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

Area of Science:

• Physical Chemistry
• Spectroscopy
• Materials Science

Background:

• Unsaturated fatty acids like oleic acid (OA) and linoleic acid (LA) are crucial in biological systems.
• Understanding their solid-state and melting behaviors is essential for various applications.
• Previous studies have lacked detailed vibrational analysis across a wide temperature range.

Purpose of the Study:

• To investigate the solid, liquid, and transition states of OA and LA using Raman spectroscopy.
• To analyze the vibrational modes and phase transitions of OA, LA, and a 3:1 LA:OA mixture.
• To characterize lipid premelting phenomena and identify flexible structures.

Main Methods:

• Continuous Gradient Temperature Raman Spectroscopy (GTRS) was employed over a temperature range of -100 to 50 °C.
• High-resolution (0.2 °C increments) spectral data and derivative analysis were used.
• Analysis focused on vibrational modes associated with fatty acid structures.

Main Results:

• OA exhibited significant spectral changes during solid-state transitions (γ to α near -4 °C) and melting (13 °C).
• LA showed distinct solid-state transitions at -50 °C and -33 °C, with major changes at melting (-7 °C).
• Melting in both OA and LA initiated at the diene structure and progressed towards the molecular ends.
• The LA:OA mixture displayed weaker or absent modes compared to individual lipids, indicating altered vibrational characteristics.
• Characteristic vibrational modes of flexible structures in OA and LA were identified starting at cryogenic temperatures.

Conclusions:

• The study provides a detailed vibrational analysis of OA and LA phase transitions.
• Data quantify lipid premelting and pinpoint flexible structural elements within these fatty acids.
• The findings offer insights into the molecular behavior of unsaturated lipids and their mixtures.