Vibrational structure of the polyunsaturated fatty acids eicosapentaenoic acid and arachidonic acid studied by infrared spectroscopy Johannes Kiefer a, * , Kristina Noack a , Juergen Bartelmess b , Christian Walter c , Heike Dörnenburg c , Alfred Leipertz a a Lehrstuhl für Technische Thermodynamik and Erlangen Graduate School in Advanced Optical Technologies, Universität Erlangen-Nürnberg, Am Weichselgarten 8, 91058 Erlangen, Germany b Lehrstuhl für Physikalische Chemie I, Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany c Lehrstuhl für Bioverfahrenstechnik, Universität Erlangen-Nürnberg, Paul-Gordan-Str. 3, 91052 Erlangen, Germany article info Article history: Received 13 August 2009 Received in revised form 19 November 2009 Accepted 19 November 2009 Available online 26 November 2009 Keywords: Vibrational structure Polyunsaturated fatty acid Eicosapentaenoic acid Eicosatetraenoic acid Arachidonic acid Infrared spectroscopy abstract The spectroscopic discrimination of the two structurally similar polyunsaturated C 20 fatty acids (PUFAs) 5,8,11,14,17-eicosapentaenoic acid and 5,8,11,14-eicosatetraenoic acid (arachidonic acid) is shown. For this purpose their vibrational structures are studied by means of attenuated total reflection (ATR) Fou- rier-transform infrared (FT-IR) spectroscopy. The fingerprint regions of the recorded spectra are found to be almost identical, while the C–H stretching mode regions around 3000 cm À1 show such significant differences as results of electronic and molecular structure alterations based on the different degree of saturation that both fatty acids can be clearly distinguished from each other. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Along with DNA and RNA bases, amino acids, carbohydrates and primary metabolites, lipids in terms of fats and fatty acids are important substances in the field of biochemistry. The latter family contains a huge number of different complex species, however, the molecular structures may be very similar hence their classification and discrimination is a challenging task from the analytical point of view [1]. The methods that are typically employed for this pur- pose are, e.g., nuclear magnetic resonance (NMR) spectroscopy [2] and mass spectrometry (MS) [3]. A large number of polyunsaturated fatty acids (PUFA) have been identified as essential nutrients as they cannot be synthesized by animals and human beings owing to a lack of appropriate enzymes. Hence these nutritive substances must be ingested together with the food. An important class of PUFAs are eicosanoic omega-3 and omega-6 fatty acids which can exert influence on the patho- genesis of many diseases like cancer, inflammation or cardiovascu- lar disease as well as the neuronal development in a positive or negative way [4–8]. Two prominent examples for C 20 omega-3 and omega-6 fatty acids are all-cis-5,8,11,14,17-eicosapentaenoic acid (EPA) and all-cis-5,8,11,14-eicosatetraenoic acid (arachidonic acid, AA), respectively. The chemical structures are displayed in Fig. 1. Both, for instance, act as precursors for prostaglandins which can cause swelling, inflammation or fever. Medical studies, however, showed that they provide the possibility to treat, e.g., cardiovascular disease, stomach ulcer, stomach cancer and Alzheimer’s disease [9,10] and, moreover, that they exert influence, e.g., on the metabolism of mammalian and leukaemia cells and human fibroblasts [11–13]. Interestingly, both AA and EPA are synthesized by the microal- gae Porphyridium purpureum [9,14]. This microalga is an already well investigated microorganism concerning its growth and the product formation during the cultivation process in bioreactors [15,16]. This opens the possibility to produce the fatty acids in bio- reactors for pharmaceutical purposes. However, as the yield is strongly dependent on the environment conditions and the com- position of the algae suspension [17], there is a need for appropri- ate analytical methods in view of process monitoring and control. Such techniques should enable quantitative or at least qualitative detection of reactants as well as product species. In particular, the discrimination of the different fatty acids produced by, e.g., Porphyridium purpureum would provide a great potential for the 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.11.052 * Corresponding author. Address: Lehrstuhl für Technische Thermodynamik, Am Weichselgarten 8, 91058 Erlangen, Germany. Tel.: +49 9131 8529766; fax: +49 9131 8529901. E-mail address: jk@ltt.uni-erlangen.de (J. Kiefer). Journal of Molecular Structure 965 (2010) 121–124 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc