Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/19/2004
Publication Date: N/A
Citation: Interpretive Summary: Nuclear magnetic resonance spectroscopy techniques are ideal for identifying chemical structures of compounds in lipids when the samples are diluted with a chemical solvent. Highly polyunsaturated lipids, such as those in sardine oil and cod muscle, are both more liquid and less dense. More liquid, less dense lipids are essential for deep sea fish because more esaturated fat would solidify under deep sea temperatures, decreasing its ability to move, making it an easier catch for the more mobile fish. The results of this study demonstrate that in diluting these polyunsaturated lipids, information about which parts of the natural lipid align and are close to each other is lost. Previous research showed that the polyunsaturated fatty acids in the lipids are U shaped. This study provides experimental evidence that when one of the ends each of two U shaped molecules is attached to a glycerol backbone, the two other ends of the U are close in space to each other. Thus, the two U shaped molecules are parallel, not anti-parallel and not perpendicular to each other. Unambiguously, it is the alignment and order among the polyunsaturated fatty acids, not the alignment and order of the glycerol backbone connector that defines the order/packing/density within the lipids.
Technical Abstract: 13C NMR spectra of natural, unenriched docosahexaenoic acid-rich sardine oil and cod muscle glycerophosphotidylcholine were obtained. Undiluted lipid and 50 vol% CDCL3 diluted samples were run and processed under identical conditions. The high viscosity of the undiluted samples did not affect data quality. We observed significant signal intensity differences across the 1D spectrum between undiluted and diluted samples. These differences identify molecular sites involved in conformational changes induced by solvent alterations to the lipid chemical environment. In undiluted samples only, we observed 13C-13C 2D NOESY crosspeaks for CH, CH2, CH3, and C=C structures over an optimal range of 2D mixing times. No C=O or glycerol crosspeaks were observed. Cross peaks from terminal methyl groups must be derived from interchain interactions. Also under identical experimental conditions, methyl group relative peak intensities were four times greater in undiluted versus diluted lipid samples. Results indicate that in undiluted natural lipids NMR signal intensity is influenced by polarization transfer from the extended lipid structure. Solvents significantly change the neighboring environment of molecules composing lipid structures, thus NOE information on lipid packing and conformation is lost. 13C-13C cross peaks in undiluted samples are likely due to perturbation of the proton-carbon NOE from viscosity effects. This polarization transfer enables NMR experiments to discern sites of relative order within an extended lipid structure.