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

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) contain nearly identical structural moieties, 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 fatty acids are at least in part conformational. Binary 3:1 mixtures of DHA with either alpha-linolenic acid (18:3n-3; ALA) or gamma linolenic acid (18:3n-6; γ-LN) and both Nuclear Magnetic Resonance and Gradient Temperature Raman Spectroscopy were utilized to probe DHA in the liquid state. Raman modes at 1970, 1778 and 1578 cm−1 definitively demonstrate DHA, since absent in γ-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)- peaks 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, γ-LN predominantly disrupts peaks at the C′ half of DHA; ALA alters the conformation of the C″ half of DHA.