Continuous gradient temperature Raman spectroscopy from -100 to 40°C yields new molecular models of arachidonic acid and 2-arachidonoyl-1-stearoyl-sn-glycero-3-phosphocholine

Authors: BROADHURST, CATHERINE - University Of Maryland; Schmidt, Walter; Nguyen, Julie; Qin, Jianwei - Tony Qin; Chao, Kuanglin - Kevin Chao; Kim, Moon

Submitted to: Prostaglandins Leukotrienes and Essential Fatty Acids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2017
Publication Date: 12/1/2017
Citation: Broadhurst, C.L., Schmidt, W.F., Nguyen, J.K., Qin, J., Chao, K., Kim, M.S. 2017. Continuous gradient temperature Raman spectroscopy from -100 to 40°C yields new molecular models of arachidonic acid and 2-arachidonoyl-1-stearoyl-sn-glycero-3-phosphocholine . Prostaglandins Leukotrienes and Essential Fatty Acids. 127: 6-15. https://doi.org/10.1016/j.plefa.2017.09.019.
DOI: https://doi.org/10.1016/j.plefa.2017.09.019

Interpretive Summary: Arachidonic acid (AA) is one of the most important biological molecules for all mammals. AA is a polyunsaturated fatty acid, meaning that the carbon chain has multiple double bonds. Brain function and reproduction are impossible without AA. Additionally, AA is converted to other biochemicals that cause and control inflammation. Despite its importance, a complete vibrational analysis of this molecule has never been reported. Continuous gradient temperature Raman spectroscopy (GTRS) has proven to be a simple, rapid technique for determining the structures of polyunsaturated fats. We applied GTRS to AA and a phospholipid containing AA that is common in brain tissue. A complete set of frequencies and intensities vs. temperature are reported, and novel 3D structures for both molecules are drawn. Basic research on AA is vitally important because chronic inflammation causes or contributes to diverse diseases ranging from arthritis to heart disease to obesity to dementia. This research benefits nutritional medicine for humans and live stock.

Technical Abstract: Despite its biochemical importance, a complete Raman analysis of arachidonic acid (AA, 20:4n-6) has never been reported. 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 utilize the GTRS technique for AA and 1-18:0, 2-20:4n-6 phosphatidyl choline (AAPC) from cryogenic to mammalian body temperatures. 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. Below -60°C AA shows strong evidence for a stable crystalline form; this feature is unique and was not observed in six other unsaturated lipids (DHA, n-3DPA, n-6DPA, LA, ALA, OA). Melting in AA occurs over a large range: (-60 to -35°C): very large frequency offsets and intensity changes correlate with premelting initiating circa -60°C, followed by melting (-37°C). Novel, unique 3D structures for both molecules reveal that AA is not symmetric as a free fatty acid, and that the free fatty acid changes significantly in the sn-2 phospholipid position. Further, different CH and CH2 sites are unequally elastic and nonequivalent.