Phenylalanine and tyrosine methyl ester intramolecular interactions and conformational analysis by 1H NMR and infrared spectroscopies and theoretical calculations

Article abstract of DOI:10.1016/j.saa.2013.12.088

Amino acid conformational analysis in solution are scarce, since these compounds present a bipolar zwitterionic structure (⁺H ₃NCHRCOO^-) in these media. Also, intramolecular hydrogen bonds have been classified as the sole interactions governing amino acid conformational behavior in the literature. In the present work we propose phenylalanine and tyrosine methyl ester conformational studies in different solvents by ¹H NMR and infrared spectroscopies and theoretical calculations. Both experimental and theoretical results are in agreement and suggest that the conformational behavior of the phenylalanine and tyrosine methyl esters are similar and are dictated by the interplay between steric and hyperconjugative interactions.

Full text of DOI:10.1016/j.saa.2013.12.088

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 482–489
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Phenylalanine and tyrosine methyl ester intramolecular interactions
1
and conformational analysis by H NMR and infrared spectroscopies
and theoretical calculations
Rodrigo A. Cormanich a, Lucas C. Ducati b, Cláudio F. Tormena a, Roberto Rittner a,
⇑
a Chemistry Institute, University of Campinas, P.O. Box 6154, 13083-970 Campinas, Brazil
b Chemistry Institute, University of São Paulo, P.O. Box 26077, 05508-900 São Paulo, Brazil
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Phenylalanine and tyrosine were
investigated by theoretical and
experimental methods.
ꢀ
ꢀ
ꢀ
The lowest energy conformers are not
stabilized by hydrogen bonding.
Steric and hyperconjugative effects
were analyzed for all conformers.
Several theoretical methods were
used to explain the conformational
preferences.
a r t i c l e i n f o
a b s t r a c t
Article history:
Amino acid conformational analysis in solution are scarce, since these compounds present a bipolar zwit-
terionic structure (+H3NACHRACOOꢁ) in these media. Also, intramolecular hydrogen bonds have been
classified as the sole interactions governing amino acid conformational behavior in the literature. In
the present work we propose phenylalanine and tyrosine methyl ester conformational studies in different
solvents by 1H NMR and infrared spectroscopies and theoretical calculations. Both experimental and the-
oretical results are in agreement and suggest that the conformational behavior of the phenylalanine and
tyrosine methyl esters are similar and are dictated by the interplay between steric and hyperconjugative
interactions.
Received 25 September 2013
Received in revised form 29 November 2013
Accepted 21 December 2013
Available online 3 January 2014
Keywords:
Conformational analysis
Amino acid methyl ester derivatives
1H NMR spectroscopy
Ó 2014 Elsevier B.V. All rights reserved.
Infrared spectroscopy
Intramolecular hydrogen bonding
Stereoelectronic effects
Introduction
considered the sole interaction governing amino acid conforma-
tional isomerism since the very beginning of amino acid conforma-
The understanding of amino acids conformational behavior in
solution is essential for a deep recognition of the complex pro-
tein geometries to which such compounds give rise [1–4]. How-
ever, intramolecular hydrogen bonds (IHB) have been commonly
tional studies in the literatures [5–18]. Recently, it was found that
the balance between steric and hyperconjugative effects and not
IHBs are actually the main forces dictating the conformational
behavior of a considerable range of amino acids and amino acid
esters [19–23].
Moreover, few to none conformational studies of amino acids in
solution can be found in the literature, particularly using NMR
⇑
Corresponding author. Tel./fax: +55 19 3521 3150; fax: +55 19 3521 3023.
E-mail address: rittner@iqm.unicamp.br (R. Rittner).
1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.12.088

R.A. Cormanich et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 482–489
483
spectroscopy. The main reason is that amino acids in solution pres-
ent a bipolar zwitterion structure (+H3NACHRACOOꢁ), which is a
rough approximation for the understanding of the behavior of an
amino acid residue in a polypeptide or protein environment [24–
26]. Consequently, experimental amino acid conformational stud-
ies have been almost limited to gas phase based spectroscopies.
Although many improvements were recently achieved by gas
phase methods, these cannot yet determine the nature and quanti-
tative distributions of amino acid conformers. Also, since amino
acid compounds are thermally unstable and have high melting
points and low vapor pressures, the vaporization of amino acid
compounds with large side chains is a challenge for such tech-
niques [27–29].
Therefore, in the present work, phenylalanine and tyrosine
methyl ester derivatives conformational behavior in solution was
studied, which does not exhibit the bipolar zwitterionic structure
and also show considerable solubility in the majority of organic
solvents, by 1H NMR spectroscopy. The obtained experimental
3JHH spin–spin coupling constants (SSCCs) were compared with
theoretical calculations in the framework of the quantum theory
of atoms in molecules (QTAIM) [30] and the natural bond orbitals
(NBO) analysis [31].
package of programs [34] and the QTAIM calculations were ob-
tained from the AIMALL software [35].
Amino acid methyl ester chloridrate deprotonation
The L-phe-OMe and the L-tyr-OMe were commercially available
(Acros Organics) as
a
chloridrate (phe-OMEꢂHCl and L-tyr-
OMeꢂHCl) and were deprotonated using commercial zinc dust
[36]. To produce a suspension, commercial zinc dust (100 mg)
was added to the phe-OMEꢂHCl and L-tyr-OMeꢂHCl (1 mmol) in
10 mL of CH2Cl2 and THF, respectively. The mixture was stirred
for ca. 10 min. Subsequently, the mixture was filtered and the sol-
vent was evaporated. The resultant phe-OMe and tyr-OMe were
obtained as a free ester crystalline solid.
1H NMR spectra
The phe-OMe and tyr-OMe 1H NMR experiments were recorded
on a Brucker Avance-III spectrometer operating at 400.13 MHz.
Spectra were recorded in solutions of ca. 10 mg in 0.7 mL of deuter-
ated commercial solvents without further purification: CDCl3, CD2-
Cl2, THF, pyridine-d5, acetone-d6, methanol-d4, acetonitrile-d3 and
DMSO-d6, referenced to internal TMS. The typical conditions used
were as follows: a probe temperature of 25 °C, 16 transients, a
spectral width of 2.6 kHz, 64k data points, an acquisition time of
12.6 s and zero-filled to 128k points (1H NMR spectra are provided
in the Supporting information section).
Materials and methods
Computational details
The phenylalanine methyl ester (phe-OMe) and tyrosine methyl
ester (tyr-OMe) conformers were obtained by scanning the
[C(Ph)ACH2ACHAC(O)] dihedral angle in 10° steps from 0° to
360° and using the 6 conformers previously obtained for ala-OMe
v
Infrared spectra
Phe-OMe and tyr-OMe samples were prepared with a concen-
tration of 0.03 M in CHCl3, CH3CN and DMSO dried solvents. The
infrared spectra were recorded on a FTIR Shimadzu IRPrestige-21
spectrometer by using a 0.5 mm width NaCl round cell window.
The infrared spectrometer conditions used were the following:
number of scans = 64, resolution = 1 cmꢁ1, spectra range = 400–
4000 cmꢁ1. The equipment was purged with dry nitrogen gas.
Deconvolution of the C@O absorption bands were carried out by
using the GRAMS curve fitting software [37].
compound, / [nN-NACAC(O)] and
w (NACAC@O) dihedral angles
[22] (dihedral angle representation showed in Fig. 1). Each poten-
tial energy curve (PEC) showed 3 minima (Figs. S1 and S2; Support-
ing information), which were subsequently optimized at the
B3LYP/aug-cc-pVDZ theoretical level. Frequency calculations
showed that all conformers, except IIIa and IV2c, had no imaginary
frequencies for both phe-OMe and tyr-OMe and are true energy
minima in each PEC. Thus, the resulting 16 conformers of the
phe-OMe and tyr-OMe compounds were optimized by using the
integral equation formalism polarizable continuum model (IEF-
Results and discussions
PCM) [32] in three different dielectric constants (
e): chloroform
(
e
= 4.8), acetonitrile ( = 37.5) and DMSO ( = 46.7). From these
e
e
The 1H NMR spectra for the deprotonated phe-OMe and tyr-
OMe were obtained in different solvents (spectra are shown in
the Supporting information): apolar (CDCl3, CD2Cl2, THF-d8), polar
aprotic (pyridine-d5, acetone-d6, acetonitrile-d3 and DMSO-d6) and
polar protic (methanol-d4) solvents. The 3JHH values were recorded
and their values for the phe-OMe compound are shown in Table 1.
Fig. 2 shows the three conformational minima (a, b and c) expected
for the side chain in relation to the main chain for the phe-OMe.
The a and b geometries have an anti relationship between the
hydrogens Ha and Hb and Ha and Hc, respectively, while the c
geometry has a gauche relationship between Ha and Hb and be-
tween Ha and Hc (Fig. 2). In this way, based in the Karplus relation-
ship [38], one would expect that the 3JHaHb and the 3JHbHc spin–spin
coupling constants to have higher values for conformers with the a
and b geometries, respectively, and smaller values for conformers
with the c geometry. Table 1 shows that the 3JHaHb values decrease
3
IEF-PCM calculations the JHH spin–spin coupling constants were
obtained for each conformer using the B3LYP functional and the
EPR-III basis set [33]. The intramolecular interactions were evalu-
ated using the natural bond orbitals (NBO) and the quantum theory
of atoms in molecules (QTAIM) methods with the obtained B3LYP/
aug-cc-pVDZ electronic density optimization calculations. The
QTAIM integrated Laplacian
q values over each atomic basin (X)
were always less than 10ꢁ3 atomic units (au) for all atoms and
indicate good integrated atomic properties. PEC, optimization,
IEF-PCM and NBO calculations were carried out in the Gaussian03
with the increase of the dielectric constant (e) of the solvent, while
the 3JHaHc values show the opposite behavior, i.e., the 3JHaHc values
increase with the increase of the dielectric constant value. This pat-
tern is interesting and shows that in non-polar solvents 3JHaHb and
3JHaHc values are far one from each other, while in solvents with
higher values of dielectric constant, the two coupling constant val-
ues are the same. Thus, two different hypotheses could explain the
Fig. 1. The / [nN-NACAC(O)],
dihedral angle representations.
w (NACAC@O), h (CH3AOAC@O) and v (HaACACAHb)

484
R.A. Cormanich et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 482–489
n
X
gi
Table 1
3JHH
¼
3Ji
ð1Þ
Experimental 1H NMR chemical shift values (ppm) and SSCCs (Hz) for the phe-OMe
compound in different solvents.
i¼1 gT
with gi
/gT representing the relative population of the conformer ‘‘i’’
3
3
and 3Ji the intrinsic JHaHb or JHaHC value of the conformer ‘‘i’’. The
2
3
3
obtained gi/gT value multiplied by the JHaHb or JHaHc for each
phe-OMe conformer in each solvent is depicted in Fig. 3a and b,
3
respectively, which show considerable value variations going from
3
one solvent to another. Fig. 3c and d shows the calculated JHaHb
and 3JHaHc values for each conformer and indicate that the variations
of these SSCCs in different solvents are negligible. Thus, the varia-
tions observed in Fig. 3a and b are indicative of the phe-OMe con-
former relative population (gi/gT parameter) changes in each
solvent. From the simultaneous observation of Fig. 3a and b, one
Solvent
e
dHa
dHb
dHc
dHd
3JHaHb 3JHaHc 2JHbHc
CDCl3
CD2Cl2
Pyridine-d5
Acetone-d6
Methanol-d4
CD3CN
4.8 4.10 3.09 3.25 3.72 7.9
9.8 4.13 3.13 3.27 3.78 7.6
4.8
4.9
5.7
5.2
6.6
6.1
6.7
14.0
14.1
13.4
13.8
–
may conclude that the second hypothesis is the correct one, i.e.,
the most stable conformers Ia and Ib acquire similar gi/gT values
12.3 3.90 2.99 3.15 3.61 7.4
20.7 4.15 3.21 3.28 3.70 6.5
32.7 3.95 3.11 3.11 3.37 6.6
37.5 4.00 3.08 3.15 3.67 6.1
46.7 3.66 2.88 2.88 3.57 6.7
a
with the increase of the solvent dielectric constant. Some small
populational variations may be observed in Fig. 3a and b for the
other conformers, but these may be considered as secondary effect.
It was also recorded experimental infrared spectra for the phe-
OMe and tyr-OMe compounds. Table 3 shows the obtained phe-
OMe C@O absorption bands in CHCl3, CH3CN and DMSO solvents.
Those bands could, in principle, be deconvoluted in all phe-OMe
16 conformers which give origin of each C@O absorbation band.
However, in practice, each experimental C@O band has only 3 com-
ponents which could be used in the deconvolution procedure.
Hence, one can deduce that each of those components could be
attributed to set of conformers. In this way, there were obtained
theoretical harmonic C@O stretching values for each phe-OMe con-
former (Table 2), which, in order to account for anharmonic vibra-
tional effects, were corrected by using different values of
correction factors (CF), and those were distributed in three differ-
ent sets A, B and C. The criterion of distribution of conformers in
each set was the C@O stretching values (Table 2), i.e., conformers
with close C@O absorption values were grouped in the sets A
(intermediate values), B (smaller values) and C (higher values).
Remarkably, the sum of the theoretically obtained conformer pop-
ulation values in each set is in excellent agreement with experi-
ment (Table 3). Also, the theoretical C@O absorption values
obtained for each set, which is given by the following equation:
13.9
–
a
DMSO-d6
a
In methanol-d4 and DMSO-d6 Hb and Hc cannot be distinguished.
3JHaHb and the JHaHc values in different solvents: one is that the
conformational equilibrium would be displaced progressively to c
geometry conformers with the increase of the dielectric constant,
so this could be the explanation for the 3JHaHb and the 3JHaHc values
approaching each other when the dielectric constant value in-
creases. The other hypothesis is: with the progressive increase of
the dielectric constant, conformers with b geometries are being fa-
3
3
3
vored over a geometry conformers until the JHaHb and the JHaHc
values become equal.
In order to get a deeper insight into the obtained experimental
results, theoretical calculations were carried out. Both the phe-
OMe and tyr-OMe conformer names were classified with Arabic
numerals followed by the letters a, b or c. The numerals indicate
the main chain geometry taken from the ala-OMe [22] and the let-
ters indicate the side chain geometries as shown in Fig. 2. From the
constructed PECs (Supporting information), 18 conformers for both
phe-OMe and tyr-OMe were obtained. At the B3LYP/aug-cc-pVDZ
theoretical level the IIIa and IV2c phe-OMe and tyr-OMe conform-
ers are not true minima and were discarded from the set of 18 con-
formers. Thus, both phe-OMe and tyr-OMe have 16 conformers
each, which are shown in Figs. S3 and S4 (Supporting information)
and the phe-OMe energies are shown in Table 2. Each of the 16
conformers was reoptimized by using the implicit solvent IEF-
PCM model with chloroform, acetonitrile and DMSO dielectric con-
stant values (Table 2 shows the values obtained for phe-OMe). The
n
X
GroupC@
¼
%P ꢃ conformerðiÞC@
ð2Þ
O
O
i¼1
with GroupC@O and conformer(i)C@O representing the values of the
C@O stretching frequency for each group (A, B and C) and of the
‘‘i’’ conformers pertaining to each group, respectively, are in agree-
ment with the experimental ones (Table 3). Then, both experimen-
tal 1H NMR and infrared theoretical data are in agreement with the
theoretical results. The theoretical data, in turn, explains the exper-
imental results.
The next step is the analysis of the intramolecular effects
which give rise to the phe-OMe conformational preferences. In
this way, the QTAIM and the NBO methods were applied. The
phe-OMe QTAIM molecular graphs are displayed in the Supporting
3JHaHb and the JHaHc SSCCs theoretical values were obtained for
each conformer in the IEF-PCM implicit solvent model by using
the B3LYP/EPR-III level.
The contribution of each ‘‘i’’ conformer in a given solvent for the
observed JHaHb SSCC may be estimated through the following
3
3
equation:
Fig. 2. Newman projections for the a, b and c phe-OMe possible side chain geometries.

R.A. Cormanich et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 482–489
485
Table 2
Relative energiesa (kcal molꢁ1), populations (percentage) for the phe-OMe conformers in the gas phase (B3LYP/aug-cc-pVDZ) and by using the IEF-PCM implicit model in CHCl3,
CH3CN and DMSO. Theoretical (B3LYP/aug-cc-pVDZ) C@O stretching values are also showed.
Conformer
Isolated
CHCl3
CH3CN
DMSO
D
E
%P
D
E
%P
C@O1
D
E
%P
C@O2
D
E
%P
C@O3
Ia
Ib
Ic
IIIb
IIIc
IV1a
IV1b
IV1c
IV2a
IV2b
V1a
V1b
V1c
V2a
V2b
V2c
0.00
0.16
0.26
1.41
0.95
0.93
0.08
1.86
2.74
0.36
2.26
0.83
2.97
2.55
1.10
2.28
20.6
15.8
13.3
1.9
4.2
4.3
18.0
0.9
0.2
11.2
0.5
5.1
0.00A
0.18A
0.66A
1.08B
1.38B
0.72C
0.30C
1.73C
2.84C
0.80C
2.29C
1.42A
3.27B
1.76C
0.97A
2.17B
25.5
18.6
8.4
4.1
2.5
7.6
15.4
1.4
0.2
6.6
0.5
2.3
0.1
1.3
4.9
0.7
1733.4
1736.8
1741.5
1727.6
1723.7
1742.1
1745.9
1750.3
1733.1
1741.9
1752.2
1740.7
1728.5
1749.1
1736.4
1729.6
0.00A
0.00A
0.74A
0.76B
1.51B
0.56C
0.19C
1.30C
1.50C
0.77C
1.79C
1.51A
3.06B
1.11C
0.82A
2.10B
15.1
15.1
6.9
6.8
3.1
8.4
12.3
3.8
3.1
6.7
2.3
3.1
0.6
4.7
6.4
1.7
1741.8
1745.9
1748.3
1731.9
1730.7
1749.7
1753.6
1755.5
1753.3
1750.9
1756.1
1746.8
1731.8
1749.6
1741.7
1737.7
0.01A
0.00A
0.86A
0.76B
1.61B
0.54C
0.24C
1.23C
1.52C
0.74C
1.52C
1.55A
3.11B
1.12C
0.91A
2.08B
21.1
21.4
5.0
5.9
1.4
8.6
14.3
2.7
1.7
6.1
1.6
1.6
0.1
3.2
4.6
0.6
1732.7
1736.6
1739.7
1723.4
1719.9
1741.8
1744.9
1746.2
1742.5
1742.4
1743.6
1738.8
1723.8
1741.6
1732.6
1727.3
0.1
0.3
3.2
0.4
a
1
2
3
A
B
C
Zero point energy correction included.
It was applied a CF of 0.989.
It was applied a CF of 0.998.
It was applied a CF of 0.993.
group A conformers.
group B conformers.
group C conformers.
Fig. 3. Calculated (B3LYP/aug-cc-pVDZ level in the IEF-PCM implicit solvent model) phe-OMe conformer parameter variations in different solvents for (a) gi
/
gT
ꢃ
3JHaHb, (b) gi
/
3
3
gT
ꢃ
3JHaHc, (c) JHaHb and (d) JHaHc
.
Information and indicate that an IHB is formed in the main chain
for all conformers, but that non-usual CAHꢂ ꢂ ꢂO interactions are
also formed between the side chain and the main chain geometries
for conformers Ic, V2a and V2c. However, conformer Ic forms a
very weak CAHꢂ ꢂ ꢂO interaction, which does not fulfill the Popelier
criteria [39] (Table 4). Thus, non-negligible CAHꢂ ꢂ ꢂO interactions
are formed only by conformers V2a and V2c, which are of high
relative energy and correspond to small populations (Table 2).
Therefore, an IHB is present only in low populated conformers
and, hence, have a secondary role in the conformational behavior
of phe-OMe according to QTAIM calculations. On the other hand,
the
Ehyper (total hyperconjugation energy) values (Table 5), obtained
by the NBO method, indicate that the interplay between steric
DELewis (conformer energy without hyperconjugation) and
D

486
R.A. Cormanich et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 482–489
Table 3
Deconvolution of the experimental C@O bond stretching absorption bands obtained by experimental infrared spectra in CHCl3, CH3CN and DMSO solvents of the phe-OMe
compound. Experimental and theoretical values of the C@O bond stretching (cmꢁ1) and population (%) for each A, B and C group are also reported.
CHCl3
CH3CN
DMSO
0.5
0.3
0.4
0.3
0.2
0.1
0.0
0.4
0.3
0.2
0.1
0.0
0.2
0.1
0.0
Group A
Group C
Group A
Group A
Group C
Group B
Group B
Group C
Group B
1780 1770 1760 1750 1740 1730 1720 1710
Wavenumber (cm-1)
1790 1780 1770 1760 1750 1740 1730 1720 1710
Wavenumber (cm-1)
1780 1770 1760 1750 1740 1730 1720 1710
Wavenumber (cm-1)
Theoretical
Experimental
Theoretical
Experimental
Theoretical
Experimental
C@O1
%P
C@O
%P
C@O2
%P
C@O
%P
C@O3
%P
C@O
%P
Group A
Group B
Group C
1736.1
1726.7
1745.9
59.8
7.4
32.8
1737.0
1725.8
1745.9
55.0
7.8
37.2
1744.4
1736.6
1752.1
46.5
15.2
38.3
1743.0
1737.0
1752.0
46.7
18.0
35.3
1735.1
1723.1
1743.5
53.7
8.1
38.2
1735.3
1725.4
1743.4
56.5
9.6
33.9
1
2
3
It was applied a correction factor (CF) of 0.989.
It was applied a correction factor (CF) of 0.998.
It was applied a correction factor (CF) of 0.993.
Table 4
Popelier parameters applied to the phe-OMe conformers which form CHꢂ ꢂ ꢂO
interactions. The qHBCP
2qHBCP and all integration parameters obtained for
Table 6
, / and
aug-cc-pVDZ theoretical level.
w
v dihedral angle values for the phe-OMe conformers calculated at the B3LYP/
,
r
hydrogen atoms are in atomic units (au). The conformer Ia is used as reference,
since it cannot form a CHꢂ ꢂ ꢂO interaction.
Conformer
w
(NACAC@O)
/ [nN-NACAC(O)]
v [HbACHACAHa]
Conformer
qHBCP
r
–
+0.016
+0.026
+0.021
2qHBCP
q(H)
E(H)
M1(H)
V(H)
Ia
Ib
Ic
IIIb
IIIc
IV1a
IV1b
IV1c
IV2a
IV2b
V1a
V1b
V1c
V2a
V2b
V2c
59.2
24.6
4.4
219.7
184.4
54.9
173.8
163.5
181.3
170.6
182.7
64.6
72.7
74.3
84.7
83.2
51.0
59.7
59.7
58.6
63.8
79.9
62.6
165.4
295.0
163.3
292.1
63.9
177.4
297.4
80.5
175.5
58.4
172.4
296.2
75.3
Ia
Ic
V2a
V2c
–
+0.004
ꢁ0.006
+0.025
+0.011
ꢁ0.6126
ꢁ0.6175
ꢁ0.6060
ꢁ0.6110
0.131
0.124
0.123
0.125
49.64
50.19
46.69
48.13
0.004
0.007
0.006
35.0
22.4
335.7
343.5
241.5
221.9
198.2
108.7
138.1
172.7
Table 5
Full energy of the real system (
hyperconjugation is removed ( ELewis) and hyperconjugative energy (
phe-OMe conformers. Energy values in kcal molꢁ1
D
EFULL)a, energy of the hypothetical case where
Ehyper) for the
D
D
.
Ia
Ib
Ic
0.26
8.38
10.36
IIIb
IIIc
0.95
10.83
12.20
IV1a
IV1b
IV1c
173.0
299.7
D
D
D
EFull
ELewis
Ehyper
0.00
5.87
8.03
0.16
3.34
5.27
1.41
4.09
4.71
0.93
7.10
8.27
0.08
4.97
7.06
1.86
10.67
10.86
IV2a
IV2b
V1a
V1b
V1c
V2a
V2b
V2c
conformers with b geometry have the greatest deviations and,
hence, lower values of ELewis (and consequently are less stabilized
D
D
D
EFull
ELewis
Ehyper
2.74
4.77
4.02
0.36
1.50
3.20
2.26
0.14
0.00
0.83
0.00
1.24
2.97
6.40
5.44
2.55
4.47
4.19
1.10
3.92
5.01
2.28
12.83
12.74
D
by hyperconjugative effects). Conformers with b geometry are in
general the most stable, while conformers with c geometry are
the least stable. Thus, one may conclude that steric effects are more
important than hyperconjugative effects for the phe-OMe confor-
mational preferences. Notwithstanding, although steric effects
have a more important role, the balance between steric and
hyperconjugative effects determine the phe-OMe conformational
energies from the NBO viewpoint.
a
ZPE corrections included.
effects and hyperconjugative effects are important in determining
the phe-OMe conformational preferences.
It is expected that deviations from 0° and/or 180° for a con-
former
w and / dihedral angles occur in order to decrease steric ef-
The only difference between phe-OMe and tyr-OMe is a hydro-
xyl group attached to the aromatic ring at the para position in
relation to the aminoester main chain geometry. Thus, due to the
distance between the hydroxyl group and the main chain geome-
try, a similar conformational behavior for tyr-OMe in comparison
with phe-OMe is expected. However, the hydroxyl group reduces
considerably the tyr-OMe solubility in organic solvents of low
dielectric constant in comparison with the phe-OMe compound
and, hence, it is possible that these compounds might have a differ-
ent behavior in solution. Indeed, tyr-OMe is not soluble in both
chloroform and dichloromethane and these two solvents were re-
placed by THF, which is the most non-polar solvent in which the
fects between two atoms or group of atoms, followed by the
consequent decrease of hyperconjugative effects, since the more
linear the bonds in one conformer the better the orbital superposi-
tions [19]. Table 6 presents the phe-OMe
values. In all cases, conformers with c side chain geometry have a
lower deviation from 0°/180° for the dihedral angle in compari-
w, / and v dihedral angle
w
son with conformers with a and b side chain geometries, and con-
formers with b geometry are the ones which show the greatest
deviations. These results are in agreement with the
Ehyper values shown in Table 5, i.e., conformers with c geometry
have higher stabilization by hyperconjugative effects, since they
have the smallest and / deviation values from 0°/180°, while
DELewis and
D
w

R.A. Cormanich et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 482–489
487
Table 7
Experimental 1H NMR chemical shift values (ppm) and SSCCs (Hz) for the tyr-OMe compound in different solvents.
2
3
Solvent
e
dHa
dHb
dHc
dHd
dHe
3JHaHb
3JHaHc
2JHbHc
THF-d8
7.6
4.10
3.86
4.12
3.88
3.98
3.61
3.04
2.97
3.16
3.00
3.04
2.77
3.12
3.10
3.23
3.04
3.09
2.77
3.65
3.60
3.73
4.87
3.71
3.57
–
6.5
7.1
6.2
6.4
5.9
6.6
5.3
5.8
5.7
6.4
6.1
6.6
13.8
13.5
14.0
13.9
14.2
–
Pyridine-d5
Acetone-d6
Methanol-d4
CD3CN
12.3
20.7
32.7
37.5
46.7
11.37
–
–
–
a
a
DMSO-d6
9.24
a
In methanol-d4 and DSMO-d6 Hb and Hc are undistinguishable.
tyr-OMe can be solubilized. Thus, the tyr-OME 1H NMR parameters
were recorded in the solvents shown in Table 7, which shows a
considerable SSCC value variation with the solvent as well as for
phe-OMe. Moreover, the JHaHb and JHaHc values show the same
trend observed for phe-OMe, i.e., these two SSCCs have different
values in solvents of low dielectric constant and become more sim-
ilar to each other, in more polar solvents, until they reach the same
values in DMSO.
the theoretical calculations suggest that the conformer Ia and Ib
population changes in these solvents are the main cause for this
observed variation. Also, the energy (and population) and the
C@O absorption values for tyr-OMe in acetonitrile and DMSO (Ta-
ble 8) are almost the same as those obtained for phe-OMe (com-
pare Tables 2 and 8 values), i.e., these two compounds have also
the same behavior in solution.
The QTAIM molecular graphs shown in Fig. S6 (Supporting
information), in the same way as phe-OMe, indicate that the tyr-
OMe V1a and V1c conformers form intramolecular CAHꢂ ꢂ ꢂO inter-
actions, which fulfill the Popelier criteria (Table 9). A CAHꢂ ꢂ ꢂO
bond path for conformer IIIc was obtained, but this interaction is
topologically very unstable (CAHꢂ ꢂ ꢂO bond ellipticity has a value
of 16.735 au) and may be considered irrelevant. By applying the
NBO analysis for tyr-OMe conformers (Table 10), one can obtain
the same results as for phe-OMe: conformers with c geometry
are, in general, the most unstable since they have the highest val-
3
3
Analogously to the phe-OMe compound, tyr-OMe has 16 true
energy minima, which were optimized at the B3LYP/aug-cc-pVDZ
theoretical level, in the isolated state, and by using the IEF-PCM
implicit solvent model for chloroform, acetonitrile and DMSO
3
dielectric constant values (Table 8). The gi
/
gT multiplied by JHaHb
3
and JHaHc values calculated at the B3LYP/EPR-III level for chloro-
form, acetonitrile and DMSO dielectric constants are depicted in
Fig. 4a and b, showing considerable variations for the calculated
3
3
solvents, together with JHaHb and JHaHc single values (Fig. 4c and
d, respectively), which are constant with the solvent variation.
ues of
geometry are the most stable ones since they experience less steric
effects than the other geometries ( ELewis values, Table 10).
DELewis (related to steric effects), while conformers with b
3
The gi
/
gT multiplied by JHaHb for the conformer Ia and the gi
/gT
3
multiplied by JHaHc for the conformer Ib (Fig. 4a and b) follow
D
the same trend as that obtained for the phe-OMe in Fig. 3. In this
Therefore, phe-OMe and tyr-OMe show similar conforma-
tional behavior in both gas phase and in solution. The QTAIM
and the NBO methods indicate that both steric effects and
3
way, as well as for phe-OMe, the tyr-OMe experimental JHaHb
3
and JHaHc values vary with the solvent dielectric constant and
Table 8
Relative energiesa (kcal molꢁ1), populations (percentage) for the tyr-OMe conformers in the gas phase (B3LYP/aug-cc-pVDZ) and by using the IEF-PCM implicit model in CHCl3,
CH3CN and DMSO. Theoretical (B3LYP/aug-cc-pVDZ) C@O stretching values are also showed.
Conformer
Isolated
THF
CH3CN
DMSO
D
E
%P
D
E
%P
D
E
%P
C@O1
D
E
%P
C@O2
Ia
Ib
Ic
IIIb
IIIc
IV1a
IV1b
IV1c
IV2a
IV2b
V1a
V1b
V1c
V2a
V2b
V2c
0.00
0.19
0.18
1.43
0.89
0.86
0.08
1.84
2.84
0.54
2.31
0.84
2.85
2.33
1.11
2.22
20.8
15.2
15.4
1.9
4.6
4.9
18.1
0.9
0.2
8.4
0.4
0.00
0.30
0.76
1.12
1.44
0.74
0.34
1.69
3.01
0.86
2.03
1.49
3.28
1.57
0.94
2.21
27.2
16.2
7.5
4.1
2.4
7.8
15.4
1.6
0.2
6.3
0.9
2.2
0.1
1.9
5.6
0.6
0.00
0.07
0.66
0.81
1.56
0.58
0.16
1.34
1.42
0.71
1.42
1.53
3.04
1.10
0.77
1.94
20.8
18.3
6.8
5.3
1.5
7.8
16.0
2.1
1.9
6.2
1.9
1.6
0.1
3.3
5.7
0.8
1742.1
1745.1
1748.3
1731.2
1730.8
1750.8
1752.6
1754.6
1751.6
1748.4
1751.0
1747.6
1730.7
1748.9
1740.6
1734.9
0.00
0.09
0.77
0.85
1.52
0.55
0.25
1.31
1.47
0.75
1.41
1.55
3.02
1.17
0.82
2.05
21.9
18.9
6.0
5.2
1.7
8.7
14.4
2.4
1.8
6.1
2.0
1.6
0.1
3.0
5.5
0.7
1732.4
1735.9
1739.1
1722.4
1720.5
1740.9
1744.0
1746.0
1742.5
1740.1
1742.5
1738.5
1721.3
1741.0
1731.8
1727.2
5.0
0.2
0.4
3.2
0.5
a
1
2
ZPE corrections included.
It was applied correction factors (CF) with values of 0.998.
It was applied correction factors (CF) with values of 0.993.

488
R.A. Cormanich et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 482–489
Fig. 4. Calculated (B3LYP/aug-cc-pVDZ level in the IEF-PCM implicit solvent model) tyr-OMe conformer parameter variations in different solvents for (a) gi/gT ꢃ /
3JHaHb, (b) gi
3
3
gT
ꢃ
3JHaHc, (c) JHaHb and (d) JHaHc
.
does. In this way, by applying experimental 1H NMR spectroscopy
together with theoretical calculations, one may obtain a deeper
understanding of the conformational behavior of these compounds
in different solvents.
Table 9
Popelier parameters applied to the tyr-OMe conformers which form CHꢂ ꢂ ꢂO interac-
tions. The
q
HBCP, r2qHBCP and all integration parameters obtained for hydrogen atoms
are in atomic units (au). The conformer Ia is used as reference, since it cannot form a
CHꢂ ꢂ ꢂO interaction.
In particular, by comparing the theoretical and experimental
Conformer
qHBCP
r
2qHBCP
q(H)
E(H)
M1(H)
V(H)
3
3
phe-OMe and tyr-OMe JHaHb and JHaHc SSCCs value trends, it
was possible to understand the conformational behavior of these
compounds in solution. Indeed, by applying the QTAIM and NBO
methods, it was highlighted that both steric and hyperconjugative
effects, and not IHB, govern both the phe-OMe and tyr-OMe con-
formational preferences, mainly steric effects. Also, it was ob-
served, both experimentally and theoretically, that the phe-OMe
and tyr-OMe compounds are conformationally similar in both the
gas phase and in solution. Thus, it was demonstrated that the hy-
droxyl group para positioned, which is the only structural differ-
ence between phe-OMe and tyr-OMe, has no significant
conformational influence.
Ia
V2a
V2c
–
–
ꢁ0.006
+0.031
+0.017
ꢁ0.6161
ꢁ0.6045
ꢁ0.6092
0.131
0.122
0.124
50.791
46.193
47.499
0.007
0.006
+0.027
+0.023
Table 10
Full energy of the real system (
hyperconjugation is removed ( ELewis) and hyperconjugative energy (
tyr-OMe conformers. Energy values in kcal molꢁ1
D
EFULL)a, energy of the hypothetical case where
Ehyper) for the
D
D
.
Ia
Ib
Ic
IIIb
IIIc
0.89
10.98
11.88
IV1a
IV1b
IV1c
D
D
D
EFull
ELewis
Ehyper
0.00
5.88
7.59
0.19
2.99
4.36
0.18
8.42
9.96
1.43
4.03
4.13
0.86
7.33
8.03
0.08
9.92
11.45
1.84
11.05
10.70
IV2a
IV2b
V1a
V1b
V1c
V2a
V2b
V2c
Acknowledgements
D
D
D
EFull
ELewis
Ehyper
2.84
3.80
2.41
0.54
1.73
2.92
2.31
0.71
0.00
0.84
0.00
0.70
2.85
6.27
4.92
2.33
6.12
5.30
1.11
4.16
4.68
2.22
13.26
12.64
The authors thank a Grant # 2012/03933-5, São Paulo Research
Foundation (FAPESP) for providing financial support for this re-
search, for a scholarship [to R.A.C. #2011/01170-1, FAPESP) and
for a fellowship (to L.C.D. #2010/15764-4, FAPESP), and the
authors thank CNPq for fellowships (to R.R. and CFT). Dr. T.R. Doi’s
assistance in revising this manuscript is also gratefully
acknowledged.
a
ZPE corrections included.
hyperconjugative effects, and not IHB, are the main causes for the
conformational preferences of these compounds.
Conclusions
Appendix A. Supplementary material
Amino ester derivatives instead of the amino acid themselves
allow the study of these compounds in organic solvents by NMR,
since they not exhibit the zwitterionic structure as an amino acid
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.12.088.

R.A. Cormanich et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 123 (2014) 482–489
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