Continuous Gradient Temperature Raman Spectroscopy of Oleic and Linoleic Acids from-100 to 50 °C

We analyzed the unsaturated fatty acids oleic (OA, 18:1n-9) and linoleic (LA, 18:2n-3), and a 3:1 LA:OA mixture from -100 to 50 degrees C with continuous gradient temperature Raman spectroscopy (GTRS). The 20 Mb three-dimensional data arrays with 0.2 degrees C increments and first/second derivatives allowed rapid, complete assignment of solid, liquid, and transition state vibrational modes. For OA, large spectral and line width changes occurred in the solid state. to a transition near -4 degrees C, and the melt (13 degrees C) over a range of only 1 degrees C. For LA, major intensity reductions from 200 to 1750 cm(-1) and some peak shifts marked one solid state phase transition at -50 degrees C. A second solid state transition (-33 degrees C) had minor spectral changes. Large spectral and line width changes occurred at the melt transition (-7 degrees C) over a narrow temperature range. For both molecules, melting initiates at the diene structure, then progresses towards the ends. In the 3:1 LA: OA mixture, some less intense and lower frequencies present in the individual lipids are weaker or absent. For example, modes assignable to C8 rocking, C9H-C10H wagging, C10H-C11H wagging, and CH3 rocking are present in OA but absent in LA:OA. Our data quantify the concept of lipid premelting and identify the flexible structures within OA and LA, which have characteristic vibrational modes beginning at cryogenic temperatures.