Unique and redundant spectral fingerprints of docosahexaenoic, alpha-linolenic and gamma-linolenic acids in binary mixtures

Authors: Schmidt, Walter; CHEN, FU - University Of Maryland; BROADHURST, CATHERINE - University Of Maryland; Qin, Jianwei - Tony Qin; CRAWFORD, MICHAEL - Imperial College; Kim, Moon

Submitted to: Journal of Molecular Liquids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2022
Publication Date: 4/26/2022
Citation: Schmidt, W.F., Chen, F., Broadhurst, C.L., Qin, J., Crawford, M.A., Kim, M.S. 2022. Unique and redundant spectral fingerprints of docosahexaenoic, alpha-linolenic and gamma-linolenic acids in binary mixtures. Journal of Molecular Liquids. 358:119222. https://doi.org/10.1016/j.molliq.2022.119222.
DOI: https://doi.org/10.1016/j.molliq.2022.119222

Interpretive Summary: One of the greatest unanswered questions in all biological science is why the polyunsaturated fatty acid docosahexaenoic acid (DHA; 22:6n-3) is absolutely necessary for constructing fast signal processing tissues in the brain, heart and retina and testes. We have combined gradient temperature Raman spectroscopy with state-of-the art nuclear magnetic resonance analysis of mixtures of DHA with the close analogy polyunsaturated fats alpha-linolenic acid (18:3n-3) and gamma-linolenic acid (18:3n-6) to probe more than one stable conformation of DHA. This results in three-dimensional structures that are accurate enough to explain the lipid biochemical differences, and to form the basis for new pharmaceuticals. Our work supports the concept that nearly all natural polyunsaturated fatty acids should be considered essential, not just two. This benefits ARS and the public because billions of dollars of medical, veterinary and scientific research funds are spent each year trying to understand and control chronic inflammation, systemic infections, diabetes and dementia. It also provides a theoretical argument supporting epidemiologically-based dietary recommendations for increasing all n-3 fats in the diet from a variety of foods.

Technical Abstract: The polyunsaturated fatty acids docosahexaenoic acid (DHA; 22:6n-3), docosapentaenoic acid (n-3DPA; 22:5n-3), and eicosapentaenoic acid (EPA; 20:5n-3) are nearly structural analogs but differ vastly in their biological activities and in their utilization in mammalian tissues. Most importantly, DHA cannot be substituted for in fast signal processing tissues such as neural retinal, cardiac and spermatogenic. Over 600 million years of evolution the difference of just a single double bond has not been overcome—DHA reigns supreme. These dramatic differences among polyunsaturated are at least in part conformational. Binary3:1 mixtures DHA with either alpha-linolenic acid (18:3n-3; ALA) or gamma linolenic acid (18:3n-6; -LN) and both Nuclear Magnetic Resonance and Gradient Temperature Raman Spectroscopy were utilized to probe DHA in the liquid state. Raman modes at 1970, 1770 and 1570 cm-1 definitively demonstrate DHA since absent in -LN and ALA. However, the latter two modes are also absent in 3:1 mixtures. This is evidence for a conformational change in DHA and of the capacity of structural analogs to disrupt the packing of DHA at the molecular level. NMR confirms chemical shifts in the middle of the DHA double bond region in the mixtures are not the same as the pure DHA chemical shifts. We propose asymmetry in torsion among =C-C-Ha , =C-C-Hb , Ha-C-C= and Hb-C-C= sites in each six carbon atom backbone moiety in =C”-C-C=C-C-C’= results in the observed non-uniformity of twisting observed spectroscopically. The distance C” C’ is minimum when the six atom moiety is fully planar, and maximum when twisting involving C” and C’ is in opposite directions from the double bond. For DHA -(H-C=C-H)-CH2- and -CH2-(H-C=C-H)- sites are conformationally identical only at C17=C16: C19” and C14’ both twist C” and C’ to the same side of C17=C16. At C14=C13 and C8=C7, torsion is similar to that at C16=C15 in ALA. Torsion related to C”-C’ distances stepwise from C2 to C22 results in planar structure more “O” shaped then “U” shaped. In 3+1 mixtures, -LN predominantly disrupts sites at the C’ half of DHA; ALA alters the conformation of the C” half of DHA.