organic compounds
L-Alanylglycylhistamine dihydrochloride
aLaboratoire SRSMC (UMR 7565 CNRS - Université de Lorraine), Groupe SUCRES, Faculté des Sciences et Technologies, BP 70239, F-54506 Vandoeuvre-lès-Nancy Cedex, France, and bLaboratoire CRM2 (UMR 7036 CNRS - Université de Lorraine), Faculté des Sciences et Technologies, BP 70239, F-54506 Vandoeuvre-lès-Nancy Cedex, France
*Correspondence e-mail: katalin.selmeczi@univ-lorraine.fr
In the title compound {systematic name: 4-[2-({N-[(2S)-2-ammoniopropanoyl]glycyl}amino)ethyl]-1H-imidazol-3-ium dichloride}, C10H19N5O22+·2Cl−, the pseudo-tripeptide L-alanylglycylhistamine is protonated at both the terminal amino group and the histidine N2 atom. The resulting positive charges are neutralized by two chloride anions. In the crystal, the organic cation adopts a twisted conformation about the CH2—CH2 bond of histamine and about the C—N bond in the main chain, stabilized by a short intramolecular C—H⋯O contact. In the crystal, N+—H⋯O and N+—H⋯Cl− hydrogen bonds link the molecules into infinite sheets parallel to the (100) plane. The stacking of these sheets along the a axis is supported by Namide—H⋯Cl− hydrogen bonds.
Related literature
For the complexation ability of L-Ala-Gly-HA, see: Gizzi et al. (2005). For bond lengths and angles in other see: Itoh et al. (1977); Selmeczi et al. (2008). For discussion of hydrogen bonding, see: Steiner (2002). For the synthesis of pseudo-peptides, see: Henry et al. (1993). For the definition of torsion angles in see: IUPAC–IUB Commission on Biochemical Nomenclature (1970).
Experimental
Crystal data
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Data collection
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Data collection: COLLECT (Nonius, 1998); cell SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: enCIFer (Allen et al., 2004).
Supporting information
10.1107/S1600536812023562/fy2052sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: 10.1107/S1600536812023562/fy2052Isup2.hkl
Supporting information file. DOI: 10.1107/S1600536812023562/fy2052Isup3.cml
The title compound was synthesized in one step according to the procedure described earlier (Henry et al., 1993). Histamine dihydrochloride and ethyl-diisopropyl-amine in chloroform were added to the commercially available N-(tert-butoxycarbonyl)-L-alanyl-glycine-OH (Boc-L-Ala-Gly-OH) at room temperature. The mixture was stirred for 10 additional hours at rt. Deprotection of the primary amine was performed with a mixture of HCl/Et2O. The title compound was obtained as white powder with 60% of yield. Suitable crystals were obtained by slow evaporation of water from an acidic aqueous solution (pH 2) of the title compound. ESI-MS+ (m/z): calculated for C10H17N5O2 239.14, found 240.20. Anal. calc. for C10H17N5O2.2HCl: C, 38.47; H, 6.13; N, 22.43. Found: C, 37.96; H, 6.08; N, 21.87%.
The absolute configurations of the title compound was known from the method of synthesis (enantiomer S) and it was also confirmed from the diffraction experiments. All H atoms were located in difference Fourier maps. The C/N-bonded H atoms were placed at calculated positions and refined using a riding model, with Cmethyl—H distance of 0.98 Å, Cmethylene—H distance of 0.99 Å, Cmethine—H distance of 1 Å, Caryl—H distance of 0.95 Å, and with N—H distance of 0.88 Å. The H-atom Uiso parameters were fixed at 1.2Ueq(C) for methine, methylene and aryl C—H, at 1.5Ueq(C) for methyl C—H, at 1.2Ueq(C) for aryl C—H and at 1.2Ueq(N) for the N—H group.
Data collection: COLLECT (Nonius, 1998); cell
SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: enCIFer (Allen et al., 2004).Fig. 1. Molecular structure of L-Ala-Gly-HA.2HCl with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. | |
Fig. 2. A view of the crystal packing of the title compound along the b axis, showing the N—H···O and N—H···Cl (orange dotted line), C—H···O (light blue dotted line) hydrogen bonds and N—O short contacts (green dotted line) in the (100) plane. |
C10H19N5O22+·2Cl− | F(000) = 328 |
Mr = 312.20 | Dx = 1.390 Mg m−3 |
Monoclinic, P21 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P2yb | Cell parameters from 1776 reflections |
a = 7.5864 (3) Å | θ = 0.4–30.0° |
b = 7.4083 (3) Å | µ = 0.44 mm−1 |
c = 13.7673 (6) Å | T = 100 K |
β = 105.337 (2)° | Prismatic, colourless |
V = 746.20 (5) Å3 | 0.45 × 0.25 × 0.11 mm |
Z = 2 |
Nonius KappaCCD diffractometer | 3459 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.060 |
Horizonally mounted graphite crystal monochromator | θmax = 28.0°, θmin = 2.8° |
Detector resolution: 9 pixels mm-1 | h = −10→10 |
ω scans | k = −9→9 |
15818 measured reflections | l = −18→18 |
3574 independent reflections |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.027 | H-atom parameters constrained |
wR(F2) = 0.070 | w = 1/[σ2(Fo2) + (0.0395P)2 + 0.2259P] where P = (Fo2 + 2Fc2)/3 |
S = 1.00 | (Δ/σ)max = 0.001 |
3574 reflections | Δρmax = 0.21 e Å−3 |
174 parameters | Δρmin = −0.23 e Å−3 |
1 restraint | Absolute structure: Flack (1983), with 1643 Friedel-pairs |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: −0.03 (4) |
C10H19N5O22+·2Cl− | V = 746.20 (5) Å3 |
Mr = 312.20 | Z = 2 |
Monoclinic, P21 | Mo Kα radiation |
a = 7.5864 (3) Å | µ = 0.44 mm−1 |
b = 7.4083 (3) Å | T = 100 K |
c = 13.7673 (6) Å | 0.45 × 0.25 × 0.11 mm |
β = 105.337 (2)° |
Nonius KappaCCD diffractometer | 3459 reflections with I > 2σ(I) |
15818 measured reflections | Rint = 0.060 |
3574 independent reflections |
R[F2 > 2σ(F2)] = 0.027 | H-atom parameters constrained |
wR(F2) = 0.070 | Δρmax = 0.21 e Å−3 |
S = 1.00 | Δρmin = −0.23 e Å−3 |
3574 reflections | Absolute structure: Flack (1983), with 1643 Friedel-pairs |
174 parameters | Absolute structure parameter: −0.03 (4) |
1 restraint |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
x | y | z | Uiso*/Ueq | ||
Cl1 | 0.75003 (4) | 0.96855 (5) | 0.94053 (3) | 0.01717 (9) | |
Cl2 | 0.14366 (5) | 0.19501 (5) | 0.69324 (3) | 0.01977 (9) | |
O2 | 0.54905 (14) | 0.49471 (17) | 0.89693 (8) | 0.0181 (2) | |
O1 | 0.73519 (18) | 0.52373 (18) | 0.61858 (9) | 0.0267 (3) | |
N3 | 0.76898 (17) | 0.74018 (19) | 0.73704 (10) | 0.0178 (3) | |
H3N | 0.8002 | 0.7654 | 0.8018 | 0.021* | |
N5 | 0.64458 (17) | 0.66369 (18) | 1.08050 (10) | 0.0161 (3) | |
H5A | 0.6377 | 0.7686 | 1.0453 | 0.024* | |
H5B | 0.6819 | 0.6876 | 1.1477 | 0.024* | |
H5C | 0.5326 | 0.6103 | 1.0657 | 0.024* | |
N4 | 0.84631 (16) | 0.4832 (2) | 0.89235 (9) | 0.0169 (3) | |
H4 | 0.9605 | 0.5056 | 0.9251 | 0.020* | |
N1 | 0.21053 (19) | 0.5728 (2) | 0.62237 (11) | 0.0213 (3) | |
H1N | 0.1619 | 0.4744 | 0.6401 | 0.026* | |
N2 | 0.2662 (2) | 0.7937 (2) | 0.53338 (11) | 0.0213 (3) | |
H2N | 0.2601 | 0.8665 | 0.4821 | 0.026* | |
C7 | 0.8047 (2) | 0.4224 (2) | 0.78845 (13) | 0.0196 (3) | |
H7A | 0.6967 | 0.3421 | 0.7760 | 0.023* | |
H7B | 0.9087 | 0.3489 | 0.7801 | 0.023* | |
C2 | 0.3860 (2) | 0.8128 (2) | 0.62861 (12) | 0.0192 (3) | |
C8 | 0.7130 (2) | 0.5057 (2) | 0.93940 (11) | 0.0156 (3) | |
C9 | 0.7782 (2) | 0.5401 (2) | 1.05237 (11) | 0.0152 (3) | |
H9 | 0.9015 | 0.5982 | 1.0689 | 0.018* | |
C4 | 0.5264 (2) | 0.9575 (2) | 0.65566 (13) | 0.0222 (3) | |
H4A | 0.5219 | 1.0119 | 0.7207 | 0.027* | |
H4B | 0.4969 | 1.0534 | 0.6038 | 0.027* | |
C3 | 0.3491 (2) | 0.6734 (2) | 0.68410 (12) | 0.0195 (3) | |
H3 | 0.4077 | 0.6494 | 0.7528 | 0.023* | |
C6 | 0.7665 (2) | 0.5683 (2) | 0.70817 (12) | 0.0189 (3) | |
C10 | 0.7887 (2) | 0.3630 (2) | 1.10983 (12) | 0.0198 (3) | |
H10A | 0.8346 | 0.3861 | 1.1822 | 0.030* | |
H10B | 0.8716 | 0.2799 | 1.0883 | 0.030* | |
H10C | 0.6666 | 0.3090 | 1.0959 | 0.030* | |
C5 | 0.7215 (2) | 0.8883 (2) | 0.66454 (13) | 0.0206 (3) | |
H5D | 0.7305 | 0.8461 | 0.5978 | 0.025* | |
H5E | 0.8095 | 0.9884 | 0.6862 | 0.025* | |
C1 | 0.1628 (2) | 0.6491 (2) | 0.53191 (13) | 0.0225 (3) | |
H1 | 0.0702 | 0.6071 | 0.4757 | 0.027* |
U11 | U22 | U33 | U12 | U13 | U23 | |
Cl1 | 0.01378 (15) | 0.01813 (17) | 0.01988 (17) | −0.00020 (13) | 0.00496 (12) | −0.00123 (13) |
Cl2 | 0.01907 (17) | 0.02083 (19) | 0.01823 (17) | −0.00004 (13) | 0.00283 (13) | 0.00210 (13) |
O2 | 0.0129 (5) | 0.0223 (6) | 0.0183 (5) | −0.0021 (4) | 0.0025 (4) | −0.0001 (5) |
O1 | 0.0369 (7) | 0.0261 (7) | 0.0169 (5) | 0.0044 (5) | 0.0070 (5) | −0.0050 (5) |
N3 | 0.0164 (6) | 0.0187 (6) | 0.0176 (6) | −0.0024 (5) | 0.0033 (5) | −0.0018 (5) |
N5 | 0.0142 (6) | 0.0174 (7) | 0.0165 (6) | 0.0004 (5) | 0.0036 (5) | −0.0007 (5) |
N4 | 0.0134 (5) | 0.0210 (7) | 0.0165 (6) | 0.0013 (5) | 0.0042 (4) | −0.0017 (5) |
N1 | 0.0191 (6) | 0.0214 (7) | 0.0222 (7) | −0.0018 (5) | 0.0036 (5) | 0.0044 (6) |
N2 | 0.0220 (7) | 0.0233 (7) | 0.0178 (6) | 0.0015 (5) | 0.0038 (5) | 0.0049 (5) |
C7 | 0.0218 (7) | 0.0188 (8) | 0.0189 (7) | 0.0021 (6) | 0.0068 (6) | −0.0036 (6) |
C2 | 0.0173 (7) | 0.0207 (8) | 0.0191 (7) | 0.0030 (6) | 0.0040 (6) | 0.0013 (6) |
C8 | 0.0150 (6) | 0.0142 (7) | 0.0174 (7) | −0.0007 (5) | 0.0040 (5) | 0.0004 (5) |
C9 | 0.0122 (6) | 0.0158 (7) | 0.0172 (7) | 0.0017 (5) | 0.0032 (5) | −0.0014 (5) |
C4 | 0.0226 (7) | 0.0175 (7) | 0.0262 (8) | −0.0001 (7) | 0.0058 (6) | 0.0016 (7) |
C3 | 0.0176 (7) | 0.0219 (8) | 0.0182 (7) | 0.0001 (6) | 0.0034 (6) | 0.0018 (6) |
C6 | 0.0150 (7) | 0.0223 (8) | 0.0201 (7) | 0.0007 (6) | 0.0061 (6) | −0.0015 (6) |
C10 | 0.0209 (7) | 0.0173 (8) | 0.0202 (7) | 0.0009 (6) | 0.0037 (6) | 0.0007 (6) |
C5 | 0.0210 (7) | 0.0184 (8) | 0.0231 (8) | −0.0017 (6) | 0.0072 (6) | 0.0016 (6) |
C1 | 0.0205 (8) | 0.0262 (9) | 0.0189 (8) | −0.0010 (6) | 0.0018 (6) | 0.0019 (6) |
O2—C8 | 1.2292 (18) | C7—C6 | 1.518 (2) |
O1—C6 | 1.238 (2) | C7—H7A | 0.9900 |
N3—C6 | 1.333 (2) | C7—H7B | 0.9900 |
N3—C5 | 1.463 (2) | C2—C3 | 1.357 (2) |
N3—H3N | 0.8800 | C2—C4 | 1.488 (2) |
N5—C9 | 1.491 (2) | C8—C9 | 1.524 (2) |
N5—H5A | 0.9100 | C9—C10 | 1.523 (2) |
N5—H5B | 0.9100 | C9—H9 | 1.0000 |
N5—H5C | 0.9100 | C4—C5 | 1.540 (2) |
N4—C8 | 1.3478 (19) | C4—H4A | 0.9900 |
N4—C7 | 1.452 (2) | C4—H4B | 0.9900 |
N4—H4 | 0.8800 | C3—H3 | 0.9500 |
N1—C1 | 1.328 (2) | C10—H10A | 0.9800 |
N1—C3 | 1.382 (2) | C10—H10B | 0.9800 |
N1—H1N | 0.8800 | C10—H10C | 0.9800 |
N2—C1 | 1.325 (2) | C5—H5D | 0.9900 |
N2—C2 | 1.391 (2) | C5—H5E | 0.9900 |
N2—H2N | 0.8800 | C1—H1 | 0.9500 |
C6—N3—C5 | 122.12 (14) | C8—C9—C10 | 110.15 (13) |
C6—N3—H3N | 118.9 | N5—C9—H9 | 109.6 |
C5—N3—H3N | 118.9 | C8—C9—H9 | 109.6 |
C9—N5—H5A | 109.5 | C10—C9—H9 | 109.6 |
C9—N5—H5B | 109.5 | C2—C4—C5 | 112.89 (14) |
H5A—N5—H5B | 109.5 | C2—C4—H4A | 109.0 |
C9—N5—H5C | 109.5 | C5—C4—H4A | 109.0 |
H5A—N5—H5C | 109.5 | C2—C4—H4B | 109.0 |
H5B—N5—H5C | 109.5 | C5—C4—H4B | 109.0 |
C8—N4—C7 | 121.08 (13) | H4A—C4—H4B | 107.8 |
C8—N4—H4 | 119.5 | C2—C3—N1 | 107.51 (14) |
C7—N4—H4 | 119.5 | C2—C3—H3 | 126.2 |
C1—N1—C3 | 108.74 (15) | N1—C3—H3 | 126.2 |
C1—N1—H1N | 125.6 | O1—C6—N3 | 122.41 (16) |
C3—N1—H1N | 125.6 | O1—C6—C7 | 119.02 (15) |
C1—N2—C2 | 109.45 (14) | N3—C6—C7 | 118.57 (14) |
C1—N2—H2N | 125.3 | C9—C10—H10A | 109.5 |
C2—N2—H2N | 125.3 | C9—C10—H10B | 109.5 |
N4—C7—C6 | 116.51 (14) | H10A—C10—H10B | 109.5 |
N4—C7—H7A | 108.2 | C9—C10—H10C | 109.5 |
C6—C7—H7A | 108.2 | H10A—C10—H10C | 109.5 |
N4—C7—H7B | 108.2 | H10B—C10—H10C | 109.5 |
C6—C7—H7B | 108.2 | N3—C5—C4 | 111.23 (14) |
H7A—C7—H7B | 107.3 | N3—C5—H5D | 109.4 |
C3—C2—N2 | 105.84 (15) | C4—C5—H5D | 109.4 |
C3—C2—C4 | 130.41 (15) | N3—C5—H5E | 109.4 |
N2—C2—C4 | 123.71 (15) | C4—C5—H5E | 109.4 |
O2—C8—N4 | 123.82 (14) | H5D—C5—H5E | 108.0 |
O2—C8—C9 | 120.74 (14) | N2—C1—N1 | 108.46 (15) |
N4—C8—C9 | 115.40 (13) | N2—C1—H1 | 125.8 |
N5—C9—C8 | 107.95 (12) | N1—C1—H1 | 125.8 |
N5—C9—C10 | 109.86 (13) | ||
C8—N4—C7—C6 | 90.63 (19) | N2—C2—C3—N1 | 0.45 (19) |
C1—N2—C2—C3 | −0.2 (2) | C4—C2—C3—N1 | −177.31 (17) |
C1—N2—C2—C4 | 177.72 (16) | C1—N1—C3—C2 | −0.5 (2) |
C7—N4—C8—O2 | −7.3 (2) | C5—N3—C6—O1 | 4.6 (2) |
C7—N4—C8—C9 | 170.48 (14) | C5—N3—C6—C7 | −175.71 (14) |
O2—C8—C9—N5 | −35.58 (19) | N4—C7—C6—O1 | 178.07 (14) |
N4—C8—C9—N5 | 146.53 (14) | N4—C7—C6—N3 | −1.6 (2) |
O2—C8—C9—C10 | 84.37 (18) | C6—N3—C5—C4 | 102.05 (17) |
N4—C8—C9—C10 | −93.53 (16) | C2—C4—C5—N3 | −56.02 (19) |
C3—C2—C4—C5 | 72.3 (2) | C2—N2—C1—N1 | −0.1 (2) |
N2—C2—C4—C5 | −105.10 (18) | C3—N1—C1—N2 | 0.4 (2) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1N···Cl2 | 0.88 | 2.21 | 3.0503 (15) | 159 |
N2—H2N···O1i | 0.88 | 1.82 | 2.6962 (19) | 175 |
N4—H4···Cl1ii | 0.88 | 2.48 | 3.3100 (13) | 157 |
N5—H5A···Cl1 | 0.91 | 2.38 | 3.2046 (14) | 151 |
N5—H5B···Cl2iii | 0.91 | 2.24 | 3.1130 (14) | 161 |
N5—H5C···Cl1iv | 0.91 | 2.37 | 3.2676 (14) | 170 |
C3—H3···O2 | 0.95 | 2.30 | 3.2074 (19) | 161 |
Symmetry codes: (i) −x+1, y+1/2, −z+1; (ii) −x+2, y−1/2, −z+2; (iii) −x+1, y+1/2, −z+2; (iv) −x+1, y−1/2, −z+2. |
Experimental details
Crystal data | |
Chemical formula | C10H19N5O22+·2Cl− |
Mr | 312.20 |
Crystal system, space group | Monoclinic, P21 |
Temperature (K) | 100 |
a, b, c (Å) | 7.5864 (3), 7.4083 (3), 13.7673 (6) |
β (°) | 105.337 (2) |
V (Å3) | 746.20 (5) |
Z | 2 |
Radiation type | Mo Kα |
µ (mm−1) | 0.44 |
Crystal size (mm) | 0.45 × 0.25 × 0.11 |
Data collection | |
Diffractometer | Nonius KappaCCD diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 15818, 3574, 3459 |
Rint | 0.060 |
(sin θ/λ)max (Å−1) | 0.661 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.027, 0.070, 1.00 |
No. of reflections | 3574 |
No. of parameters | 174 |
No. of restraints | 1 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.21, −0.23 |
Absolute structure | Flack (1983), with 1643 Friedel-pairs |
Absolute structure parameter | −0.03 (4) |
Computer programs: COLLECT (Nonius, 1998), DENZO and SCALEPACK (Otwinowski & Minor, 1997), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), Mercury (Macrae et al., 2008), enCIFer (Allen et al., 2004).
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1N···Cl2 | 0.880 | 2.212 | 3.0503 (15) | 159 |
N2—H2N···O1i | 0.880 | 1.818 | 2.6962 (19) | 175 |
N4—H4···Cl1ii | 0.880 | 2.484 | 3.3100 (13) | 157 |
N5—H5A···Cl1 | 0.910 | 2.377 | 3.2046 (14) | 151 |
N5—H5B···Cl2iii | 0.910 | 2.240 | 3.1130 (14) | 161 |
N5—H5C···Cl1iv | 0.910 | 2.369 | 3.2676 (14) | 170 |
C3—H3···O2 | 0.949 | 2.297 | 3.2074 (19) | 161 |
Symmetry codes: (i) −x+1, y+1/2, −z+1; (ii) −x+2, y−1/2, −z+2; (iii) −x+1, y+1/2, −z+2; (iv) −x+1, y−1/2, −z+2. |
Acknowledgements
Technical support (NMR, ESI-MS and X-ray measurements) from the Université de Lorraine is gratefully acknowledged.
References
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The metal complexation ability of the N-terminal sequence in serum albumin (HSA, involved in the transport of metal ions in blood) is among the best studied examples of peptide–metal interactions. The so-called ATCUN-motif of HSA (Amino Terminal Cu(II) and Ni(II) binding site) was mimicked, among others, by the ligand L-Alanyl-Glycyl Histamine (L-Ala-Gly-HA) (Gizzi et al., 2005). We report here the molecular structure of the dihydrochloride salt of the pseudo-tripeptide L-Ala-Gly-HA. In the title compound, the L-Ala-Gly-HA part is doubly positively charged on the amino and the imidazole groups. This double charge is neutralized by the presence of two chloride ions (Fig. 1). The absolute configuration (S) of atom C9 was assumed from the stereochemistry of the precursor Boc-L-Ala-Gly-OH. In the organic cation the bond distances and angles of the peptide bonds and of the protonated imidazole ring are close to the values measured for other oligopeptides (Itoh et al., 1977; Selmeczi et al., 2008). The conformation of the title tripeptide can be determined by analysis of the torsion angles about the C—C (ψ), C—N (ω) and N—C (ϕ) bonds (IUPAC–IUB Commission on Biochemical Nomenclature, 1970). The conformation may be considered as fully extended (open) if the magnitude of ψ, ω and ϕ angles is near 180° (e.g. for Gly-β-Ala-Histamine; Selmeczi et al., 2008). The torsion angle ψ about the C4—C5 bond in L-Ala-Gly-HA is -56.01° resulting in the folding back of the imidazole ring on the N3—C6—O1 bonds. In addition, the main chain of the tripeptide adopts a twisted conformation defined by the small value of torsion angle of ϕ about N4—C7 (90.63°) and of ψ about C7—C6 (-1.62°). These torsion angle values show the folded (closed) conformation of L-Ala-Gly-HA. All protons attached to the N1, N2, N4 and N5 nitrogen atoms are involved in moderate hydrogen bonding (Steiner, 2002). The N1 and N5 nitrogen atoms form N—H···Cl hydrogen bonds with the Cl2 and Cl1, Cl2iii, Cl1iv atoms, respectively [symmetry codes: (iii) -x + 1, y + 1/2, -z + 2, (iv) -x + 1, y - 1/2, -z + 2]. The N2 nitrogen atom forms stronger H-bond with the O1i carbonyl oxygen atom of a neighbouring peptide molecule [symmetry code: (i) -x + 1, y + 1/2, -z + 1]. The C3—H···O2 intramolecular contact is also included in the H-bond list. This interaction results from the bending back of imidazole ring on the peptide main chain and it stabilizes the `closed' conformation of the molecule. The O2 carbonyl oxygen atom participates in relatively close interaction with the neighbouring N5iii nitrogen atom, reflecting a partial positive charge on the latter (distance O2···N5 is 2.917 Å). These hydrogen bonds link the molecules into infinite two-dimensional sheets parallel to the (100) plane forming a stacking structure along the a axis (Fig. 2). These horizontal layers are interlinked by an another N—H···Cl hydrogen bond present in the structure between N4 and Cl1ii [symmetry code: (ii) -x + 2, y - 1/2, -z + 2], thus forming a three-dimensional framework.