Terahertz Absorption Spectra of Fatty Acids and Their Analogues

Transcription

1 Journal of Oleo Science Copyright 2011 by Japan Oil Chemists Society Terahertz Absorption Spectra of Fatty Acids and Their Analogues Feng Ling Jiang 1, Ikuo Ikeda 1, Yuichi Ogawa 2 and Yasushi Endo 3 1 Graduate School of Agricultural Science, Tohoku University (Sendai , JAPAN) 2 Graduate School of Agriculture, Kyoto University (Kyoto , JAPAN) 3 School of Bioscience and Biotechnology, Tokyo University of Technology (Tokyo , JAPAN) Abstract: Absorption spectra in the terahertz (THz) region between 10 and 400 cm -1 were measured for fatty acids and their analogues at room temperature. Saturated fatty acids such as palmitic and stearic acids had some sharp peaks, while unsaturated fatty acids such as oleic, linoleic and linolenic acids had two distinct peaks at 247 and 328 cm -1. These peaks apparently derived from the carboxylic group because oleyl alcohol had no distinct peak. The THz absorption spectra of fatty acids may be affected by the crystalline as well as the chemical structure. The THz absorption spectra of oleic acid esters depended on ester types, although all oleic acid esters had some peaks due to the ester group. THz absorbance of fatty acids positively correlated with concentration. Based on these results, THz spectrometry may be a good analytical method for the non-destructive qualitative and quantitative evaluation of fatty acids and their analogues. Key words: fatty acid, non-destructive analytical method, terahertz spectrometry 1 INTRODUCTION Terahertz THz waves are between millimeter radio and far infrared light waves and their range is typically defined as THz cm 1. THz waves have characteristics of radio and light waves. They can be transmitted through packing materials such as paper, cloth, ceramics, plastics, wood, and so on. They can be easily effected, focused, and refracted like ultraviolet and visible lights waves. Moreover, THz radiation does not change the chemical structure and it is not harmful to humans, in contrast to X-rays and UV radiation. Especially, it is very interesting that many organic and inorganic compounds have specific spectral characteristics, commonly called fingerprint in the THz regions 1 3. In recent years, many researchers in biophysics and biochemistry have adopted THz spectroscopy 4 7. The THz radiation excites the crystalline phonon vibrations, hydrogenbonding stretches, and torsions of molecules. The THz spectra provide insight into the structural dynamics of biomolecules including intra- and inter-interactions via hydrogen bonds or Van der Waals forces. Thus, THz spectroscopy is applicable as a non-destructive analytical method. There are several reports for the use of THz absorption spectra in chemicals 8 10, drugs 10, 11, carbohydrates 12, 13, amino acids and polypeptides 14 17, fats and oils 18, 19, vita- mins 20, 21 and so on. However, there are few reports concerning fatty acids and their analogues. The RIKEN THz Database and the NICT Terahertz Spectral Database include saturated fatty acids such as stearic acid 18:0 and palmitic acid 16:0 but not unsaturated fatty acids 22. In this paper, we measured the THz absorption spectra of various fatty acids and their analogues to build a THz spectral database in the range 10 to 400 cm 1, although the THz absorption spectra were measured for edible fats and oils at wave numbers under 100 cm 1 18, 19. Moreover, we discuss the relation between the THz spectra and the chemical structure of fatty acids and their analogues, and the applicability of THz spectrometry in the field of fats and oil. 2 EXPERIMENTAL PROCEDURES 2.1 Materials Palmitic, stearic, oleic, linoleic and linolenic acids, methyl oleate, ethyl oleate, propyl oleate, oleyl alcohol, monoolein, diolein, triolein purity 95, and liquid paraffin for IR measurements were supplied from Wako Pure Chemical Industries, Ltd. Osaka, Japan Correspondence to: Yasushi Endo, School of Bioscience and Biotechnology, Tokyo University of Technology, Katakuramachi, Hachioji, Tokyo , JAPAN endo@bs.teu.ac.jp Accepted March 16, 2011 (received for review February 7, 2011) Journal of Oleo Science ISSN print / ISSN online 339

2 F. L. Jiang, I. Ikeda, Y. Ogawa and Y. Endo 2.2 Terahertz spectrometry A fourier transform-infrared FT-IR spectrophotometer made by JASCO Corp. Tokyo, Japan was remodeled using a silicon broadband beam splitter and a deuterated l-alanine-doped triglycine sulfate DLATGS room-temperature detector, to obtain spectra in the cm 1 range. The light source was a high-pressure mercury lamp, which has a higher intensity than a ceramic lamp below 3 THz. Fatty acids and their analogues were dissolved in liquid paraffin at concentrations of 18 to 855 mm, and were placed in a polyethylene cell with 0.5 mm path. The polyethylene cell and liquid paraffin are appropriate well suited for THz spectrometry, because they have no notable absorption at 10 to 400 cm 1. As room-temperature reference, the THz absorption spectra of the polyethylene cell were measured befor the collection of the THz absorption spectra of fatty acids and their analogues. The wavenumber resolution was 16 cm 1 and the accumulation was 100 times. The data represent the average of triplicate measuements. 3 RESULTS AND DISCUSSION Figure 1 shows the THz absorption spectra of palmitic acid 16:0 and stearic acid 18:0 at 10 to 400 cm 1. The THz absorption spectrum of palmitic acid was slightly different from that of stearic acid. Palmitic acid has four distinct peaks at 177, 235, 309, and 378 cm 1, while stearic acid has five distinct peaks at 154, 201, 258, 316, and 386 cm 1 and a broad peak at 112 cm 1. Moreover, the THz absorbance of the palmitic and stearic acids increased with increasing wavenumbers. The results agree with the RIKEN Terahertz Database22, even though the latter does not contain spectra beyond 200 cm 1. This is the first report of distinct peaks for the palmitic and stearic acids Fig. 1 Terahertz spectra of palmitic and stearic acid at 200 mm between 200 and 400 cm 1. Figure 2 shows the THz absorption spectra of oleic 18:1, linoleic 18:2, and linolenic 18:3 acids. All of them have similar spectra patterns, and their absorbance is increasing with increasing wavenumbers. They have two distinct peaks at 247 and 328 cm 1 and three peaks at 77, 166 and 378 cm 1. The spectra of the unsaturated and saturated fatty acids differ, eg. palmitic and stearic acids. Probably, the THz absorption spectra of the saturated fatty acids are associated with their crystalline as well as chemical structure, because the palmitic and stearic acids are solid at room temperature, whereas the oleic, linoleic, and linolenic acids are liquid. However, the double bonds on the carbon chain did not affect the THz absorption spectral pattern, although they reduced the absorbance Fig. 2. Figure 3 shows the THz absorption spectrum of oleyl alcohol. Oleyl alcohol has a broad band at 231 cm 1. Therefore, the oleic acid peaks at 328 and 378 cm 1 apparently Fig. 2 Terahertz spectra of oleic, linoleic, and linolenic acid at 200 mm. Fig. 3 Terahertz spectrum of oleyl alcohol at 200 mm. 340

3 Terahertz Spectra of Fatty Acids derive from the carboxylic group. Especially, the peak at 77 cm 1 may be due to the carboxylic group, because saturated and unsaturated fatty acids have a peak at 77 cm 1, but fatty alcohols such as oleyl alcohol do not. Actually, this peak is also observed in amino acids, aromatic carboxylic acids, and short chain fatty acids such as butanoic acid 22. In contrast, the peaks observed at cm 1 for both oleic acid and oleyl alcohol may be associated with hydrogen bond vibrations. Figure 4 shows the THz absorption spectra of methyl, ethyl, and propyl esters of oleate. These compounds show completely different spectra patterns. Methyl oleate has three peaks at 46, 189, and 328 cm 1. Ethyl oleate has five peaks at 50, 166, 258, 347, and 374 cm 1, and propyl oleate has five peaks at 62, 98, 272, 314, and 347 cm 1. The ester structure strongly affected the THz absorption spectra of fatty acids. The oleic acid peak at 77 cm 1 shifted due to presence of the ester groups. Moreover, the peaks observed at about 330 cm 1 of oleic acid esters may derive from the ester group the ester carbonyl bond, because these peaks are also observed for oleic acid but not oleyl alcohol. These peaks may be associated with C O vibrations. Figure 5 shows the THz absorption spectra of monoolein, diolein and triolein. They have similar spectra patterns, although the absorbance depends on the glyceride type. Triolein have two peaks at 77 and 328 cm 1 and diolein have two peaks at 328 and 69 cm 1. Conversely, monoolein have sharp peaks at 309, 328, and 378 cm 1. The peak observed at 328 cm 1 may derive from the ester group the ester carbonyl bond. The absorbance is the highest for monoolein, followed by diolein and triolein, and it decrease with the numbers of ester groups in the glycerol molecules. Figure 6 shows the variation in the absorbance and the second derivative of oleic acid at different concentrations. The absorbance increased as the oleic acid concentration increased. Among the spectra of the second derivative, the intensity at 77, 110, 243, 278, and 328 cm 1 varied with the concentration of oleic acid. Especially, the intensity of the second derivative at 77 cm 1 was distinct. Thus, a calibra- Fig. 5 Terahertz spectra of monoolein, diolein, and triolein at 200 mm. Fig. 4 Terahertz spectra of methyl oleate, ethyl oleate and propyl oleatae at 200 mm. Fig. 6 Changes in absorbance (A) and intensity of the second derivatives (B) of oleic acid at concentrations of 18, 36 53, 62, 89, 177, 266, 355, and 855 mm. 341

4 F. L. Jiang, I. Ikeda, Y. Ogawa and Y. Endo harmful organic solvents such as chloroform. However, THz spectroscopy is a non-destructive method and it needs no toxic organic solvents. Infrared spectrometry is also available for determining fatty acids and their analogues, but it is not always able to distinguish the palmitic acid from stearic acid, or the methyl esters from ethyl esters. However, THz spectroscopy can distinguish the carbon chains of the saturated fatty acids and the alcohol side chains of the fatty acid esters, as shown in Figs 1 and 4. We believe that the THz spectrometry is an effective tool for the non-destructive qualitative and quantitative analysis of fatty acids and their analogues. Fig. 7 Calibration curve of oleic acid by the second derivative of absorbance at 77 cm -1. tion curve was established using the intensity of the second derivative at 77 cm 1 and the concentration between 18 and 855 mm. As shown in Fig. 7, the intensity of the second derivative for oleic acid correlates positively with the concentration r 0.999, although the determination limit was almost 10 mm. Such strong correlations are obtained for not only for the fatty acids but also for esters and glycerides. From these results, the concentration of fatty acids can be determined with THz spectrometry. We applied the THz absorption spectrometry to determine the content of fatty acids in oily foods. In this study, the fatty acids and their analogues have comparatively broad and fewer peaks, and lower THz absorptivity than sugars and amino acids in the range 10 to 400 cm 1. Fatty acids and their analogues are liquid, while sugar and amino acids are solid at room temperature. Probably the crystalline structure affects the THz absorptivity of these compounds. Upadhya et al. 12 reported that D-glucose has sharp absorption peaks at 1.45 and 2.1 THz 48 and 70 cm 1 and minor peak at 1.26 THz 41 cm 1. Among amino acids, tryptophan has broad bands at 1.4 and 1.8 THz 47 and 60 cm 1 15, and serine and cystein have several bands from 50 to 400 cm These peaks are assigned to torsional motion. Probably, the peak at 77 cm 1 may derive from the torsional motion of the carboxyl group but not the crystalline structure, because the absorbance at 77 cm 1 follows the Lambert-Beer law Fig. 7. Hu et al. 18 observed that vegetable oils have broad peaks at 0.2 to 1.6 THz 7 to 53 cm 1. However, the THz region between 100 and 400 cm 1 is not measured for fats and oils. We have shown that fatty acids and their analogues have several absorption peaks between 10 and 400 cm 1, and that these peaks may be assigned to the carboxylic or the ester group. Fatty acid esters are generally quantified by gas liquid chromatography, mass spectrometry, or nuclear magnetic resonance. These methods are destructive or they use References 1 Ferguson, S.; Zhang, X. C. Materials for terahertz science and technology. Nature Materials 1, Kawase, K.; Ogawa, Y.; Watanabe, Y.; Inoue, H.; Nondestructive terahertz imaging of illicit drugs using spectral fingerprints. Optics Express 11, Pickwell, E.; Wallacce, V. P. Biomedical applications of terahertz technology. J. Phys. D: Appl. Phys. 39, R301-R Ogawa, Y. Application of terahertz imaging to analysis. BUNSEKI 2007, Nazarov, M. M.; Shkurinov, A. P.; Tuchin, V. V.; Zhernovaya, O. S. Modification of terahertz spectrometer to study biological samples. Proc. SPIE 6535, 65351J Plusquellic, D. F.; Siegrist, K.; Heilweil, E. J.; Esenturk, O. Applications of terahertz spectroscopy in biosystems. Chem. Phys. Chem., 8, Ueno, Y.; Ajito, K. Analytical terahertz spectroscopy. Anal. Sci. 24, Xu, H.; Han, J. G.; Yu, X. H.; Li, W. X.; Zhu, Z. Y.; Li, L. F.; Ji, T. Terahertz time-domain spectroscopy of naphthalene and naphthols. Struct. Chem. 15, Ikeda, T.; Matsushita, A.; Tatsuno, M.; Minami, Y. Investigation of inflammable liquids by terahertz spectroscopy. Appl. Phys. Lett. 87, Kawase, K.; Watanabe, Y.; Ogawa, Y.; Ito, H. Component special pattern analysis of chemicals using terahertz spectroscopic imaging. IEEJ Trans EIS 124, Kishi, T.; Kanamori, T.; Tsujikawa, K.; Iwata, Y.T.; Inoue, H.; Ohtsuru, O.; Hoshina, H.; Otani, C.; Kawase, K. Differentiation of optical active form and racemic form of amphetamine-type stimulants by terahertz spectroscopy. Spectroscopy Material Properties 1, Upadhya, P. C.; Shen, Y. C.; Davies, A. G.; Linfield, E. H. 342

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