Submitted to: Journal of Lipids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2016
Publication Date: 9/23/2016
Citation: Broadhurst, L., Schmidt, W.F., Kim, M.S., Nguyen, J.K., Qin, J., Chao, K., Bauchan, G.R., Shelton, D.R. 2016. Continuous gradient temperature Raman spectroscopy of oleic and linoleic acids from -100 to 50°C. Journal of Lipids. 51:1289-1302.

Interpretive Summary: Raman spectroscopy has advantages for analysis of biological, agricultural and food chemicals: it is fast, nondestructive, solvent-free and nearly transparent to water. Rapid Raman analyses suitable for in-plant processing of meat/fish proteins, carcass lipids, produce and dairy products have been developed. Pesticide and other contaminant residues can also be determined. Our research group has developed the technique of continuous gradient temperature Raman spectroscopy (GTRS) which applies the precise temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy. GTRS can easily detect changes that occur within 1°C temperature increments; more precise measurements can be made utilizing shallower gradients over specific temperature ranges of interest. Thus, GTRS provides a very rapid and straightforward technique to identify theoretically proposed molecular rearrangements that occur just near or at phase transitions. Polyunsaturated fatty acids are essential biochemicals for both human and animal health and are part of the phospholipid and triacylglycerol components of many foods. Fully euclidating the structures of the various long chain polyunsaturated fatty acids via infrared (IR) or Raman spectroscopy can be a daunting task due to the repetitive nature of the acyl/olefin chains and drastic decreases in melting points as the number of double bonds increases. Consequently only the vibrational modes of the monounsaturated fatty acid oleic acid have been well characterized over a range of temperatures. OA is one of the most common fatty acids in food, cosmetic and pharmaceutical lipids. Currently, a complete and interpreted Raman spectrum of LA is not available. In this contribution we extend the GTRS technique from cryogenic to warm liquid temperatures (-100 to 50°C) for OA and LA. A complete set of Raman frequencies and intensities vs. temperature are reported with three-dimensional contour plots, and correlated to phase changes observed with DSC. This information will be of interest to other scientists.

Technical Abstract: Gradient Temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry (DSC) to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur near and at phase transitions. Herein we apply GTRS and DSC to the unsaturated fatty acids oleic (OA, 18:1n-9) and linoleic (LA, 18:2n-3) from -100 to 50°C. 20 Mb three-dimensional data arrays with 0.2°C increments and first/second derivatives allowed complete assignment of solid, liquid and transition state vibrational modes. For OA, large spectral and line width changes occurred in the solid state ' to ' transition near -4ºC, and the melt (13ºC) over a range of only 1°C. For example, below -7ºC, the methyl rocking vibrational mode at 895 cm-1 is strong; from -7ºC to 13ºC, intensity increases to very strong; above 13ºC, intensity falls precipitously and a new, very sharp frequency arises at 858 cm-1. For LA, major intensity reductions from 200 to 1750 cm-1 and some peak shifts marked one solid state phase transition at -50°C. A second solid state transition (-33°C) had minor spectral changes. Large spectral and line width changes occurred at the melt transition (-7°C). For example, below -7ºC, CH2 twisting occurs primarily on the carbonyl sided chain: HOOC-(CH2)7- and extends into CH wagging op at C10. Above -7ºC CH2 rocking at C11 becomes markedly stronger, C=CH rocking ip at C10 and C12 intensifies, and C=CH wagging op at C9 and C13 becomes undifferentiated. Melting initiates at the diene structure, then progresses towards the ends of the molecule.