Continuous gradient temperature Raman spectroscopy of N-6DPA and DHA from -100 to 20°C.
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 CH2twisting (1265cm-1) with no noticeable CH2bending, 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 more parallel or perpendicular to the hydrophilic head group.
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705, United States; Department of Nutrition and Food Science, University of Maryland, College Park, MD 20742, United States. Electronic address: leigh.broadhurst@ars.usda.gov.
C Leigh
CL
Broadhurst
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705, United States.
Walter F
WF
Schmidt
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705, United States.
Moon S
MS
Kim
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705, United States.
Julie K
JK
Nguyen
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705, United States.
Jianwei
J
Qin
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705, United States.
Kuanglin
K
Chao
Electron and Confocal Microscopy Unit, Plant Sciences Institute, USDA ARS, Beltsville, United States.
Gary L
GL
Bauchan
Environmental Microbiology and Food Safety Laboratory, U.S. Department of Agriculture Agricultural Research Service, 10300 Baltimore Avenue, Beltsville, MD 20705, United States.
Daniel R
DR
Shelton
D004281:Docosahexaenoic Acids / Q000737:chemistry*; D004537:Eicosanoic Acids / Q000737:chemistry*; D015394:Molecular Structure; D013059:Spectrum Analysis, Raman; D013696:Temperature
D004537:Eicosanoic Acids; C015876:eicosa-11,14-dienoic acid; D004281:Docosahexaenoic Acids
DHA; Docosahexaenoic acid; Gradient temperature Raman spectroscopy; Raman spectroscopy; n-6 Docosapentaenoic acid; n-6DPA