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Title: Continuous gradient temperature Raman spectroscopy of the long chain polyunsaturated fatty acids Docosapentaenoic (DPA, 22:5n-6) and Docosahexaenoic (DHA; 22:6n-3) from -100 to 20° C

Author
item BROADHURST, C - University Of Maryland
item Schmidt, Walter
item Kim, Moon
item Nguyen, Julie
item Qin, Jianwei - Tony Qin
item Chao, Kuanglin - Kevin Chao
item Bauchan, Gary
item Shelton, Daniel

Submitted to: Proceedings of SPIE
Publication Type: Proceedings
Publication Acceptance Date: 3/28/2016
Publication Date: 5/17/2016
Citation: Broadhurst, C.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 the long chain polyunsaturated fatty acids Docosapentaenoic (DPA, 22:5n-6) and Docosahexaenoic (DHA; 22:6n-3) from -100 to 20° C. Proceedings of the SPIE 9864, Sensing for Agriculture and Food Quality and Safety VIII, 98640E.

Interpretive Summary: ARS researchers have developed continuous gradient temperature Raman spectroscopy (GTRS), which correlates vibrational frequency and intensity changes in solid/liquid samples with the temperature ranges where elastic parts of molecules absorb heat. GTRS is the first straightforward technique to identify molecular rearrangements occurring near phase transitions. Using GTRS will provide additional basic analytical data to improve understanding of subtle structural differences and functions arising from those differences, which will greatly benefit researchers across the life science fields. This study demonstrates use of GTRS, applied to analysis of docosahexaenoic acid (DHA; 22:6n-3) and docosapentaenoic acid (DPA; 22:5n-6). Understanding the absolute necessity of DHA for constructing the fast signal processing tissues in the mammalian brain, eye, and heart has long been a great challenge. DPA, equal in length but with one less diene group, is abundant in food chains yet cannot substitute for DHA, as illustrated (from extreme deprivation of DHA in test animals) by DPA incorporation into neural and retinal tissues leading to developmental disorders, neurodegeneration, dementia, memory loss, subnormal retinal signaling and loss of visual acuity. DHA and n-6 DPA are indistinguishable with conventional Raman techniques, but very different when examined by GTRS. Detailed analysis revealed three-dimensional structures: DHA is a flat helical molecule with small pitch, while only half the DPA molecule follows this helical model. 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. Using this newly developed technique to improve understanding of subtle structural differences and related the functions arising from those differences will greatly benefit researchers across the life science fields.

Technical Abstract: The structural, cognitive and visual development of the human brain and retina strictly require long-chain polyunsaturated fatty acids (LC-PUFA). Excluding water, the mammalian brain is about 60% lipid. One of the great unanswered questions with respect to biological science in general is the absolute necessity of the LC-PUFA docosahexaenoic acid (DHA; 22:6n-3) in these fast signal processing tissues. 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. Gradient Temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry to Raman spectroscopy, providing a straightforward technique to identify molecular rearrangements that occur near and at phase transitions. Herein we apply GTRS to DPA, and DHA from -100 to 20°C. 20 Mb three-dimensional data arrays with 1°C increments and first/second derivatives allows 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. The DHA molecule contains major CH2 twisting (1265 cm-1) with no noticeable CH2 bending, consistent with a flat helical structure with small pitch. Further modeling of neuronal membrane phospholipids must take into account this structure for DHA, which would be configured parallel to the hydrophilic head group line.