Continuous gradient temperature Raman spectroscopy of n-6 DPA and DHA from -100 C to 20°C

Authors: BROADHURST, LEIGH - University Of Maryland; Schmidt, Walter; Kim, Moon; Nguyen, Julie; Qin, Jianwei - Tony Qin; Chao, Kuanglin - Kevin Chao; Bauchan, Gary; Shelton, Daniel

Submitted to: Chemistry and Physics of Lipids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2016
Publication Date: 6/18/2016
Publication URL: http://handle.nal.usda.gov/10113/63250
Citation: Broadhurst, L., Schmidt, W.F., Kim, M.S., Nguyen, J.K., Qin, J., Chao, K., Bauchan, G.R., Shelton, D.R. 2016. Continuous gradient temperature Raman spectroscopy of n-6 DPA and DHA from -100 C to 20°C. Chemistry and Physics of Lipids. 200:1-10.

Interpretive Summary: One of the greatest challenges for biological science is understanding the absolute necessity of the long chain polyunsaturated fatty acid docosahexaenoic acid (DHA; 22:6n-3) for constructing the fast signal processing tissues in the brain, eye and heart. A lipid of the same chain length with just one less diene group, docosapentaenoic acid (DPA; 22:5n-6), is fairly abundant in terrestrial food chains yet cannot substitute for DHA. For animal testing with extreme n-3 polyunsaturated fatty acid deprivation, n-6 DPA is incorporated into neural and retinal tissues, leading to developmental disorders, neurodegeneration, dementia, memory loss, subnormal retinal signaling and loss of visual acuity. We have developed the technique of continuous gradient temperature Raman spectroscopy (GTRS), which correlates vibrational frequency and intensity changes in solid or liquid samples with the temperature ranges where elastic parts of molecules absorb heat. GTRS provides the technique to identify molecular rearrangements that occur near phase transitions. DHA and n-6 DPA are indistinguishable with conventional Raman techniques, but very different when examined in a temperature gradient. Detailed analysis reveals the three-dimensional structure of the two lipids. The DHA molecule is a flat helical structure with small pitch. DPA follows this helical model for only half the molecule. Further modeling of neuronal membrane phospholipids must take into account a helical DHA, which would be configured parallel to the hydrophilic head group line, rather than perpendicular as has been the traditional model. This research benefits towards various medical and animal testing to help determine various treatments or cures for various health disorders.

Technical Abstract: One of the great unanswered questions with respect to biological science in general is the absolute necessity of DHA in fast signal processing tissues. N-6 DPA, with just one less diene, group, is fairly abundant in terrestrial food chains yet cannot substitute for DHA. 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 apply GTRS and DSC to DPA (22:5n-6) and DHA (22:6n-3) from -100 to 50°C. 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, including low intensity/frequency vibrations that cannot be readily analyzed with conventional Raman. DPA and DHA show significant spectral changes with premelting (-33 and -60°C, respectively) and melting (-27 and -44°C, respectively). The CH2-(HC=CH)-CH2 moieties are not identical in the second half of the DHA and DPA structures. n-6 DPA has bending (1450 cm-1 ) over almost the entire temperature range. In contrast, DHA contains major CH2 twisting (1265 cm-1) with no noticeable CH2 bending, consistent with a flat helical structure with a small pitch. Further modeling of neuronal membrane phospholipids must take into account torsion present in the DHA structure, which is essential in determining whether the lipid chain is configured more parallel or perpendicular to the hydrophilic head group.