radicals is also plausible. The experimental
studies so far have led to the conclusion that capsaicin reacts via C7-
benzylic radical in nonpolar and neutral medium (pH 7) [8,11,16],
DMSO-d6 solution of N-(4-hydroxy-3-methoxybenzyl)benzamide
(0.12 mol.l−1) was mixed with excess of dry CD3ONa and shacked for a
couple of minutes. The remains of solid CD3ONa were filtered out from
the reaction mixture and the solution was transferred to the IR
2
Anichina, K.
