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 °C with continuous gradient temperature Raman spectroscopy (GTRS). The 20 Mb three-dimensional data arrays with 0.2 °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 γ to α transition near -4 °C, and the melt (13 °C) over a range of only 1 °C. For LA, major intensity reductions from 200 to 1750 cm-1and 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) 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.
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, Bldg. 161 Rm. 116, 10300 Baltimore Avenue, Beltsville, MD, 20705, USA. leigh.broadhurst@ars.usda.gov.|Department of Nutrition and Food Science, University of Maryland, College Park, MD, 20742, USA. leigh.broadhurst@ars.usda.gov.
C Leigh
CL
Broadhurst
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, Bldg. 161 Rm. 116, 10300 Baltimore Avenue, Beltsville, MD, 20705, USA.
Walter F
WF
Schmidt
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, Bldg. 161 Rm. 116, 10300 Baltimore Avenue, Beltsville, MD, 20705, USA.
Moon S
MS
Kim
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, Bldg. 161 Rm. 116, 10300 Baltimore Avenue, Beltsville, MD, 20705, USA.
Julie K
JK
Nguyen
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, Bldg. 161 Rm. 116, 10300 Baltimore Avenue, Beltsville, MD, 20705, USA.
Jianwei
J
Qin
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, Bldg. 161 Rm. 116, 10300 Baltimore Avenue, Beltsville, MD, 20705, USA.
Kuanglin
K
Chao
Electron and Confocal Microscopy Unit, Plant Sciences Institute, USDA, ARS, Beltsville, MD, 20705, USA.
Gary L
GL
Bauchan
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, Bldg. 161 Rm. 116, 10300 Baltimore Avenue, Beltsville, MD, 20705, USA.
Daniel R
DR
Shelton
D008041:Linoleic Acids / Q000737:chemistry*; D019301:Oleic Acid / Q000737:chemistry*; D044367:Phase Transition; D013059:Spectrum Analysis, Raman / Q000379:methods*; D013696:Temperature
D008041:Linoleic Acids; D019301:Oleic Acid
Gradient temperature Raman spectroscopy; Linoleic acid; Oleic acid; Polyunsaturated fatty acids; Raman spectroscopy
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