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

• Published in: Chemistry and physics of lipids 2016-10, Vol.200, p.1-10
• Main Authors: Broadhurst, C. Leigh, Schmidt, Walter F., Kim, Moon S., Nguyen, Julie K., Qin, Jianwei, Chao, Kuanglin, Bauchan, Gary L., Shelton, Daniel R.
• Format: Article
• Language: English
• Subjects: DHA; Docosahexaenoic acid; Docosahexaenoic Acids - chemistry; Eicosanoic Acids - chemistry; Gradient temperature Raman spectroscopy; Molecular Structure; n-6 Docosapentaenoic acid; n-6DPA; Raman spectroscopy; Spectrum Analysis, Raman; Temperature
• ISSN: 0009-3084; 1873-2941
• DOI: 10.1016/j.chemphyslip.2016.06.003

Three dimensional gradient temperature Raman spectroscopy contour plot for n-6 docosapentaenoic acid showing solid state, premelting (−33°C) and melting (−27°C) spectral changes. Melting initiates at C16C17, the aliphatic chain subsequent to the last double bond, with corresponding mode δ (CC16C)+δ (CC17C). [Display omitted] •GTRS applies the temperature gradients utilized in differential scanning calorimetry to Raman spectroscopy, identifying molecular rearrangements at phase transitions.•We apply GTRS to n-6DPA and DHA from −100 to 20°C.•20Mb three-dimensional data arrays with 0.2°C increments and first/second derivatives allow complete assignment of solid, liquid and transition state vibrational modes.•DPA has bending over almost the entire temperature range. In contrast, DHA contains major CH2 twisting with no noticeable CH2 bending, consistent with a flat helical structure with a small pitch. One of the great unanswered questions with respect to biological science in general is the absolute necessity of docosahexaenoic acid (DHA, 22:6n-3) in fast signal processing tissues. N-6 docosapentaenoic acid (n-6DPA, 22:5n-6), with just one less double bond, 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 n-6DPA and DHA from −100 to 20°C. 20Mb 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. N-6DPA and DHA show significant spectral changes with premelting (−33 and −60°C, respectively) and melting (−27 and −44°C, respectively). The CH2(HCCH)CH2 moieties are not identical in the second half of the DHA and DPA structures. DPA has bending (1450cm−1) over almost the entire temperature range. In contrast, DHA contains major CH2 twisting (1265cm−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 essential in determining whether the lipid chain is configured