Trans and positional ethylenic bonds in two dominant isomers of eicosapentaenoic acid from the freshwater sponge Baicalospongia bacillifera

Abstract

Reinvestigation of the current FA composition of the regional freshwater sponge Baicalospongia bacillifera showed that the main measured isomer of EPA (14% of the total FA now detected) is, in fact, an unusual 5Z,8Z,11Z,14Z,18E-EPA. Two other isomers of this acid also present were identified as a novel 5Z,8Z,11Z,15Z,18E-EPA and the common methylene-interrupted 5Z,8Z,11Z,14Z,17Z-EPA (usually written simply as EPA). Isolation of these acids as their methyl ester derivatives was accomplished with the use of a combination of silver-ion column chromatography and HPLC. The structure of the two new compounds was deduced from GC-MS and detailed NMR data. Partial hydrazine reduction of all three isolated EPA esters followed by separation of cis/trans isomers of the resulting monoenoic acids and GC-MS analysis of their dimethyl disulfide adducts were used for determination of the configuration and position of the double bonds. We may assume that the sponge B. bacillifera cannot receive these unusual EPA isomers directly from food sources (e.g., algal diatoms), and accordingly restructuring of ordinary EPA to novel acids may take place in the sponge tissue.

Preferential formation of mono‐dimethyl disulfide adducts for determining double bond positions of poly‐unsaturated fatty acids

“…Determination of double bond positions in highly unsaturated lipids like EPA is challenging. Such determination has previously been achieved through methods including the DMDS derivatization on corresponding mono‐unsaturated compounds after partial hydrogenation (Imbs & Rodkina, 2005; Yamamoto et al, 1991), 4,4‐dimethyloxazoline (DMOX) derivatization (Toral et al, 2018), Paternò‐Büchi Reaction (Murphy et al, 2017) and formation of acetonitrile covalent adduct (Alves et al, 2011). However, it is difficult to control the reaction products for the partial hydrogenation method.…”

“…One solution to this problem, as demonstrated by Yamamoto et al (1991), Imbs and Rodkina (2005), and Tanaka et al (1997), is to partially hydrogenate poly‐unsaturated fatty acid methyl esters (FAMEs) into mono‐unsaturated FAMEs with hydrazine, followed by silver nitrate silica gel isolation of mono‐unsaturated FAMEs and subsequent DMDS reaction. However, due to the difference in reduction rates for double bonds at different locations (e.g., terminal double bond is often the easiest to reduce) (Ratnayake et al, 1990), it is challenging to obtain sufficient quantity of partially hydrogenated mono‐unsaturated FAMEs for every double bond in a poly‐unsaturated lipid.…”

Elucidation of double‐bond positions of polyunsaturated alkenes through gas chromatography/mass spectrometry analysis of mono‐dimethyl disulfide derivatives

“…As one alternative solution, partial hydrogenation of samples into mono-unsaturated compounds has been applied before subsequent DMDS reaction. [9][10][11] This solution avoids the formation of cyclized or poly-DMDS adducts. However, the isolation and purification of mono-unsaturated compounds from a mixture of partially hydrogenated products require use of a chromatography column packed with silver nitrate silica gel, which is often cumbersome and can greatly increase the sample preparation time.…”

“…acids 7,[9][10][11] and alkenes. 5,12 Traditionally, the reaction between DMDS and unsaturated compounds was performed under heated conditions for an extended period of time (e.g., 40 C for 4 h or overnight) to ensure all double bonds are fully derivatized.…”