A contribution from Raman spectroscopy for investigations of trans and cis Isomers of Cholesteryl Esters in biological samples

The cis-trans geometry conversion of double bonds in lipids is an endogenous process that can be mediated by sulfur-centered free radicals [1,2]. Trans isomers of poly-unsaturated fatty acids can be assumed as marker of free radical stress and their recognition in biological samples is a crucial step that can be helped by synthesis and characterization of appropriate reference compounds [3]. Cholesteryl linoleate and arachidonate esters are a fraction of plasma lipids and are interesting targets for their connection with membrane phospholipid turnover and their roles in cardiovascular health. In this context Raman spectroscopy can give a useful contribution, since Raman analysis can be performed directly on the lipid extracts without any derivatization reaction, it is non-destructive, and can rapidly supply some important information. A Raman spectral library of different cis and trans isomers of cholesteryl esters was conveniently generated to be used as references for the examination of complex biological samples and facilitate the isomer recognition. Unsaturated cholesteryl esters having differences in the alkyl chain length and/or the double bonds number, were analysed and characteristic Raman band patterns were identify. In particular, the Raman C=C stretching region (1640-1680 cm-1) provides the most convenient spectral range for evaluating the total unsaturation degree of a sample, as well as information about the presence of trans isomers (Figure 1). In fact, both the ratiometric analysis and the curve fitting procedure in this spectral range gave rise to valuable data in agreement with the gas chromatographic results. In addition, the potential of Raman analysis for trans isomer detection in biological samples was tested in some plasma cholesteryl ester fractions, whose fatty acid composition was previously estimated by gas chromatography analysis. References [1] Chatgilialoglu, C., Ferreri, C., Melchiorre, M., Sansone, A., Torreggiani, A. Chem. Rev. 114 255 (2014) [2] Torreggiani A, Domènech J, Orihuela R, Ferreri C, Atrian S, Capdevila M, Chatgilialoglu C. Chem Eur J 15 6015 (2009) [3] Melchiorre, M.; Torreggiani, A.; Chatgilialoglu, C.; Ferreri, C. J. Am. Chem. Soc., 133, 15184 (2011