I. INTRODUCTION
Benzene, toluene, and xylene (BTX) are common organic solvent widely used in the chemical industry. It has been reported that exposure to high concentrations of volatile organic compounds such as BTX will lead to a series of diseases, causing acute and chronic respiratory effects, functional alterations of the central nervous system, mucous and skin irritations, and, in extreme cases, chromosome aberrations (Bilban, Reference Bilban2004; Celik and Akbas, Reference Celik and Akbas2005). Hippuric acid (HA) and methylhippuric acids (mHA) are metabolites of toluene and xylene produce in the human body, since they are found as physiological components of the human urine if toluene or xylene was inhaled. Thus, quantification of HA and mHA in urine is actually used as a diagnostic marker of exposure to toluene and xylene (Sperlingová et al., Reference Sperlingová, Dabrowská, Stránsky, Kucera and Tichy2007; Antunes et al., Reference Antunes, Niederauer and Linden2013).
A search in the Cambridge Structural Database (CSD, v 5.36, February 2015) (Allen, Reference Allen2002) and PDF-ICDD database (ICDD, 2011) showed no entries for these HA derivatives. The aim of this work is to report the X-ray powder diffraction data of the three N-acylamino acids derivatives of the HA; 2, 3, and 4-methylhippuric acids (Figure 1), which are the specific metabolites of the 2, 3, and 4-xylene solvent contaminants.
Figure 1. Structural diagram of the ortho, meta, and para-methylhippuric acids.
II. EXPERIMENTAL
A. Specimen preparations
For the X-ray analysis, small quantities of ortho, meta, and para-methylhippuric acids at 99% of purity (obtained from commercial source, Aldrich) were ground mechanically in an agate mortar and pestle. The resulting fine powders, sieved to 106 µm, were mounted on a flat zero-background holder covered with a thin layer of petroleum jelly.
B. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) characterization
Melting points were measured in an electro thermal apparatus. The FTIR absorption spectra were obtained as KBr pellet using a Perkin-Elmer 1600 spectrometer. 1H-NMR spectra were recorded on a Bruker Avance 400 model spectrometer in DMSO-d 6 solution.
2 mHA: mp 164–165 °C, FTIR (cm−1); C–H 749.9, 727.9, N–H 3479.3, O–H 3296.8, 1H-NMR (400 MHz, DMSO-d 6) δ (ppm); 11.0 (s, COOH), 7.45 (s, CONH), 7,44 (d, H5), 7.32 (t, H7), 7.19 (d, H8), 7.23 (t, H6), 4.11 (s, CH 2), 2.43 (s, CH 3).
3 mHA: mp 138–140 °C, FTIR (cm−1); C–H 809.4, N–H 3450.6, O–H 3356.0, 1H-NMR (400 MHz, DMSO-d 6) δ (ppm); 11.1 (s, COOH), 8.1 (s, CONH), 7.84 (s, H5), 7.74 (d, H9), 7.48 (t, H7), 7.37 (d, H6), 3.94 (s, |CH 2), 2.35 (s, CH 3).
4 mHA: mp 163–165 °C, FTIR (cm−1); C–H 832.7, N–H 3458.6, O–H 3354.4, 1H-NMR (400 MHz, DMSO-d 6) δ (ppm); 11.8 (s, COOH), 7.99 (s, CONH), 7.82 (d, H5, H9), 7.27 (d, H6, H8), 4.12 (s, CH 2), 2.37 (s, CH 3).
C. Powder diffraction data collection
X-ray powder diffraction patterns were collected at room temperature in a Philips PW-1150/25 diffractometer with Bragg–Brentano geometry using CuKα radiation (λ = 1.5418 Å; 30 KV, 15 mA) and a diffracted beam graphite monochromator. The specimens were scanned from 5 to 55°2θ, with a step size of 0.02 and counting time of 10 s. Silicon (SRM 640) was used as an external standard. The analytical software package HIGHSCORE PLUS v2.0 (PANalytical, Almelo, The Netherlands) was used to establish the positions of the peaks from the α 1 component, strip mathematically the α 2 component from each reflection, and to determine the peak intensities of the diffraction peaks (Tables I–III).
Table I. X-ray powder diffraction data of ortho-methylhippuric acid (2 mHA).
Table II. X-ray powder diffraction data of meta-methylhippuric acid (3 mHA).
Table III. X-ray powder diffraction data of para-methylhippuric acid (4 mHA).
III. RESULTS AND DISCUSSION
The experimental powder diffraction patterns are depicted in Figures 2–4. Automatic indexing of the experimental X-ray diffraction patterns were done using DICVOL06 (Boultif and Louër, Reference Boultif and Louër2004), which gave unique solutions in monoclinic (2 mHA, 3 mHA) and orthorhombic (4 mHA) cells. These results are consistent with the space groups P21/n (2 mHA), C2/c (3 mHA), and P212121 (4 mHA) obtained in a single-crystal analysis, which will be published elsewhere.
Figure 2. X-ray powder diffraction pattern of ortho-methylhippuric acid.
Figure 3. X-ray powder diffraction pattern of meta-methylhippuric acid.
Figure 4. X-ray powder diffraction pattern of para-methylhippuric acid.
The complete powder diffraction dataset were reviewed in the mentioned space groups, using the program NBS*AIDS83 (Mighell et al., Reference Mighell, Hubbard and Stalick1981). The X-ray powder diffraction patterns of three N-acylamino acids are given in Tables I–III. The crystal data and indexing figures of merit M N (de Wolff, Reference de Wolff1968) and F N (Smith and Snyder, Reference Smith and Snyder1979) for the three compounds are shown in Table IV.
Table IV. X-ray crystal structural data for the mHA isomers.
SUPPLEMENTARY MATERIAL
The supplementary material for this article can be found at http://dx.doi.org/10.1017/S0885715616000312.
ACKNOWLEDGEMENTS
This work was supported by Consejo de Desarrollo Científico, Humanístico, Tecnológico y de las Artes (CDCHTA-ULA, grant No. C-1921-15-08-AA), and FONACIT (Fondo Nacional de Investigaciones Científicas, grant No. LAB-97000821) in Venezuela.
