Gradient Temperature Raman Spectroscopy of Fatty Acids with One to Six Double Bonds Identifies Specific Carbons and Provides Systematic Three Dimensional Structures

Specialized pro-resolving mediators provide promising targets for new drugs and natural products. Much work has been accomplished on the structure/ function of the lipoxygenase and cyclooxygenase enzymes but not on the substrates. A better visualization of three-dimensional lipid structures will allow increased refinement of the interactions that produce the pro-resolving mediators, and lead to improvements in synthetic pathways. We present systematic analysis of oleic (18:1n-9), linoleic (18:2n-6), alpha-linolenic (18:3n-3), arachidonic (20:4n-6), docosapentaenoic (22:5n-3), and docosahexaenoic (22:6n-3) acids. Continuous gradient temperature Raman spectroscopy (GTRS) applies the temperature gradients utilized in differential scanning calorimetry to Raman spectroscopy. GTRS can identify and differentiate specific carbon chain sites, finally allowing Raman analysis to explain why the long-chain polyunsaturated fatty acids (LC-PUFA) exhibit such extreme functional differences despite minimal changes in chemical structure. Detailed vibrational analysis of the important frequency ranges 1450 - 1200 cm-1 (includes CH2 bending and twisting) and 1750 - 1425 cm-1 (includes C=C stretching and C-C stretching plus H-C in-plane rocking) shows for the first time that each molecule has its own characteristic set of modes with only some redundancy/commonality. The number and frequency of modes correlates with three-dimensional molecular structure, not the degree of unsaturation. The high degree of specificity of lipoxygenase and cyclooxygenase enzymes should be reconsidered in light of the fact that individual sites on the polyunsaturated fatty acid chain are nonequivalent, and each LC-PUFA molecule has an individual, specific three dimensional structure incorporating torsion.

Inspection of maleic anhydride in starch powder using line-scan hyperspectral Raman chemical imaging technique

Excessive use of maleic anhydride(MAN)in starch production is potentially harmful for consumers’health.This study presents a macro-scale Raman chemical imaging method for detection and quantification of MAN particles mixed in starch powder.MAN was mixed into corn starch at eight concentration levels from 50 ppm to 6400 ppm(w/w).Each mixture was put in a sample holder with a 150 mm×100 mm area and a 2 mm depth to create a large surface and a thin layer of the powdery sample for inspection.A 30 W 785 nm line laser was projected on the sample surface,from which hyperspectral images were obtained by a line-scan Raman imaging system with a spatial resolution of 0.2 mm.Fluorescence signals generated by laser-sample interactions were eliminated by a mathematical baseline correction method.A unique Raman peak was selected at 1839 cm-1 for the MAN detection,at which single-band fluorescence-corrected images were extracted from the mixture of each concentration and used to generate chemical images for MAN detection and mapping.The MAN detection limit was estimated at 100 ppm based on the Raman imaging measurement results.Pixel concentrations of the MAN in the chemical images were found linearly correlated with mass concentrations of the MAN particles in the starch powder,suggesting the Raman chemical imaging method has the potential for quantitative detection of the MAN in the starch-MAN mixtures.