organic compounds
L-Methionyl-L-tyrosine monohydrate
aDepartment of Biological Science, Hampton University, Hampton, VA 23668, USA, bSchool of Science, Hampton University, Hampton, VA 23668, USA, cDepartment of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA, and dDepartment of Environmental Toxicology, Southern University and A&M College, Baton Rouge, LA 70813, USA
*Correspondence e-mail: biosainath@gmail.com
The study of the oxidation of various proteins necessitates scrutiny of the amino acid sequence. Since methionine (Met) and tyrosine (Tyr) are easily oxidized, L-Met–L-Tyr monohydrate, C14H20N2O4S·H2O (systematic name: (2S)-2-{[(2S)-2-amino-4-methylsulfanylbutanoyl]amino}-3-(4-hydroxyphenyl)propanoic acid monohydrate) is presented herein and provides information about the zwitterionic nature of the dipeptide. We suspect that the zwitterionic state of the dipeptide and its interaction within the solvent medium may play a major role in the oxidation of the dipeptide. In the crystal, all the potential donor atoms interact via strong N—H⋯O, C—H⋯O, O—H⋯S, and O—H⋯O hydrogen bonds.
that contain these amino acids are frequently studied using a variety of oxidation methods, including, but not limited to, pulse radiolysis, electrochemical oxidation, and laser To date, the oxidation of the Met–Tyr dipeptide is not fully understood. Several investigators have proposed a mechanism of intramolecular between the sulfide radical of Met and the Tyr residue. Our elucidation of the structure and ofKeywords: crystal structure; zwitterion; oxidation; nitration.
CCDC reference: 2260065
Structure description
Protein oxidation is an important physiological and pathological mechanism (Berlett & Stadtman, 1997; Wojcik et al., 2008). Oxidation of tyrosine (Tyr) and methionine (Met) residues play a role in the etiology of inflammatory diseases (Gu et al., 2015; Meredith et al., 2014). Studies have shown that the Met–Tyr dipeptide has a significant antioxidant activity against the of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS+.) and peroxyl radicals, while the Tyr–Met dipeptide does not have any reaction with those radicals (Torkova et al., 2015). The presence of a C-terminal Met group to Tyr had somewhat conflicting results with many oxidation systems (Zhang et al., 2009; Wojcik et al., 2008; Nagy et al., 2009). Several studies have suggested that the mechanism of oxidation is through intramolecular from Met to Tyr phenoxy radicals (Bergès et al., 2011; Houée-Lévin et al., 2015; Kciuk et al., 2005; Zhang et al., 2009). The diverse oxidation ability of the dipeptides could be attributed to the structural differences, particularly the configuration of the zwitterion and their interaction with solvent molecules. With this in mind, we have elucidated the structure of L-Met–L-Tyr to better understand its role in the oxidation and nitration process.
The title compound, L-Met–L-Tyr monohydrate, C14H20N2O4S·H2O (Fig. 1), has been analyzed as part of broader studies on the redox properties of Met-containing dipeptides. Within the dipeptide, the amine group of Met is protonated and the carboxyl group of tyrosine is deprotonated, thereby generating a zwitterionic configuration. The conformation of the dipeptide molecule can be quantified by four torsion angles. Besides the expected essentially planar peptide linkage, the tyrosine portion has C10—N1—C8—C7 = 165.36 (17)° and N1—C8—C7—C4 = −71.1 (2)°. The methionine portion has C10—C11—C12—C13 = −45.6 (2)° and the sulfur-containing substituent shows an extended conformation with C11—C12—C13—S1 = −173.31 (13)°.
This structure has been reported recently (Babu et al., 2023). The title compound (Fig. 1), derived from two amino acids, L-Met and L-Tyr, crystallized as a monohydrate and was structurally analyzed as a part of broader studies on the redox properties of Met dipeptides. The determined from the X-ray data agrees with that of the starting materials.
In the crystal, the molecules interact with each other via strong intermolecular N—H⋯O, C—H⋯O, O—H⋯S, and O—H⋯O hydrogen bonds, forming a three-dimensional network (Table 1 and Fig. 2). All the hydrogen-bond donors in the hydroxy, amidine, and carboxylate groups, as well as the solvent water molecule, are involved. It is interesting to note that the dipeptide crystallized as a monohydrate. The water molecule is approximately tetrahedrally surrounded by four hydrogen bonds. In particular, a hydrogen bond exists between atom O1W of the water molecule and atom O3 of the Tyr carboxylate group and atom S1 of Met. The amine N1 group of Met forms hydrogen bonds with atoms O2 and O3 of the Tyr carboxylate group, while the protonated amine N2 group of Met hydrogen bonds not only with atoms O2 and O3 of the Tyr carboxylate group, but also with atom O1W of the water molecule. Several weak C—H⋯O hydrogen bonds also occur (Table 1). The is shown in Fig. 3.
Synthesis and crystallization
The dipeptide L-Met–L-Tyr was obtained commercially (Chemimpex International, Inc., Wood Dale, IL, USA). To about 100 mg of the dipeptide in a small tube, 2 ml of ethanol was added and mixed throughly on a vortex mixer. Additional solvent was added as required in small increments, while mixing on a vortex mixer and keeping the contents at 60 °C in a water bath. A small amount of water was added at the end to dissolve the peptide completely. The solution was left undisturbed at room temperature for slow evaporation and crystallization.
Refinement
Crystal data, data collection and structure .
details are summarized in Table 2
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Structural data
CCDC reference: 2260065
https://doi.org/10.1107/S2414314623005515/hb4434sup1.cif
contains datablocks I, global. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314623005515/hb4434Isup2.hkl
Supporting information file. DOI: https://doi.org/10.1107/S2414314623005515/hb4434Isup3.cml
Data collection: APEX2 (Bruker, 2016); cell
SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2020) and ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).C14H20N2O4S·H2O | Dx = 1.360 Mg m−3 |
Mr = 330.39 | Cu Kα radiation, λ = 1.54184 Å |
Orthorhombic, P212121 | Cell parameters from 7969 reflections |
a = 5.4826 (3) Å | θ = 3.8–69.4° |
b = 12.4971 (7) Å | µ = 2.01 mm−1 |
c = 23.5451 (13) Å | T = 100 K |
V = 1613.23 (15) Å3 | Lath, colourless |
Z = 4 | 0.29 × 0.08 × 0.02 mm |
F(000) = 704 |
Bruker Kappa APEXII DUO CCD diffractometer | 3002 independent reflections |
Radiation source: IµS microfocus | 2854 reflections with I > 2σ(I) |
QUAZAR multilayer optics monochromator | Rint = 0.040 |
φ and ω scans | θmax = 69.4°, θmin = 4.0° |
Absorption correction: multi-scan (SADABS; Krause et al., 2015) | h = −4→6 |
Tmin = 0.777, Tmax = 0.961 | k = −14→15 |
14990 measured reflections | l = −28→28 |
Refinement on F2 | Hydrogen site location: mixed |
Least-squares matrix: full | H atoms treated by a mixture of independent and constrained refinement |
R[F2 > 2σ(F2)] = 0.025 | w = 1/[σ2(Fo2) + (0.033P)2 + 0.2455P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.064 | (Δ/σ)max = 0.001 |
S = 1.06 | Δρmax = 0.22 e Å−3 |
3002 reflections | Δρmin = −0.18 e Å−3 |
221 parameters | Absolute structure: Flack x determined using 1154 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013) |
0 restraints | Absolute structure parameter: 0.031 (7) |
Primary atom site location: structure-invariant direct methods |
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. |
Refinement. All H atoms were located in difference maps and those on C atoms were treated thereafter as riding in geometrically idealized positions, with C—H = 0.95 Å for phenyl, 0.99 Å for CH2 and 0.98 Å for methyl H atoms. The coordinates of the N—H and O—H H atoms were refined. The Uiso(H) values were assigned as 1.5Ueq of the attached atom methyl, OH, and ammonium H atoms, and as 1.2Ueq otherwise. |
x | y | z | Uiso*/Ueq | ||
S1 | 0.45672 (10) | 0.72942 (4) | 0.10139 (2) | 0.01932 (13) | |
O1 | 0.6522 (3) | 0.64462 (13) | 0.60560 (7) | 0.0234 (3) | |
H1OH | 0.526 (6) | 0.613 (2) | 0.6171 (13) | 0.035* | |
O2 | 1.2758 (2) | 0.52566 (10) | 0.29568 (6) | 0.0150 (3) | |
O3 | 0.9501 (3) | 0.54361 (10) | 0.23911 (6) | 0.0152 (3) | |
O4 | 0.8826 (2) | 0.79919 (11) | 0.28382 (6) | 0.0159 (3) | |
N1 | 0.6958 (3) | 0.65289 (13) | 0.32141 (7) | 0.0131 (3) | |
H1N | 0.543 (5) | 0.6144 (19) | 0.3235 (10) | 0.016* | |
N2 | 0.4517 (3) | 0.90575 (13) | 0.27842 (7) | 0.0143 (3) | |
H21N | 0.507 (5) | 0.927 (2) | 0.3129 (12) | 0.022* | |
H22N | 0.298 (5) | 0.938 (2) | 0.2718 (11) | 0.022* | |
H23N | 0.557 (5) | 0.941 (2) | 0.2526 (11) | 0.022* | |
C1 | 0.6961 (4) | 0.61434 (16) | 0.55052 (9) | 0.0177 (4) | |
C2 | 0.9078 (4) | 0.65037 (18) | 0.52474 (9) | 0.0206 (4) | |
H2 | 1.0162 | 0.6960 | 0.5448 | 0.025* | |
C3 | 0.9612 (4) | 0.61967 (16) | 0.46948 (9) | 0.0188 (4) | |
H3 | 1.1061 | 0.6452 | 0.4520 | 0.023* | |
C4 | 0.8066 (4) | 0.55226 (16) | 0.43912 (8) | 0.0151 (4) | |
C5 | 0.5910 (4) | 0.52002 (16) | 0.46491 (9) | 0.0183 (4) | |
H5 | 0.4796 | 0.4765 | 0.4444 | 0.022* | |
C6 | 0.5353 (4) | 0.55030 (17) | 0.52031 (9) | 0.0193 (4) | |
H6 | 0.3874 | 0.5272 | 0.5374 | 0.023* | |
C7 | 0.8751 (4) | 0.51252 (15) | 0.38059 (9) | 0.0153 (4) | |
H7A | 0.7422 | 0.4661 | 0.3663 | 0.018* | |
H7B | 1.0237 | 0.4679 | 0.3838 | 0.018* | |
C8 | 0.9225 (4) | 0.60106 (15) | 0.33686 (8) | 0.0127 (4) | |
H8 | 1.0308 | 0.6558 | 0.3549 | 0.015* | |
C9 | 1.0589 (4) | 0.55304 (14) | 0.28579 (8) | 0.0127 (4) | |
C10 | 0.6979 (3) | 0.74683 (14) | 0.29371 (8) | 0.0117 (4) | |
C11 | 0.4474 (4) | 0.78691 (14) | 0.27481 (8) | 0.0136 (4) | |
H11 | 0.3216 | 0.7592 | 0.3018 | 0.016* | |
C12 | 0.3816 (3) | 0.75003 (16) | 0.21469 (8) | 0.0156 (4) | |
H12A | 0.3336 | 0.6737 | 0.2163 | 0.019* | |
H12B | 0.2376 | 0.7912 | 0.2017 | 0.019* | |
C13 | 0.5835 (4) | 0.76236 (17) | 0.17061 (8) | 0.0171 (4) | |
H13A | 0.7207 | 0.7137 | 0.1796 | 0.020* | |
H13B | 0.6454 | 0.8368 | 0.1706 | 0.020* | |
C14 | 0.7332 (5) | 0.7195 (2) | 0.06034 (11) | 0.0367 (6) | |
H14A | 0.8149 | 0.7893 | 0.0596 | 0.055* | |
H14B | 0.6935 | 0.6976 | 0.0215 | 0.055* | |
H14C | 0.8415 | 0.6664 | 0.0776 | 0.055* | |
O1W | 0.2853 (3) | 0.50422 (12) | 0.64496 (7) | 0.0195 (3) | |
H1W | 0.240 (5) | 0.447 (2) | 0.6281 (13) | 0.029* | |
H2W | 0.372 (6) | 0.487 (2) | 0.6762 (14) | 0.029* |
U11 | U22 | U33 | U12 | U13 | U23 | |
S1 | 0.0240 (2) | 0.0196 (2) | 0.0143 (2) | −0.0005 (2) | −0.0008 (2) | −0.00063 (19) |
O1 | 0.0273 (8) | 0.0285 (8) | 0.0144 (7) | −0.0058 (7) | 0.0056 (7) | −0.0028 (6) |
O2 | 0.0133 (7) | 0.0133 (6) | 0.0185 (7) | 0.0011 (5) | 0.0002 (6) | −0.0008 (6) |
O3 | 0.0163 (7) | 0.0156 (6) | 0.0136 (6) | −0.0017 (6) | −0.0017 (6) | −0.0014 (5) |
O4 | 0.0127 (7) | 0.0136 (7) | 0.0214 (7) | −0.0007 (5) | 0.0007 (5) | 0.0018 (5) |
N1 | 0.0116 (8) | 0.0115 (7) | 0.0161 (8) | −0.0002 (7) | 0.0001 (6) | 0.0010 (6) |
N2 | 0.0142 (8) | 0.0134 (8) | 0.0154 (8) | 0.0014 (7) | 0.0003 (7) | 0.0010 (7) |
C1 | 0.0229 (11) | 0.0181 (10) | 0.0123 (10) | 0.0032 (8) | 0.0012 (8) | 0.0002 (8) |
C2 | 0.0220 (11) | 0.0228 (10) | 0.0171 (10) | −0.0029 (9) | −0.0004 (9) | −0.0016 (8) |
C3 | 0.0165 (9) | 0.0229 (10) | 0.0169 (10) | −0.0017 (9) | 0.0018 (9) | 0.0018 (8) |
C4 | 0.0181 (10) | 0.0133 (9) | 0.0139 (10) | 0.0042 (8) | 0.0005 (8) | 0.0038 (8) |
C5 | 0.0210 (11) | 0.0179 (9) | 0.0160 (10) | −0.0031 (8) | −0.0017 (8) | 0.0009 (8) |
C6 | 0.0180 (10) | 0.0226 (10) | 0.0175 (10) | −0.0008 (9) | 0.0023 (9) | 0.0021 (8) |
C7 | 0.0181 (10) | 0.0120 (9) | 0.0158 (10) | 0.0015 (8) | 0.0012 (8) | 0.0017 (7) |
C8 | 0.0141 (10) | 0.0117 (8) | 0.0124 (9) | 0.0009 (8) | −0.0007 (8) | −0.0006 (7) |
C9 | 0.0135 (9) | 0.0076 (8) | 0.0169 (10) | −0.0029 (8) | 0.0000 (8) | 0.0010 (7) |
C10 | 0.0136 (9) | 0.0109 (9) | 0.0105 (9) | 0.0012 (7) | 0.0015 (7) | −0.0018 (7) |
C11 | 0.0141 (9) | 0.0122 (9) | 0.0145 (9) | 0.0001 (8) | 0.0010 (8) | 0.0019 (7) |
C12 | 0.0149 (9) | 0.0162 (9) | 0.0158 (10) | −0.0023 (7) | −0.0025 (7) | 0.0014 (8) |
C13 | 0.0186 (10) | 0.0177 (9) | 0.0149 (10) | −0.0014 (8) | 0.0012 (8) | −0.0016 (7) |
C14 | 0.0377 (14) | 0.0502 (16) | 0.0222 (13) | 0.0036 (13) | 0.0064 (10) | −0.0097 (12) |
O1W | 0.0249 (8) | 0.0188 (7) | 0.0147 (7) | −0.0028 (6) | −0.0006 (6) | −0.0001 (6) |
S1—C14 | 1.802 (3) | C5—C6 | 1.392 (3) |
S1—C13 | 1.819 (2) | C5—H5 | 0.9500 |
O1—C1 | 1.372 (3) | C6—H6 | 0.9500 |
O1—H1OH | 0.84 (3) | C7—C8 | 1.534 (3) |
O2—C9 | 1.259 (3) | C7—H7A | 0.9900 |
O3—C9 | 1.256 (2) | C7—H7B | 0.9900 |
O4—C10 | 1.228 (2) | C8—C9 | 1.538 (3) |
N1—C10 | 1.343 (2) | C8—H8 | 1.0000 |
N1—C8 | 1.448 (3) | C10—C11 | 1.528 (3) |
N1—H1N | 0.97 (3) | C11—C12 | 1.532 (3) |
N2—C11 | 1.488 (2) | C11—H11 | 1.0000 |
N2—H21N | 0.91 (3) | C12—C13 | 1.525 (3) |
N2—H22N | 0.95 (3) | C12—H12A | 0.9900 |
N2—H23N | 0.95 (3) | C12—H12B | 0.9900 |
C1—C2 | 1.385 (3) | C13—H13A | 0.9900 |
C1—C6 | 1.387 (3) | C13—H13B | 0.9900 |
C2—C3 | 1.388 (3) | C14—H14A | 0.9800 |
C2—H2 | 0.9500 | C14—H14B | 0.9800 |
C3—C4 | 1.392 (3) | C14—H14C | 0.9800 |
C3—H3 | 0.9500 | O1W—H1W | 0.85 (3) |
C4—C5 | 1.388 (3) | O1W—H2W | 0.90 (3) |
C4—C7 | 1.512 (3) | ||
C14—S1—C13 | 100.05 (11) | N1—C8—C9 | 113.30 (16) |
C1—O1—H1OH | 109 (2) | C7—C8—C9 | 109.01 (15) |
C10—N1—C8 | 120.38 (16) | N1—C8—H8 | 108.0 |
C10—N1—H1N | 117.9 (14) | C7—C8—H8 | 108.0 |
C8—N1—H1N | 120.5 (14) | C9—C8—H8 | 108.0 |
C11—N2—H21N | 110.3 (16) | O3—C9—O2 | 125.79 (19) |
C11—N2—H22N | 113.9 (16) | O3—C9—C8 | 119.33 (17) |
H21N—N2—H22N | 109 (2) | O2—C9—C8 | 114.87 (17) |
C11—N2—H23N | 116.0 (16) | O4—C10—N1 | 124.39 (18) |
H21N—N2—H23N | 104 (2) | O4—C10—C11 | 120.75 (16) |
H22N—N2—H23N | 104 (2) | N1—C10—C11 | 114.86 (16) |
O1—C1—C2 | 118.13 (19) | N2—C11—C10 | 107.23 (16) |
O1—C1—C6 | 122.15 (19) | N2—C11—C12 | 110.91 (15) |
C2—C1—C6 | 119.7 (2) | C10—C11—C12 | 112.47 (16) |
C1—C2—C3 | 119.8 (2) | N2—C11—H11 | 108.7 |
C1—C2—H2 | 120.1 | C10—C11—H11 | 108.7 |
C3—C2—H2 | 120.1 | C12—C11—H11 | 108.7 |
C2—C3—C4 | 121.3 (2) | C13—C12—C11 | 115.31 (16) |
C2—C3—H3 | 119.3 | C13—C12—H12A | 108.4 |
C4—C3—H3 | 119.3 | C11—C12—H12A | 108.4 |
C5—C4—C3 | 117.98 (19) | C13—C12—H12B | 108.4 |
C5—C4—C7 | 120.98 (19) | C11—C12—H12B | 108.4 |
C3—C4—C7 | 121.01 (19) | H12A—C12—H12B | 107.5 |
C4—C5—C6 | 121.2 (2) | C12—C13—S1 | 108.02 (14) |
C4—C5—H5 | 119.4 | C12—C13—H13A | 110.1 |
C6—C5—H5 | 119.4 | S1—C13—H13A | 110.1 |
C1—C6—C5 | 119.9 (2) | C12—C13—H13B | 110.1 |
C1—C6—H6 | 120.1 | S1—C13—H13B | 110.1 |
C5—C6—H6 | 120.1 | H13A—C13—H13B | 108.4 |
C4—C7—C8 | 114.64 (16) | S1—C14—H14A | 109.5 |
C4—C7—H7A | 108.6 | S1—C14—H14B | 109.5 |
C8—C7—H7A | 108.6 | H14A—C14—H14B | 109.5 |
C4—C7—H7B | 108.6 | S1—C14—H14C | 109.5 |
C8—C7—H7B | 108.6 | H14A—C14—H14C | 109.5 |
H7A—C7—H7B | 107.6 | H14B—C14—H14C | 109.5 |
N1—C8—C7 | 110.24 (16) | H1W—O1W—H2W | 110 (3) |
O1—C1—C2—C3 | 178.66 (19) | C4—C7—C8—C9 | 163.90 (16) |
C6—C1—C2—C3 | −1.9 (3) | N1—C8—C9—O3 | −13.3 (2) |
C1—C2—C3—C4 | −0.5 (3) | C7—C8—C9—O3 | 109.8 (2) |
C2—C3—C4—C5 | 2.7 (3) | N1—C8—C9—O2 | 167.13 (16) |
C2—C3—C4—C7 | −175.37 (19) | C7—C8—C9—O2 | −69.7 (2) |
C3—C4—C5—C6 | −2.7 (3) | C8—N1—C10—O4 | −6.3 (3) |
C7—C4—C5—C6 | 175.42 (19) | C8—N1—C10—C11 | 174.12 (16) |
O1—C1—C6—C5 | −178.65 (19) | O4—C10—C11—N2 | −33.4 (2) |
C2—C1—C6—C5 | 1.9 (3) | N1—C10—C11—N2 | 146.24 (16) |
C4—C5—C6—C1 | 0.4 (3) | O4—C10—C11—C12 | 88.8 (2) |
C5—C4—C7—C8 | 123.7 (2) | N1—C10—C11—C12 | −91.57 (19) |
C3—C4—C7—C8 | −58.3 (3) | N2—C11—C12—C13 | 74.5 (2) |
C10—N1—C8—C7 | 165.36 (17) | C10—C11—C12—C13 | −45.6 (2) |
C10—N1—C8—C9 | −72.2 (2) | C11—C12—C13—S1 | −173.31 (13) |
C4—C7—C8—N1 | −71.1 (2) | C14—S1—C13—C12 | −167.63 (16) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1N···O2i | 0.97 (3) | 1.95 (3) | 2.863 (2) | 156 (2) |
N2—H21N···O1Wii | 0.91 (3) | 2.01 (3) | 2.805 (2) | 145 (2) |
N2—H22N···O3iii | 0.95 (3) | 1.91 (3) | 2.827 (2) | 162 (2) |
N2—H23N···O2iv | 0.95 (3) | 1.80 (3) | 2.743 (2) | 171 (2) |
C6—H6···O1W | 0.95 | 2.61 | 3.290 (3) | 129 |
C11—H11···O1v | 1.00 | 2.66 | 3.359 (2) | 127 |
C11—H11···O4i | 1.00 | 2.49 | 3.108 (2) | 119 |
C12—H12A···O2i | 0.99 | 2.65 | 3.440 (2) | 137 |
C13—H13B···O2iv | 0.99 | 2.53 | 3.472 (2) | 159 |
O1W—H1W···S1vi | 0.85 (3) | 2.54 (3) | 3.3674 (16) | 165 (3) |
O1W—H2W···O3vii | 0.90 (3) | 1.82 (3) | 2.716 (2) | 179 (3) |
Symmetry codes: (i) x−1, y, z; (ii) x+1/2, −y+3/2, −z+1; (iii) −x+1, y+1/2, −z+1/2; (iv) −x+2, y+1/2, −z+1/2; (v) x−1/2, −y+3/2, −z+1; (vi) −x+1/2, −y+1, z+1/2; (vii) −x+3/2, −y+1, z+1/2. |
Funding information
Funding for this research was provided by: National Science Foundation, Division of Chemistry (award No. 0847742 to M. O. Claville; award No. 1238838 to M. O. Claville); Louisiana Board of Regents (grant No. LEQSF (1999–2000)-ENH-TR-13 to Frank Fronczek).
References
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