Formyl-l-arginine monohydrate

Article abstract of DOI:10.1016/j.molstruc.2006.07.025

A new compound N^α-formyl-l-arginine ⁺(H₂N)₂CNH(CH₂)₃CH(HN(HCO))COO^- with zwitter-ionic structure has been found out in the system l-arginine + HCOOH + H₂O. The title compound crystallizes in monoclinic system (space group P2₁) as monohydrate. Crystal structure has been determined by single crystal X-ray diffraction method and FTIR ATR, FT Raman spectra are reported. Dehydration process has been studied.

Full text of DOI:10.1016/j.molstruc.2006.07.025

Journal of Molecular Structure 831 (2007) 90–96
www.elsevier.com/locate/molstruc
q
Formyl-L-arginine monohydrate
a
b
a
a,
*
H.A. Karapetyan , M.Yu. Antipin , R.P. Sukiasyan , A.M. Petrosyan
a
Molecule Structure Research Center, Yerevan, Armenia
New Mexico Highlands University, Las Vegas, NM 87701, USA
b
Received 7 June 2006; received in revised form 13 July 2006; accepted 22 July 2006
Available online 7 September 2006
Abstract
A new compound Na-formyl-L-arginine +(H2N)2CNH(CH2)3CH(HN(HCO))COOꢀ with zwitter-ionic structure has been found out in
the system L-arginine + HCOOH + H2O. The title compound crystallizes in monoclinic system (space group P21) as monohydrate. Crys-
tal structure has been determined by single crystal X-ray diffraction method and FTIR ATR, FT Raman spectra are reported. Dehydra-
tion process has been studied.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Formyl-L-arginine; Crystal structure; Vibrational spectra
1. Introduction
L-ArgÆHCOOH with orthorhombic symmetry was
obtained. Earlier in Ref. [4] the salt L-ArgÆHCOOH has
been obtained in form of powder. During crystal growth
of L-ArgÆHCOOH a new compound Na-formyl-L-arginine
(NFLA) has been discovered [5]. The present work deals
with investigation of this compound by X-ray diffraction
and vibrational spectroscopic methods.
In the system L-Arg + H2C2O4 + H2O, where L-Arg is L-ar-
ginine and H2C2O4 is oxalic acid, in addition to salts with
various compositions, a compound with non-salt nature Na-
oxalyl-L-arginine (NOLA) was discovered [1]. The crystal
NOLA has been found out as a result of long standing of the
mother solution at 35 ꢁC during crystal growth of L-arginine
oxalate. In the molecule of NOLA guanidyl group is protonat-
ed due to deprotonation of oxalyl group, while carboxyl group
remains neutral. So, NOLA has a zwitter-ionic structure:
þðH2NÞ2CNHðCH2Þ3CHðNHC2O3ꢀÞCOOH. Similar com-
pound has been found out by us in the system
L-Arg + HCOOH + H2O.
2. Experimental
The starting reagents were L-arginine (Sigma Chem. Co.)
and formic acid (MERCK 98–100% GR). X-ray diffraction
data for the structure analysis were collected by a CAD-4
Enraf Nonius diffractometer. For the structure solution
and refinement we used the programs SHELXS 97 and
SHELXL 97 [6,7]. Fourier transform Raman spectra were
registered by a NXP FT-Raman Module of a Nicolet 5700
spectrometer (number of scans 512, resolution 4 cmꢀ1).
The same spectrometer was used for measuring IR spectra
with nujol (4000–400 cmꢀ1, number of scans 32, resolution
2 cmꢀ1). Attenuated total reflection Fourier transform
infrared spectra (FTIR ATR) were registered by the aid of
a Nicolet ‘‘Nexus’’ FT-IR spectrometer with ZnSe prism
(4000–650 cmꢀ1, number of scans 16, resolution 4 cmꢀ1).
From this system a salt L-ArgÆHCOOH with monoclinic
symmetry has been obtained [2]. In Ref. [3] another form of
q
CCDC 609291 contains the supplementary crystallographic data for
this paper. These data 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).
*
Corresponding author. Tel.: +374 10 285139; fax: +374 10 282267.
E-mail address: apetros@msrc.am (A.M. Petrosyan).
0022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.07.025

H.A. Karapetyan et al. / Journal of Molecular Structure 831 (2007) 90–96
91
þðH2NÞ2CNHðCH2Þ3CHðNH2ÞCOOꢀ þ HCOOH
For the study of thermal properties we employed a Paulik–
Paulik–Erdey Derivatograph (MOM, Hungary) and also a
Boe¨tius type microscope with heating table.
! þðH2NÞ2CNHðCH2Þ3CHðHNðHCOÞÞCOOꢀ þ H2O
Crystals formed at 38 ꢁC proved to be monohydrate
(NFLAÆH2O). Recrystallization at room temperature results
in the same phase. At 60 ꢁC anhydrous NFLA in powder
form can be obtained from solution (see Section 3.4). In con-
trast to oxalyl-L-arginine [1], formyl-L-arginine monohy-
drate has very high solubility. It is possible to obtain
NFLAÆH2O in form of bulk crystals. Due to presence of opti-
cally active L-arginine moiety NFLAÆH2O crystallizes in
non-centrosymmetric group (space group P21) and displays
second harmonic generation signal of a Nd:YAG laser [5].
3. Results and discussion
3.1. Formation of Na-formyl-L-arginine
Na-Formyl-L-arginine (NFLA) is formed in aqueous
solution containing L-arginine and formic acid in mole
ratio 1:2 at a temperature above 30 ꢁC. It was discovered
during crystal growth of L-ArgÆHCOOH and can be
formed according to the following scheme:
3.2. Crystal and molecular structure
Table 1
Crystal data and structure refinement for Na-formyl-L-arginine
monohydrate
Formyl-L-arginine crystallizes in monoclinic system.
Crystallographic data and details of structure refinement
are listed in Table 1. Independent part of unit cell con-
tains one formyl-arginine molecule and a water molecule
(Fig. 1). As in case of NOLA the molecule of formyl-L-
arginine has zwitter-ionic structure. However, here pro-
tonation of guanidyl group occurs due to carboxyl group
of L-arginine moiety, in contrast to NOLA, where pro-
tonation of guanidyl group occurs due to oxalyl group.
So, the molecule of NOLA can be compared with doubly
charged L-Arg2+ cation, while NFLA can be compared
with singly charged L-Arg+ cation. Intramolecular bond
Identification code
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Crystal system
Space group
NFLAÆH2O
C7H16N4O4
220.24
293(2)
0.71073
Monoclinic
P21
a = 7.3099(15) A, a = 90ꢁ;
b = 8.8550(18) A, b = 94.93(3)ꢁ;
c = 8.3716(17) A, c = 90ꢁ
˚
˚
˚
˚
Unit cell dimensions
3
˚
Volume (A )
539.88(19)
2
1.355
0.111
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
236
Crystal size (mm3)
0.14 · 0.17 · 0.22
Table 2
a
Theta range for data collection 2.44–29.96ꢁ.
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 29.96ꢁ 100.0%
˚
Bond lengths [A] and angles [ꢁ] for N -formyl-L-arginine monohydrate
0 6 h 6 10, ꢀ12 6 k 6 12, ꢀ11 6 l 6 11
C(1)–O(1)
C(1)–O(2)
C(7)–O(3)
N(1)–C(7)
C(2)–N(1)
C(5)–N(2)
C(6)–N(2)
C(6)–N(3)
C(6)–N(4)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(7)–N(1)–C(2)
1.2578(16)
1.2519(16)
1.232(2)
1.316(2)
1.457(2)
1.459(2)
1.325(2)
1.326(2)
1.332(2)
1.540(2)
1.529(2)
1.529(2)
1.521(2)
124.8(1)
C(6)–N(2)–C(5)
O(2)–C(1)–O(1)
O(2)–C(1)–C(2)
O(1)–C(1)–C(2)
N(1)–C(2)–C(3)
C(3)–C(2)–C(1)
N(1)–C(2)–C(1)
C(4)–C(3)–C(2)
C(5)–C(4)–C(3)
N(2)–C(5)–C(4)
N(2)–C(6)–N(3)
N(2)–C(6)–N(4)
N(3)–C(6)–N(4)
O(3)–C(7)–N(1)
125.1(1)
124.8(1)
116.5(1)
118.8(1)
109.7(1)
110.3(1)
112.2(1)
112.6(1)
112.9(1)
113.3(1)
122.7(1)
118.1(1)
119.3(1)
126.1(1)
3358
3141 [Rint = 0.0119]
Refinement method
Full-matrix least-squares on F2
3141/1/200
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
1.032
R1 = 0.0340, wR2 = 0.0823
R1 = 0.0409, wR2 = 0.0864
0.205 and ꢀ0.151 e A
ꢀ3
˚
Largest diff. peak and hole
Table 3
a
˚
Hydrogen bonds parameters (A, ꢁ) in the structure of N -formyl-L-
arginine monohydrate (NFLAÆH2O)
D–H
d(D–H) d(Hꢁ ꢁ ꢁA) ÆDHA d(Dꢁ ꢁ ꢁA) A
N1–H2 0.87(3) 2.13(3) 151(2) 2.919(2) O1 [ꢀx,yꢀ1/2,ꢀz + 1]
N2–H10 0.86(2) 2.00(2) 170(2) 2.856(2) O2 [ꢀx,yꢀ1/2,ꢀz]
N3–H11 0.81(2) 2.20(2) 164(2) 2.980(2) O3 [ꢀx + 1,yꢀ1/2,ꢀz + 1]
N3–H12 0.92(2) 1.98(2) 170(2) 2.887(2) O2 [ꢀx + 1,yꢀ1/2,ꢀz]
N4–H13 0.83(2) 2.09(2) 168(2) 2.903(2) O1 [ꢀx,yꢀ1/2,ꢀz]
N4–H14 0.82(2) 2.05(2) 162(2) 2.841(2) O4 [ꢀx + 1, y + 1/2,ꢀz]
O4–H15 0.89(3) 1.91(3) 174(2) 2.802(2) O3 [x,yꢀ1,z]
Fig. 1. Independent part of unit cell of Na-formyl-L-arginine monohydrate
(NFLAÆH2O) and atomic numbering scheme.
O4–H16 0.78(3) 2.09(3) 167(3) 2.854(2) O1 [ꢀx,yꢀ1/2,ꢀz + 1]

92
H.A. Karapetyan et al. / Journal of Molecular Structure 831 (2007) 90–96
lengths and angles are given in Table 2. Bond lengths
angles: O1C1C2N1(u1) (ꢀ6.3(2)ꢁ), N1C2C3C4(v1)
(ꢀ65.0(1)ꢁ), C2C3C4C5(v2) (176.4(1)ꢁ), C3C4C5N2(v3)
(61.6(1)ꢁ), C6N2C5C4(v4) (85.0(2)ꢁ), C5N2C6N4(v5)
(175.1(1)ꢁ) determine the conformation of L-arginine
moiety, while the C2N1C7O3 (ꢀ2.7(2)ꢁ) angle that of
formyl moiety. Hydrogen bonds parameters are given
in Table 3. All active hydrogen atoms are involved in
hydrogen bonds (Fig. 2). Oxygen atom O(1) forms three
hydrogen bonds, while each of O(2) and O(3) form two
hydrogen bonds. Water molecule binds together three
neighboring molecules forming two hydrogen bonds as
a donor (O4–H15ꢁꢁꢁO3 and O4–H16ꢁꢁꢁO1) and one hydro-
gen bond as an acceptor (N4–H14ꢁꢁꢁO4). Conformation
˚
˚
C(1)–O(1) (1.258 A), C(1)–O(2) (1.252 A) and O(1)–
C(1)–O(2) angle (125ꢁ) are characteristic for carboxylate
group (compare with respective values of L-ArgÆ
HNO3Æ0.5H2O [8] and L-ArgÆHClO3 and L-ArgÆHBrO3
[9]). In formyl group C(7)–O(3) bond length is equal to
˚
1.232 A. C–C distances are also typical. C–N bond
lengths in guanidyl group have expected values, while
N–C(6)–N angles indicate that guanidyl group slightly
deviates from expected planarity. Bond length C(2)–
˚
N(1) (1.457 A) is somewhat shorter than the respective
+
+
˚
value C(2)–NH3 (ca. 1.500 A) of L-Arg cation and is
˚
close to the respective value of NOLA (1.452 A). Torsion
Fig. 2. Hydrogen bonding scheme in the structure of NFLAÆH2O.
Fig. 3. A stereoscopic view of packing in the crystal structure of NFLAÆH2O.

H.A. Karapetyan et al. / Journal of Molecular Structure 831 (2007) 90–96
93
and hydrogen bonds scheme of the NFLA molecule can
3.3. FTIR ATR and raman spectra
be seen from stereoscopic view of packing in the crystal
structure shown in Fig. 3. As can be seen from Table 3
and Fig. 2 nitrogen atoms N(2) and N(4) form hydrogen
bonds N(2)–Hꢁ ꢁ ꢁO(2) and N(4)–Hꢁ ꢁ ꢁO(1) with oxygen
atoms O(1) and O(2) of the same neighboring molecule.
Such type of hydrogen bond is designated as type B in
Ref. [10].
FTIR and FT Raman spectra are shown in Figs. 4
and 5. Wavenumbers with tentative assignment of some
characteristic bands are collected in Table 4. Absorption
bands in the high-frequency region (3500–2800 cmꢀ1) are
related to stretching vibrations of O–H, N–H and C–H
bonds. In the Raman spectrum, strong characteristic
Fig. 4. FTIR spectra of NFLAÆH2O [ATR (a), in nujol (b)].
Fig. 5. FT Raman spectrum of NFLAÆH2O.

94
H.A. Karapetyan et al. / Journal of Molecular Structure 831 (2007) 90–96
Table 4
formyl group, is shifted towards low-frequency region
due to two formed hydrogen bonds. A strong band at
Wavenumbers (cmꢀ1) of IR absorption peaks and Raman lines in the
spectra of NFLAÆH2O
1385 cmꢀ1
,
which has
a
Raman-counter part at
1390 cmꢀ1, can be assign to symmetric stretching vibra-
tion of carboxylate group. Absorption band in the
500–700 cmꢀ1 region (measured in nujol) may be
assigned to libration vibrations of water molecule and
deformation vibrations of carboxylate group.
IR
Raman
Assignment
3423
3286
3092; 3063 3061
2962
2880
2795
2195
1674
1650
1548
1464; 1452 1458
1434 1437
1398;1385 1390
1366
1329
1318
1269
1258
1238
1197
1177
1131
1107
1090
1070
960
3425
3297
H2O stretching
N–H stretching of NH and NH2 groups
N–H stretching of NH and NH2 groups
2979; 2952; 2922 C–H stretching of CH2 groups
2884; 2871 C–H stretching of CH groups
2765; 2736; 2713 Overtones
Combinational
3.4. Dehydration of NFLAÆH2O
1678
1654
1549
C@O stretching of formyl group
Asymmetric stretching of COOꢀ group
Deformation of NH2 groups
Thermogravimetric (TG), derivative thermogravimetric
(DTG) and differential thermoanalytical (DTA) curves
are shown in Fig. 6. The first loss of mass having endo-
thermic character occurs near 110 ꢁC. Apparently, this
process is caused by dehydration. However, the loss of
mass (6.6%) is less than 8.18% expected for one water
molecule. Respective quantity is equal to n = 0.8. The
second loss of mass also having endothermic character
occurs near 175 ꢁC. For elucidation of the nature of
these processes, we heated NFLAÆH2O at 125 ꢁC and
190 ꢁC. Registered IR spectra of obtained specimens
are shown in Figs. 7(a and b). Comparison of these spec-
tra shows their great similarity. The main distinction is
the presence of a band at 3284 cmꢀ1 which disappears
at complete dehydration. So one can suppose that at
partially dehydration a compound with definite composi-
tion (5NFLAÆH2O) is formed and the band at 3284 cmꢀ1
Deformation of CH2 groups
Deformation of CH2 groups
Symmetric stretching of COOꢀ group
Wagging of CH2 groups
1305
1265
Twisting vibration of CH2 groups
Twisting vibration of CH2 groups
1244
1200
1169
1119
C–N stretching of C–NHCHO group
Wagging of NH2 groups
Rocking of CH2 groups
1093
1073; 1034; 1015 Stretching of C–N bonds
963
930
925
891
855
801
770
755; 739
692
895
862
815
774
759
695
Stretching of C–C bonds
Stretching of C–C bonds
Deformation of COOꢀ group
Deformation of COOꢀ group
646
609
618
Libration of H2O
536
480
526
479
Libration of H2O
Rocking of COOꢀ group
410
413; 400
315
281
221
peaks at 2884, 2919, 2950, 2976 cmꢀ1 are related to
stretching vibrations of C–H bonds, hence narrow bands
at 2880 and 2962 cmꢀ1 in the IR spectrum are related to
C–H bonds as well. We assign absorption band at
3423 cmꢀ1 (corresponding Raman-line is at 3425 cmꢀ1
)
to stretching vibration m(O–H) of water molecule. From
the correlation of m(O–H) vs. R(Oꢁ ꢁ ꢁO) [11] one can esti-
mate m(O–H) ca. 3400 cmꢀ1 for 2.802 and 2.854 A
˚
(O4ꢁ ꢁ ꢁO3 and O4ꢁ ꢁ ꢁO1, respectively), which is in good
agreement with observed value 3423 cmꢀ1. Other bands
(3063, 3092, 3286 cmꢀ1) are related to stretching vibra-
tions of N–H bonds of NH2 and N–H groups. Deforma-
tion vibrations of water molecule, NH2 group and also
asymmetric stretching vibrations of carboxylate and for-
myl groups are in the 1700–1500 cmꢀ1 region. The peak
at 1674 cmꢀ1, which probably corresponds to m(C@O) of
Fig. 6. Thermogravimetric (TG), derivative thermogravimetric (DTG)
and differential thermoanalytical (DTA) curves of NFLAÆH2O.

H.A. Karapetyan et al. / Journal of Molecular Structure 831 (2007) 90–96
95
Fig. 7. ATR-IR spectra of NFLAÆH2O heated at 125 ꢁC (a) and 190 ꢁC (b) and specimen obtained from solution at 60 ꢁC (c) [spectra in the 650–400 cmꢀ1
region are measured in nujol].

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H.A. Karapetyan et al. / Journal of Molecular Structure 831 (2007) 90–96
corresponds to stretching vibration of water molecule. It
is possible to notice also changes in the region of libra-
tion vibrations of water molecule. We tried to obtain
anhydrous NFLA from water solution at higher temper-
ature and have succeeded to obtain anhydrous NFLA at
60 ꢁC in form of white powder. IR spectrum (Fig. 7c) of
this specimen completely coincides with spectrum of
anhydrous NFLA obtained by dehydration at 190 ꢁC.
This spectrum contains a shoulder at 1707 cmꢀ1, which
we assign to m(C@O) of formyl group. We recrystallized
from water solution at room temperature specimens
obtained by heating at 125 ꢁC and 190 ꢁC, as well
obtained from solution at 60 ꢁC. In all cases recrystal-
lized specimens proved to be NFLAÆH2O by IR spectra.
This shows that the process at 175 ꢁC is caused by com-
plete dehydration and not by decomposition of NFLA
molecule. Decomposition of the NFLA molecule occurs
near 266 ꢁC.
Acknowledgements
This work was supported by CRDF Grant No. AE2-
2533-YE-03. The authors thank Dr. Martirosyan G.G.
for his help.
References
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4. Conclusions
A new L-arginine derivative with non-salt nature, namely,
Na-formyl-L-arginine monohydrate (NFLAÆH2O) has been
obtained and studied by single crystal X-ray diffraction
and vibrational spectroscopic methods. Anhydrous NFLA
in form of powder has been obtained by dehydration of
NFLAÆH2O at 190 ꢁC and also from aqueous solution at
60 ꢁC.