L-Tryptophan l-tryptophanium chloride

Article abstract of DOI:10.1016/j.saa.2014.09.090

l-Tryptophan l-tryptophanium chloride is a new salt with (A?A⁺) type dimeric cation. It crystallizes in the monoclinic system (space group P2₁, Z = 2). The asymmetric unit contains one zwitterionic l-tryptophan molecule, one l-tryptophanium cation and one chloride anion. The dimeric cation is formed by a OH?O hydrogen bond with the O?O distance equal to 2.5556(18) ?. The infrared and Raman spectra of the crystal are studied and compared with the spectra of l-tryptophanium chloride.

Full text of DOI:10.1016/j.saa.2014.09.090

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 743–750
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
L-Tryptophan L-tryptophanium chloride
V.V. Ghazaryan a, M. Fleck b, A.M. Petrosyan a,
⇑
a Institute of Applied Problems of Physics, NAS of Armenia, 25 Nersessyan Str., 0014 Yerevan, Armenia
b Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria
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
ꢀ
ꢀ
ꢀ
ꢀ
Single crystal of 2
obtained for the first time.
Crystal and molecular structure of
L
-TrpꢁHCl was
2
L
-TrpꢁHCl has been determined.
The existence of dimeric
-Trpꢁ ꢁ ꢁ -TrpH) cation is established.
IR and Raman spectra of 2
(
L
L
L-TrpꢁHCl
were studied and compared with
spectra of previously known
L-TrpꢁHCl.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 August 2014
Received in revised form 14 September 2014
Accepted 19 September 2014
Available online 30 September 2014
L
-Tryptophan
L
-tryptophanium chloride is a new salt with (Aꢁ ꢁ ꢁA+) type dimeric cation. It crystallizes in
the monoclinic system (space group P21, Z = 2). The asymmetric unit contains one zwitterionic L-trypto-
phan molecule, one L-tryptophanium cation and one chloride anion. The dimeric cation is formed by a
OAHꢁ ꢁ ꢁO hydrogen bond with the Oꢁ ꢁ ꢁO distance equal to 2.5556(18) Å. The infrared and Raman spectra
of the crystal are studied and compared with the spectra of
L
-tryptophanium chloride.
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
L
-Tryptophan
Structure
Dimeric cation
L-tryptophanium chloride
Vibrational spectra
Introduction
dimeric cation can form all amino acids except amino acids capable
to form doubly charged cation (e.g. lysine, ornithine, histidine and
arginine), while these amino acids can form the A+ꢁ ꢁ ꢁA2+ type cat-
ion. These two type dimeric cations can form short and strong
hydrogen bonds, while the A+ꢁ ꢁ ꢁA+ type dimeric cation is formed
mainly by b-alanine and is formed by relatively long hydrogen
bonds due to additional repulsion between positively charged cat-
ions. The largest number of salts is known for Aꢁ ꢁ ꢁA+ type dimeric
cation. For some amino acids large number of salts with Aꢁ ꢁ ꢁA+
type dimeric cation is known. For some others only one salt is
known and there are amino acids for which such salts were not
obtained to date.
Reactions of amino acids with inorganic and organic acids result
in various type of compounds: simple salts with definite cations
and anions, mixed salts with different anions and/or cations, salts
with dimeric cations and adducts without forming of salts [1].
Three type dimeric cations are known: Aꢁ ꢁ ꢁA+, A+ꢁ ꢁ ꢁA+ and
A+ꢁ ꢁ ꢁA2+, where A, A+, A2+ are amino acids in zwitterionic state,
singly and doubly charged cations respectively. The Aꢁ ꢁ ꢁA+ type
⇑
Corresponding author. Tel.: +374 10 241106; fax: +374 10 281861.
E-mail
addresses:
aram.m.petrosyan@gmail.com,
apetros@iapp.sci.am
(A.M. Petrosyan).
http://dx.doi.org/10.1016/j.saa.2014.09.090
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

744
V.V. Ghazaryan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 743–750
The largest amino acid
-alanine molecules, in which one hydrogen atom of the methyl
group is substituted by an indolyl group. For -tryptophan some
simple salts were obtained and structurally characterized. Takiga-
wa et al. [2] obtained chloride and bromide of -tryptophanium,
L
-tryptophan may be considered as an
was and mounted on a glass needle with laboratory grease.
Single-crystal X-ray intensity data of 2
-TrpꢁHCl were obtained
by a measurement on a Bruker APEX II diffractometer, equipped
L
L
L
0
with a graphite monochromator and using Mo K
a (k = 0.71073 ÅA)
L
radiation. The structure was solved using direct methods; subse-
quent difference Fourier syntheses and least-square refinements
yielded the positions of the remaining atoms. Non-hydrogen atoms
were refined with independent anisotropic displacement parame-
ters, hydrogen atoms with isotropic displacement parameters.
The hydrogen atoms were treated as riding on their parent atoms,
except for the hydrogen atoms of the oxygen and nitrogen atoms.
All calculations were performed using the Bruker instrument soft-
ware and the SHELX97 program package [11–13].
The crystallographic data as well as details of the measure-
ment are listed in Table 1. Further crystallographic data have
been deposited with the Cambridge Crystallographic Data Centre
and can be obtained free of charge via www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033), citing the title of this paper and the CCDC No.
1017250.
determined their structures and showed that chloride and bromide
are isostructural. In addition, they obtained one more salt with
2L
-TrpꢁHBrꢁ0.5H2O composition according to chemical analysis
and determined its symmetry and unit cell parameters without
structure determination, however. The structure of -tryptophani-
um bromide was determined also in [3] and more accurately in
[4] at 100 K. The authors of [5] obtained crystals of -tryptophani-
um picrate, while in [6,7] crystals of -tryptophanium phosphite
and -tryptophanium hydrogen selenite were obtained and their
L
L
L
L
crystal structures were determined. Bakke and Mostad [8]
determined the structure of DL-tryptophan and also obtained
D-tryptophanium hydrogen oxalate, when they tried to obtain
DL-tryptophanium hydrogen oxalate. The first and during long
time the only structurally characterized salt with
-tryptophanium dimeric cation was the salt
(HgCl3)ꢂ obtained by Book et al. [9]. In our paper [10] we reported
a reinvestigation of the structure of -tryptophanium picrate,
where we showed that there is a -tryptophan -tryptophanium
dimeric cation in its structure. Inspired by this result, we supposed
that the salt 2
-TrpꢁHBrꢁ0.5H2O also may be formed by this mech-
anism and contain -tryptophan -tryptophanium dimeric cations,
so we decided to return to study the -Trp + HCl + H2O and
-Trp + HBr + H2O systems. Our research showed that in addition
to -TrpꢁHBr and 2 -TrpꢁHBrꢁ0.5H2O at least two more
-TrpꢁHCl,
salts, namely, 2 -TrpꢁHBr exist in these systems
-TrpꢁHCl and 2
and we optimized the conditions of -TrpꢁHBr. Based
-TrpꢁHCl and
on spectroscopic data we supposed that they contain -tryptophan
-tryptophanium dimeric cations. Recently we could obtain the salt
L
-tryptophan
L
(
L
-Trpꢁ ꢁ ꢁ
L
-Trp+)
L
L
L
Vibrational spectra
L
Attenuated total reflection Fourier-transform infrared spectra
(FTIR ATR) were registered by a Nicolet 5700 spectrometer (ZnSe
prizm, Happ–Genzel apodization, ATR distortion is corrected, num-
ber of scans 32, resolution 4 cmꢂ1). Part of the IR spectrum in the
region 500–400 cmꢂ1 was taken from FTIR spectra registered with
L
L
L
L
L
L
L
Nujol mull (4000–400 cmꢂ1
2 cmꢂ1).
,
number of scans 32, resolution
L
L
L
L
L
Fourier-transform Raman spectra were registered by a NXR
FT-Raman Module of a Nicolet 5700 spectrometer at room temper-
ature with resolution 4 cmꢂ1. Number of scans and laser power at
L
2
L
-TrpꢁHCl in form of single crystal and determined its crystal and
molecular structure.
the sample for
L-tryptophanium chloride (I) were 256 and 0.29 W,
In the present paper we report the structure and vibrational
while for -tryptophan
L
L
-tryptophanium chloride (II) were: 512
spectra of 2
L
-TrpꢁHCl and compare with structure and vibrational
and 0.45 W.
spectra of
L
-TrpꢁHCl.
Experimental
Table 1
Crystal data and details of the refinement for L-tryptophan L-tryptophanium chloride.
Synthesis and crystal growth
Formula
Mr
C22H25ClN4O4
444.91
As initial reagents we used
TLC) purchased from ‘‘Sigma–Aldrich’’ Chem. Co. and hydrochloric
acid (‘‘chemically pure’’ grade, 32%) from ‘‘Reakhim’’ Co. Both
L
-tryptophan (reagent grade, P98%,
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21
13.284(3)
5.5844(11)
14.691(3)
90.00
2
L
-TrpꢁHCl and -TrpꢁHCl compounds have incongruent solubility.
L
They have been obtained at room temperature by evaporation of
aqueous solutions containing non-stoichiometric ratios of solved
a
(°)
b (°)
99.46(3)
90.00
1075.0(4), 2
1.374
components:
M-ratio first
L
-tryptophan and hydrochloric acid. From 2:1
-tryptophan is formed. From 1.7:1 and 1.5:1
c (°)
V (Å3), Z
L
Dcalc (gcmꢂ3
)
M-ratios 2
L
-TrpꢁHCl in form of needle crystals were obtained with
-tryptophan. At equimolar ratio of
-tryptophan and HCl first the
-TrpꢁHCl crystal is formed
-tryptophan. The needle-shaped crystals
l(Mo K
a
) (cmꢂ1
)
0.210
negligible admixture of
L
L
F (000)
468
2L
T (K)
296(2)
hkl range
Reflections measured
Reflections unique
ꢂ19/19, ꢂ8/8, ꢂ21/21
without admixture of
L
7335
of
obtain the chloride analog 2
-TrpꢁHBrꢁ0.5H2O [2]. At 0 °C, however, anhydrous 2
was formed.
L
-TrpꢁHCl were obtained at 1:1.5 M-ratio. We also tried to
-TrpꢁHClꢁ0.5H2O of known compound
-TrpꢁHCl
7335
L
Data with (Fo > 4r(Fo))
5858
2L
L
Rint
0.0000
Parameters refined
293
Flack parameter [14]
ꢂ0.04(4)
R(F)a (for Fo > 4
r(Fo))
0.0377
wR(F2)a (all reflections)
0.0930
Crystal structure determination
Weighting parameters a, b
D
0.052/0.013
0.179/ꢂ0.252
qfin (max/min) (e Å–3
)
Suitable single crystals of 2
L-TrpꢁHCl were manually selected
a
w(F2o ꢂ F2c)2/
R
wF4o]1/2, w = 1/[
r
2(F2o) + (a ꢃ P)2
+
and checked for irregularities under the microscope. A well-devel-
R1 =
R
||Fo| ꢂ |Fc||/
R
|Fo|, wR2 = [
R
b ꢃ P], P = (F2o + 2Fc2)/3.
oped crystal with the approximate size of 0.1 ꢃ 0.08 ꢃ 0.08 mm3

V.V. Ghazaryan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 743–750
745
Table 2
Results and discussion
Selected bond lengths (Å) and angles (°) in L-tryptophan L-tryptophanium chloride.
Molecular and crystal structure of
chloride
L
-tryptophan
L
-tryptophanium
C1AAO1A
C1AAO2A
C1AAC2A
C2AAN1A
C2AAC3A
C3AAC4A
C4AAC5A
C4AAC7A
C5AAN2A
N2AAC6A
C6AAC11A
C6AAC7A
C7AAC8A
C8AAC9A
C9AAC10A
C10AAC11A
O2AAC1AAO1A
O2AAC1AAC2A
O1AAC1AAC2A
N1AAC2AAC1A
N1AAC2AAC3A
C1AAC2AAC3A
C4AAC3AAC2A
C5AAC4AAC7A
C5AAC4AAC3A
C7AAC4AAC3A
C4AAC5AAN2A
C6AAN2AAC5A
N2AAC6AAC11A
N2AAC6AAC7A
C11AAC6AAC7A
C8AAC7AAC6A
C8AAC7AAC4A
C6AAC7AAC4A
C9AAC8AAC7A
C8AAC9AAC10A
C11AAC10AAC9A
C10AAC11AAC6A
1.3134(18)
1.2032(18)
1.5148(19)
1.4882(18)
1.5372(19)
1.498(2)
1.370(2)
1.439(2)
1.374(2)
1.363(2)
1.400(2)
1.416(2)
1.402(2)
1.382(2)
1.401(3)
1.375(3)
124.74(14)
123.34(13)
111.90(12)
107.76(11)
111.35(11)
113.70(11)
115.32(12)
106.08(13)
127.28(14)
126.60(13)
110.03(14)
109.39(13)
130.43(14)
107.47(13)
122.10(14)
118.85(13)
134.13(14)
107.02(13)
118.72(15)
121.54(16)
121.25(16)
117.54(15)
C1BAO1B
C1BAO2B
C1BAC2B
C2BAN1B
C2BAC3B
C3BAC4B
C4BAC5B
C4BAC7B
C5BAN2B
N2BAC6B
C6BAC11B
C6BAC7B
C7BAC8B
C8BAC9B
C9BAC10B
C10BAC11B
O2BAC1BAO1B
O2BAC1BAC2B
O1BAC1BAC2B
N1BAC2BAC1B
N1BAC2BAC3B
C1BAC2BAC3B
C4BAC3BAC2B
C5BAC4BAC7B
C5BAC4BAC3B
C7BAC4BAC3A
C4BAC5BAN2A
C6BAN2BAC5A
N2BAC6BAC11B
N2BAC6BAC7B
C11BAC6BAC7B
C8BAC7BAC6B
C8BAC7BAC4B
C6BAC7BAC4B
C9BAC8BAC7B
C8BAC9BAC10B
C11BAC10BAC9B
C10BAC11BAC6B
1.245(2)
1.2473(18)
1.527(2)
1.4896(18)
1.528(2)
1.489(2)
1.368(2)
1.438(2)
1.360(2)
1.369(2)
1.393(2)
1.412(2)
1.404(2)
1.378(3)
1.402(3)
1.375(3)
126.28(15)
116.45(15)
117.25(13)
109.87(11)
111.37(12)
107.22(12)
117.43(13)
105.84(14)
126.16(15)
127.78(14)
110.51(14)
109.29(14)
130.69(15)
107.28(13)
122.03(15)
118.61(14)
134.33(15)
107.06(13)
119.20(16)
121.07(16)
121.15(16)
117.92(16)
The salt 2
group P21, details in Table 1). The unit cell contains one asymmet-
ric formula unit (Fig. 1) comprising one -tryptophanium moiety,
one zwitterionic -tryptophan molecule and one chloride anion.
L-TrpꢁHCl crystallizes in the monoclinic system (space
L
L
Selected bond lengths and valence angles are shown in Table 2.
Bond lengths of carboxyl and carboxylate groups are typical for
these groups. The bond lengths of the carboxyl group C1A@O2A
(1.2032(18) Å) and C1AAO1A (1.3134(18) Å) are in good agree-
ment with respective data (1.204(3) Å and 1.316(3) Å) of
phanium cation in the structure of -tryptophanium bromide [4],
which are more accurate than that of [2,3]. Hydrogen bond param-
eters are shown in Table 3. -Tryptophanium moiety and zwitter-
ionic
-tryptophan are bonded with O1AAH1Aꢁ ꢁ ꢁO2B hydrogen
L-trypto-
L
L
L
bond with Oꢁ ꢁ ꢁO distance equal to 2.5556(18) Å thus forming
dimeric cation. The bond length C1BAO2B (1.2473(18) Å) is
slightly longer than C1BAO1B (1.245(2) Å) probably due to the
formed hydrogen bond. The Oꢁ ꢁ ꢁO distance (2.5556(18) Å) being
short should be considered, however, as relatively long among
Oꢁ ꢁ ꢁO known distances of Aꢁ ꢁ ꢁA+ type dimeric cations. The shortest
Oꢁ ꢁ ꢁO contact (2.381(12) Å) was found in the structure of 2
L-
AlaꢁHBr, while the longest Oꢁ ꢁ ꢁO distance was found in the struc-
tures of 2GlyꢁHBr (2.562(3) Å) and 2 -ValꢁHClO4ꢁH2O (2.562(4) Å)
L
(see [1] and references therein). The majority of hydrogen bonds
fall in the 2.42–2.48 Å interval (ca. 71%) with typical value of
2.45 0.01 Å [1]. In the structure of (
Oꢁ ꢁ ꢁO (2.41(2) Å) distance was found [9], while in the structure
of -tryptophanium picrate picric acid the Oꢁ ꢁ ꢁO
-tryptophanꢁ ꢁ ꢁ
distance makes 2.470(6) Å [10]. So, one of three known Oꢁ ꢁ ꢁO dis-
tances of -tryptophanium cations is among the rel-
L
-Trpꢁ ꢁ ꢁ
L
-Trp+)HgClꢂ3 the
L
L
L-tryptophanꢁ ꢁ ꢁ
L
atively short contacts, the second is among the most frequent
distances, and the Oꢁ ꢁ ꢁO distance of the present structure is among
the relatively long distances.
The
L
-tryptophanium moiety in addition to mentioned OAHꢁ ꢁ ꢁO
Table 3
Hydrogen bonds parameters for L-tryptophan L-tryptophanium chloride (in Å and °).
hydrogen bond forms also hydrogen bonds by the N(1A)H+3 group.
The N(1A)H+3 group forms one weak hydrogen bond with O2B oxy-
gen atom of symmetry-related zwitterionic L-tryptophan moiety
and two hydrogen bonds with chloride ion (Table 3). The N2AAH
DAHꢁ ꢁ ꢁA
DAH
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
<DHA
O1AAH1Aꢁ ꢁ ꢁO2Bi
N1AAH11Aꢁ ꢁ ꢁO2Bii
N1AAH12Aꢁ ꢁ ꢁCl1
N1AAH13Aꢁ ꢁ ꢁCl1i
N1BAH11Bꢁ ꢁ ꢁCl1ii
N1BAH12Bꢁ ꢁ ꢁO1Bi
N1BAH13Bꢁ ꢁ ꢁO1Biii
N2BAH21Bꢁ ꢁ ꢁCl1iv
0.87(3)
0.89
0.89
0.89
0.89
0.89
0.89
0.81(2)
1.69(3)
2.23
2.33
2.40
2.30
1.89
2.13
2.62(2)
2.5556(18)
2.9959(19)
3.1910(14)
3.2588(15)
3.1682(16)
2.7410(18)
2.9621(17)
3.2762(17)
171(3)
144
164
161
166
158
155
139.1(19)
group of indole group does not form any usual hydrogen bonds.
The N(1B)H+3 group of
L-tryptophan moiety forms one hydrogen
bond with chloride ion and two hydrogen bonds with O1B oxygen
atoms of two nearest symmetry-related molecules (Table 3). In
Symmetry code: (i) x, y ꢂ 1/2, ꢂz; (ii) ꢂx + 1, y ꢂ 1/2, ꢂz + 1; (iii) ꢂx + 1, y ꢂ 3/2,
ꢂz + 2; (iv) x ꢂ 1, y ꢂ 3/2, ꢂz.
contrast to the L-tryptophanium moiety here the N2BAH group
of indole group also forms hydrogen bond with chloride ion. Thus,
chloride ion forms four hydrogen bonds of NAHꢁ ꢁ ꢁCl type.
In dimeric cation zwitterionic
L-tryptophan molecule and
L
-tryptophanium moiety have different conformations. Disposition
of amino and carboxyl groups are determined by N1C2C3C4 (v1
)
and C1C2C3C4 (v L-tryptophanium moiety these
1,2) [15]. For
values make 63.63°(v1) and (ꢂ58.31°)(v1,2), that is, have gauche
disposition, while in case of zwitterionic moiety these values are
(ꢂ58.76°)(v1) and (ꢂ179.00°)(v1,2), that is, carboxylate group has
trans disposition. This conformation is similar to one in the
structure of
(L-Trpꢁ ꢁ ꢁ
L L-tryptophanium
-Trp+)HgClꢂ3 , where for
Fig. 1. Molecular structure of L-tryptophan L-tryptophanium chloride. Note that
molecule A is in cation form, molecule B is a zwitterion, the charge is counterbal-
anced by the chloride anion.
moiety these values make 65.98°(
v
1) and (ꢂ54.28°)(
v
1,2), while
for zwitterionic moiety these values make (ꢂ76.33°)(v1) and

746
V.V. Ghazaryan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 743–750
Fig. 2. Packing diagram of L-tryptophan L-tryptophanium chloride, viewed along [010].
167.10°)(
v
1,2) [9]. In the structure of
L
-tryptophanꢁ ꢁ ꢁ
L
-tryptophani-
(v
1,2), while in the case of the zwitterionic moiety the situation
um picrate picric acid conformations are different. For the
is opposite: (172.62°)(v1) and (69.07°)(v1,2) [10]. In the case of
L
-tryptophanium moiety the NH+3 group has gauche disposition
L
-tryptophanium chloride these values are equal to (ꢂ62.72°)(v1
)
(ꢂ72.15°)(v1), the carboxyl group trans disposition (168.50°)
and (57.85°)(v1,2) [2].
Fig. 3. Infrared and Raman spectra of L-tryptophanium chloride.

V.V. Ghazaryan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 743–750
747
Fig. 4. Infrared and Raman spectra of L-tryptophan L-tryptophanium chloride.
In Fig. 2 a packing diagram of 2
L
-TrpꢁHCl is shown, viewed along
characteristic spectroscopic feature of indole group in the molecule
of tryptophan is the stretching vibration of NAH group in high-
frequency region (see e.g. [16,17]). It is easy to find it in Fig. 3 at
3383 cmꢂ1 (IR) and 3385 cmꢂ1 (R) and in Fig. 4 at 3380 cmꢂ1 (IR)
and 3384 cmꢂ1 (R). Stretching vibrations of CH and CH2 groups
are better revealed in Raman spectrum. The lines at 3123 cmꢂ1
(Fig. 3) and 3119 cmꢂ1 (Fig. 4) we assign to stretching vibration
of CAH bond of pyrrole ring. The lines at 3062 cmꢂ1 (Fig. 3) and
3060 cmꢂ1 (Fig. 4) we assign to CAH bonds of benzene ring, the
lines at 2966 cmꢂ1 and 2941 cmꢂ1 (Fig. 3) and 3038, 2960, 2938,
2924 cmꢂ1 (Fig. 4) to aliphatic CH and CH2 groups. It may be noted
that these lines are superimposed on broad bands in the region
3300–2500 cmꢂ1, which we assign to stretching vibrations of
NAH bonds of NH+3 groups. In contrast to the Raman spectra, in
the infrared spectra stretching vibrations of NAH bonds of NH3+
groups are revealed as strong absorption bands in 3300–
2500 cmꢂ1 (Fig. 3) and 3300–2100 cmꢂ1 (Fig. 4) regions, while
[010]. The units are packed in a double layer-like pattern, with the
amino and acid groups facing toward each other (in the middle of
Fig. 2) and the indolyl groups facing outwards. The structure is
polar; all amino acids molecule are inclined in the same direction
(in Fig. 2, the indolyl groups are inclined toward the viewer, the
amino and acid ends away from the viewer). The chloride anions
are located in the voids of the structure (the discussion of the
hydrogen bonding network is given above and further elaborated
on in ‘Infrared and Raman spectra’).
Infrared and Raman spectra
For discussion of the 2
ister and compare with the
spectra of
-TrpꢁHCl (I) and 2
with the spectrum of -alaninium chloride (
with published spectra of indole and -tryptophan is useful too.
As it was shown in [1] the published spectra of
-AlaꢁHCl actually
-alanine. So, in Fig. 5 we show also the IR and Raman
-AlaꢁHCl. Crystals of -AlaꢁHCl were obtained similarly
L
-TrpꢁHCl spectra we found useful to reg-
-TrpꢁHCl spectra. In Figs. 3 and 4 the
-TrpꢁHCl (II) are shown. Comparison
-AlaꢁHCl) as well as
L
L
L
L
L
L
m
(CH) as weak peaks. In the structure of
L
-TrpꢁHCl the carboxyl
L
group forms hydrogen bond with chloride anion OAHꢁ ꢁ ꢁCl with
Oꢁ ꢁ ꢁCl distance 3.04 Å and the NH+3 group forms three NAHꢁ ꢁ ꢁCl
hydrogen bonds with 3.17 Å, 3.19 Å and 3.24 Å distances. For these
belong to
spectra of
L
L
L
to
L
L
-TrpꢁHCl by evaporation from aqueous solution containing
hydrogen bonds one may expect [18] m(OH) and m(NH) near
-alanine and HCl in M-ratio 1:1.5. We expect to find in spectra
2900 cmꢂ1, 2950 cmꢂ1, 3000 cmꢂ1 and 3100 cmꢂ1 respectively,
which well correspond to the peaks at 3019 cmꢂ1, 2937 cmꢂ1
of (I) and (II) the presence of characteristic bands of indole group,
carboxyl group, protonated NH+3 group and CH, CH2 groups, as well
as reflection in the spectra of (II) the presence of OAHꢁ ꢁ ꢁO
and 2920 cmꢂ1 (Fig. 3). In the IR spectrum of 2
L
-TrpꢁHCl (Fig. 4)
there is a broad absorption band with peak at 2882 cmꢂ1, which
also we assign to
(NH) of two NH+3 groups. This peak may be
hydrogen bond of
L
-tryptophan
L-tryptophanium dimer. The
m

748
V.V. Ghazaryan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 743–750
Fig. 5. Infrared and Raman spectra of
L-AlaꢁHCl.
caused by
m
(NH) of N1BAH12Bꢁ ꢁ ꢁO1B with shortest NAHꢁ ꢁ ꢁO bond
correlates with torsion v2,1 angle [21] (see also [20]):
1542 + 6.7(cos3|
2,1| + 1)1/2, where v2,1 is the C2C3C4C5 torsion
angle (see Fig. 1). For
-TrpꢁHCl [2] this angle is equal to ꢂ77.96°
and
(1545 cmꢂ1) value. In case of 2
m
W3(cmꢂ1) =
equal to 2.7410(18) Å. The peak at 3159 cmꢂ1 we also assign to
v
m
(NH) and not to
in the Raman spectrum. The
expected in this region, while
at lower wavenumbers. The presence of COOH carboxyl group of
-tryptophanium cation in
-TrpꢁHCl is confirmed by presence of
a strong absorption band at 1734 cmꢂ1 and a line at 1738 cmꢂ1
m
(CH) because of absence of corresponding line
(NH) of all other NAHꢁ ꢁ ꢁO are
(OH) of OAHꢁ ꢁ ꢁO bond is expected
L
m
m
W3 = 1546 cmꢂ1, which is in good agreement with observed
m
L
-TrpꢁHCl the value of v2,1 for zwit-
terionic moiety makes 101.89° and respective calculated value
L
L
m
W3 = 1550 cmꢂ1, while for
L
-tryptophanium moiety v2,1 = 82.95°
and respectively
m
W3 = 1547 cmꢂ1. However, in this case the
caused by
m
(C@O) (Fig. 3). In the spectra of 2
L
-TrpꢁHCl these values
observed value is equal to 1554 cmꢂ1. In the IR spectra the band
at 1580 cmꢂ1 (Fig. 3) and 1567 cmꢂ1 (Fig. 4) may also be caused
(or mainly be caused) by asymmetric deformation vibration of
NH+3 groups, while rather strong bands at 1511 cmꢂ1 (Fig. 3) and
1496 cmꢂ1 (Fig. 4) we assign to symmetric deformation vibration
of the same groups. It should be noted that in the IR spectrum of
are somewhat lower: 1728 cmꢂ1 (IR) and 1732 cmꢂ1 (R) (Fig. 4).
The region 1700–400 cmꢂ1 also contains modes caused by both
the main alaninyl group and side indolyl group. The Raman-lines
in the spectrum of 2
L
-TrpꢁHCl at 1620 cmꢂ1, 1578 cmꢂ1 and
1554 cmꢂ1 (Fig. 4) we assign to vibrations of indolyl group. One
may assume that the line at 1620 cmꢂ1 is caused by asymmetric
L
-AlaꢁHCl (Fig. 5) there are similar peaks at 1606 cmꢂ1 and
stretching vibration of carboxylate group of zwitterionic
L-trypto-
1582 cmꢂ1, which are caused probably by skeletal vibration of
phan moiety. Indeed, the authors of [17] assigned the line at
L
-alaninium cation and asymmetric deformation vibration of NH3+
1622 cmꢂ1 to asymmetric stretching vibration of carboxylate
group, while the strong band at 1493 cmꢂ1 may be assigned to
symmetric deformation vibration of NH+3 group. Rather intensive
Raman-lines at 1431 cmꢂ1 (Fig. 3) and 1428 cmꢂ1 (Fig. 4) are char-
acteristic for deformation scissoring vibration of CH2 group. The
absorption band at 1411 cmꢂ1 (Fig. 4) may be caused by symmetric
stretching of COOꢂ carboxylate group. However, it should be noted
group of zwitterionic L-tryptophan. However, we cannot agree
with this assignment because this vibration is characteristic also
for indole (see e.g. [19]). In addition, these three lines are present
in the spectrum of
L
-TrpꢁHCl (Fig. 3) at 1621 cmꢂ1, 1581 cmꢂ1
and 1545 cmꢂ1 with similar intensities, where carboxylate COOꢂ
group is absent. The lines at 1545 cmꢂ1 and 1554 cmꢂ1 (labeled
as W3) are caused mainly by stretching vibration of C4AC5 bond
of pyrrole ring (see Fig. 1) (see e.g. [20]. Two other lines are caused
by stretching vibrations of benzene ring. The value of W3
that in the IR spectrum of
L
-TrpꢁHCl (Fig. 3) there is also a peak at
1418 cmꢂ1 as well as in the spectra of
L
-AlaꢁHCl (Fig. 5) and indole
[19]. There is an absorption band with peaks at 1362 cmꢂ1 and
1341 cmꢂ1 (Fig. 4) with respective Raman lines at 1365 cmꢂ1 and

V.V. Ghazaryan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 743–750
749
Table 4
Wavenumbers (in cmꢂ1) and assignment of peaks in IR and Raman spectra of L-tryptophanium chloride (I) and L-tryptophan L-tryptophanium chloride (II).
I
I
II
II
IR
Raman
IR
Raman
Assignment
3383
3385
3380
3159
3117
3076; 3055
3384;3363sh
m
m
m
m
m
m
m
(NH) Indole
(NH), NH+3
(CH) pyrrole
(CH) benzene
(NH), NH+3
3123
3062
3119
3060
3038
3019; 2937; 2920
2966; 2941
2880
2681
3014; 2965; 2935
2882
2711; 2634; 2605
1922
1728
1651
1606
1567
3038; 2960; 2938; 2924
2861
2682
(CH), CH, CH2
(NH), NH+3
2634; 2608
1938; 1903
1734
1663
1604
Combi
Combi
m(C@O)
1738
1732
1621; 1607
1581
1545
1620; 1600
1578
1554
BR str
1580
1542
das(NH+3), BR str
Indole stretch
1531sh; 1518
1495
1519
1496
1511
1493
ds(NH+3)
1461; 1445
1418
1464; 1447
1431
1456
1422
1411
1362; 1341
1460
1428
1408sh
1365; 1344
d(CH2)
m
s(COOꢂ)
1370; 1352; 1340
1328
1371; 1356; 1343
1330
1303
1304
1306
1303
d(CH)
1260
1290; 1262
1236; 1215; 1190
1140; 1129; 1099
1055
1259
1276; 1261
1233; 1211; 1190
1148; 1135; 1117
1075; 1054
1013
1233; 1212; 1190
1138; 1126; 1096
1058; 1032
1014
1243; 1229; 1209
1146; 1130; 1115; 1095
1072; 1058; 1050
1011
q
m
(NH+3), d(NH), Indole
(CAN)
1016
b(CH), Indole
954; 937
902
1002; 955; 940
904
987; 965; 953; 933; 921
912
991; 968
877
850; 830; 812
773
880
896; 877
851; 828; 813
778; 757
898; 881
858; 830
780
q(CH2)
865; 853; 833; 817
774
748
761
745sh; 739
761
c
(CH), Indole
747; 722
702
690; 646
593
575; 557; 543
517
741
700
648
595
553
514
681; 637
594
573; 547; 534
521
705; 684; 645
604; 594
576; 554; 538
520
c
(NH), Indole
472; 459; 429
416
467; 462; 428; 423
407
463; 427
406
367; 345; 322
299; 243; 217
353; 327
250; 192
Combi – combination, overtone, sh – shoulder, s – symmetric, as – asymmetric,
rocking, BR – benzene ring.
m – stretching, d – deformation, x – wagging, s – torsion, b – in-plane, c – out of plane, q –
1344 cmꢂ1. This is the region of wagging vibration of CH2 group.
Similar peaks are present in the spectra of
-TrpꢁHCl (Fig. 3) and
-AlaꢁHCl (Fig. 5). However, the spectra of indole also contain peaks
in this region; hence they may be caused also by indolyl group. In
the region of
(CAOH) (ca. 1250 cmꢂ1) there is a band with peaks
additional absorption in the region 1700–800 cmꢂ1. We consider
that this additional absorption is caused by
hydrogen bond of dimeric cation. This absorption band is not
strong enough. In addition, it is difficult to point out its center.
Nevertheless, its position is somewhat shifted toward lower wave-
numbers compared to that expected according to the curve of
L
m(OH) of OAHꢁ ꢁ ꢁO
L
m
at 1259, 1243, 1229, 1209 cmꢂ1 and Raman lines at 1233,
1211 cmꢂ1 (Fig. 4). This is also the region of twisting vibration of
CH2 group and also deformation vibration of CH bonds of indolyl
group. The absorption peak at 1146 cmꢂ1 (Fig. 4) may be caused
by rocking vibration of NH+3 group, however, there are vibrations
of indole group also in this region. There are rather intensive
Raman-lines at 1013 cmꢂ1 and 761 cmꢂ1 in the spectrum of
known correlation between
m
(OH) and R(Oꢁ ꢁ ꢁO) [22] (ca.
2000 cmꢂ1). However, it should be noted that there are not exper-
imental results for R(Oꢁ ꢁ ꢁO) value near 2.55 Å [22]. There are only
two points for ca. 2.58 Å with
m
(OH) values ca. 2000 cmꢂ1 and
1800 cmꢂ1. Later reinvestigated or investigated a number of crys-
tals with dimeric cations with OAHꢁ ꢁ ꢁO hydrogen bonds [1]. For
example in the structures of 2GlyꢁHCl and 2Gly.HBr the Oꢁ ꢁ ꢁO dis-
tances are close to this value: 2.552(5) Å [23] and 2.562(3) Å [1]
respectively. The IR spectra of these crystals provided in [24,25]
consist each two broad absorption bands: 3118–2241 cmꢂ1 and
ca. 2200–500 cmꢂ1 centered at ca. 3000 cmꢂ1 and ca. 1500 cmꢂ1
for 2Gly.HCl [24] and 3116–2230 cmꢂ1 and ca. 2200–550 cmꢂ1
centered at ca. 3000 cmꢂ1 and ca. 1500 cmꢂ1 for 2Gly.HBr [25].
The authors of [24,25] assigned the bands at 3118–2241 cmꢂ1
and 3116–2230 cmꢂ1 to stretching vibrations of OAH and NH3+
groups. In our opinion these bands are caused by stretching
2
L
-TrpꢁHCl (Fig. 4) and similar lines in the spectrum of -TrpꢁHCl
L
at 1016 cmꢂ1 and 761 cmꢂ1 (Fig. 3). Respective absorption band
of the line at 761 cmꢂ1 is at 748 cmꢂ1 (Fig. 3) and doublet with
peaks at 745 cmꢂ1 and 739 cmꢂ1 (Fig. 4). Both bands are rather
strong. These Raman-lines and absorption bands are caused by
vibrations of indole group. They are present also in the spectra of
L-tryptophan [16,17] and indole [19] and probably are caused by
deformation of CH groups and ring breathing.
Let us now compare the two IR spectra in Figs. 3 and 4 entirely.
One can note that absorption peaks in Fig. 4 are superimposed on

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V.V. Ghazaryan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 743–750
vibration of NH+3 groups, while the bands centered at ca. 1500 cmꢂ1
are caused by stretching vibration of OAH groups. More detailed
study of the spectra of 2GlyꢁHCl was carried out in [26]. These
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m
(OH) the peak at 2026 cmꢂ1. The value
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R(Oꢁ ꢁ ꢁO) = 2.55 Å falls into the region which separates the regions
with weak and intermediate hydrogen bonds from the region with
strong hydrogen bonds [22].
[10] A.M. Petrosyan, M. Fleck, V.V. Ghazaryan, Reinvestigation of
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L-tryptophan
L
L
[11] NONIUS, Kappa CCD Program Package: COLLECT, DENZO, SCALEPACK, SORTAV,
Nonius BV, Delft, The Netherlands, 1999.
[12] Z. Otwinowski, W. Minor, Processing of X-ray diffraction data collected in
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Conclusions
-Tryptophan L-tryptophanium chloride, a new salt with an
(AꢁLꢁ ꢁA+) type dimeric cation, crystallizes in monoclinic system,
with space group P21. The Oꢁ ꢁ ꢁO distance in OAHꢁ ꢁ ꢁO hydrogen
bond of dimeric cation makes 2.5556(18) Å. Being short this hydro-
gen bond is relatively long compared to other known salts with
dimeric cation, particularly, compared to two previously known
[16] X. Cao, G. Fischer, Infrared spectral, and conformational studies of zwitterionic
L-tryptophan, J. Phys. Chem. A 103 (1999) 9995–10003.
salts with (
L
-Trpꢁ ꢁ ꢁ
L
-Trp+) dimeric cations: (
L
-Trpꢁ ꢁ ꢁ
L
-Trp+)HgCl3
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L
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[10]. Conformations of zwitterionic and cationic moieties in
dimeric cation are different. Vibrational (IR and Raman) spectra
of 2L-Trp.HCl were studied and compared with spectra of L-Trp.HCl
(see Table 4) and L-Ala.HCl.
L
-Trpꢁ ꢁ ꢁ
L
-Trp+) picrate picric acid (2.470(6) Å)
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