Raman Spectroscopy And DFT Study Of Unsaturated Fatty Acid In Edible Oil

Unsaturated fatty acids in edible oils are important nutrients and play an important role in our daily lives..Unsaturated fatty acids are classified into monounsaturated fatty acids and polyunsaturated fatty acids.Unsaturated fatty acids have functions such as protecting the heart,lowering blood sugar and cholesterol,regulating blood lipids,and preventing memory decline.Among them,monounsaturated fatty acids have the effects of delaying cell aging,protecting skin and lowering plasma cholesterol.Polyunsaturated fatty acids have improved blood microcirculation and enhance the ability of thinking.In this paper,laser Raman spectroscopy combined with DFT calculation was used to study edible oil,and a method for distinguishing different unsaturated fatty acids in edible oil was found.Firstly,the difference in Raman spectra of seven different edible oils was studied.The relative intensity ratios of the Raman peaks at 1655cm^-1 and 1440cm^-1,1265cm^-1 and1300cm^-1(I₁₂₆₅/I₁₃₀₀,I₁₆₅₅/I₁₄₄₀)in the spectra of the seven samples were found to be related to the polyunsaturated fatty acid content.The Raman peak at 1655 cm^-1 is more closely related to the C=C stretching vibration than the peak at 1265 cm^-1,and the relative intensity ratio of 1655 cm^-1 and 1440 cm^-1(I₁₆₅₅/I₁₄₄₀)can better distinguish the fatty acid with different contents in edible oils.Then,the theoretical Raman spectroscopy and molecular structure?bond length,bond angle and dihedral angle?of oleic acid?OA?,linoleic acid?LA?and linolenic acid?ALA?were calculated and explained through DFT.Combined the theoretical spectrum of fatty acids with the experimental spectroscopy,the relationship between the C=C number of the molecules and the relative intensity ratio of the spectra at 1440cm^-1and 1660cm^-1(I₁₆₆₅/I₁₄₄₀)was discussed.It was found that the relative intensities of the peaks at 1660cm^-1 and 1440cm^-1 are varied with those three acids containing different number of C=C,which can distinguish those three different kinds of unsaturated fatty acids?oleic acid?OA?,linoleic acid?LA?and linolenic acid?ALA??.If rapid quantitative analysis of edible oil through this method is required,more systematic experiments and calculations are needed in the future.Finally,the theoretical Raman spectroscopy and molecular structure?bond length,bond angle and dihedral angle?of stearidonic acid?SDA?,eicosapentaenoic acid?EPA?and docosahexaenoic acid?DHA?were calculated by using DFT method.It is found that there is no obvious correlation between the ratio of the relative peak intensities of the peaks closely related to C=C stretching vibration in the theoretical Raman spectrum and the number of C=C in the molecule.If we want to study further on the relationship between Raman spectroscopy and three kinds of unsaturation,a combination of theory and experiment is needed for making a specific analysis.