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

Abstract

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 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) 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.

A Simple Surface-Enhanced Raman Spectroscopic Method for on-Site Screening of Tetracycline Residue in Whole Milk

“…Comparing Figure 2 a, milk-tetracycline solution has an overlap of 1285 cm − 1 with milk at 1256 cm − 1 to form a broader 1260 cm − 1 peak. Additionally, 1265 cm − 1 is a lipid peak, corresponding to CH 2 twisting in the double bond region of linoleic acid [ 58 ]. Thus, tetracycline may also be binding with any polyunsaturated fats present in milk fat.…”

Using torsional forces to explain the gradient temperature Raman spectra of endosulfan isomers and its irreversible isomerization

“…Vibrational modes resulting from dihedral (torsion) angles that are not 0° or 180° always result in localized sites of torsion, curvature, and torque. Using oleic acid as an example, the wavenumber of C-C asymmetrical stretching (1057 cm -1 ) is different than C-C symmetrical stretching (1093 cm -1 ) [26,27]. C-C asymmetrical stretching requires at least a three atom model C1-C2-C3, where C1-C2 stretching is not equal to C2-C3 stretching.…”

“…Curvature, torsion, and torque are analogous to distance, velocity, and acceleration on the linear scale. Inertia is the straight line a car will travel going into a curve, whereas torque is the force that enables a car to travel at a constant speed from the beginning to the end of a uniform curve [22][23][24].…”

Continuous Gradient Temperature Raman Spectroscopy of Fish Oils Provides Detailed Vibrational Analysis and Rapid, Nondestructive Graphical Product Authentication

“…To the first order, all the long-chain PUFAs appear nearly identical, with the vibrational modes of most moieties degenerate, redundant or very broad [12,13,14,15,16]. We have developed continuous gradient temperature Raman spectroscopy (GTRS), which is a simple, rapid technique for determining the unique structures of the unsaturated fatty acids [17,18,19,20,21]. 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.…”