Preparation of Dimethyl Disulfide Adducts from the Mono‐Trans Octadecadienoic Acid Methyl Esters

Abstract

The dimethyl disulfide (DMDS) adduct method is one of the more effective methods for determining double bond positions of dienoic acid. The DMDS method can be simply used to obtain the characteristic ions in which cleavage occurs between the methylthio group-added double-bond carbons as can be seen in the mass spectrum obtained using gas chromatography/electron ionization-mass spectrometry. In the case of the methylene-interrupted di-cis type and di-trans type dienoic acid, the DMDS addition reaction only occurs at one double-bond position, and cannot occur at the remaining double-bond position due to steric hindrance. As a result, two types of adducts are produced in the addition reaction. However, in the case of the methylene-interrupted mono-trans (mono-cis) type dienoic acid, the DMDS addition reaction only occurs at the cis-double bond. As a result, one type of adduct is produced in the addition reaction. In this report, we investigate the cause of the reaction selectivity by focusing on the addition reaction time.

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

“…We have recently demonstrated that, by adjusting reagent amounts and performing the DMDS adduction at low temperatures, sufficient amounts of mono-DMDS alkenone (poly-unsaturated long-chain ketone containing trans double bonds) adducts can be obtained for determining their double bond positions (Dillon et al, 2016;Liao et al, 2021;Richter et al, 2017;Zhao et al, 2014), including those possessing five double bonds at Δ 4 , Δ 7 , Δ 14 , Δ 21 , Δ 28 (Liao et al, 2021). Importantly, our research on alkenones demonstrates that it is not necessary to form mono-DMDS adducts of unsaturated compounds exclusively for the purpose of determining the double bond positions, as appeared to be implied from the studies of Shibamoto et al (2016Shibamoto et al ( , 2018. Even if di-or higher DMDS adducts are also formed, modern gas chromatograph-mass spectrometer can easily separate the mono-DMDS adducts from poly-DMDS adducts, allowing for convenient determination of double bond positions.…”

“…Another solution, as a more convenient one, is to obtain mono-DMDS adducts for poly-unsaturated lipids. One special circumstance, as demonstrated by Shibamoto et al (2016Shibamoto et al ( , 2018, is that mono-DMDS adducts are formed preferentially (and possibly exclusively) for 9,12-C 18:2 fatty acids (double bonds at Δ 9 , Δ 12 can be either cis or trans). In both c9, c12-C 18:2 and t9, t12-C 18:2 fatty acids, DMDS is only (or mostly) added onto one, rather than both, double bond (Shibamoto et al, 2016(Shibamoto et al, , 2018.…”

“…One special circumstance, as demonstrated by Shibamoto et al (2016Shibamoto et al ( , 2018, is that mono-DMDS adducts are formed preferentially (and possibly exclusively) for 9,12-C 18:2 fatty acids (double bonds at Δ 9 , Δ 12 can be either cis or trans). In both c9, c12-C 18:2 and t9, t12-C 18:2 fatty acids, DMDS is only (or mostly) added onto one, rather than both, double bond (Shibamoto et al, 2016(Shibamoto et al, , 2018. Based on these observations, Shibamoto et al (2016Shibamoto et al ( , 2018 propose that the preferential formation of mono-DMDS adducts for these fatty acids with single methylene-interrupted double bonds is primarily due to steric hindrance (Shibamoto et al, 2016(Shibamoto et al, , 2018.…”

“…In both c9, c12-C 18:2 and t9, t12-C 18:2 fatty acids, DMDS is only (or mostly) added onto one, rather than both, double bond (Shibamoto et al, 2016(Shibamoto et al, , 2018. Based on these observations, Shibamoto et al (2016Shibamoto et al ( , 2018 propose that the preferential formation of mono-DMDS adducts for these fatty acids with single methylene-interrupted double bonds is primarily due to steric hindrance (Shibamoto et al, 2016(Shibamoto et al, , 2018. In other words, when DMDS is added onto one of the two double bonds on 9,12-C 18:2 , adding DMDS onto the remaining double bond is prohibited due to the proximity of the remaining double bond to the bulky DMDS group already added onto the molecule.…”

“…In other words, when DMDS is added onto one of the two double bonds on 9,12-C 18:2 , adding DMDS onto the remaining double bond is prohibited due to the proximity of the remaining double bond to the bulky DMDS group already added onto the molecule. Additionally, cis double bond reacts preferentially with DMDS, thus in c9, t12-C 18:2 or t9, c12-C 18:2 , only mono-DMDS adducts on c9 or c12 double bond are formed (Shibamoto et al, 2016(Shibamoto et al, , 2018. If the idea of steric hindrance is true, it is possible that similar reaction conditions can also result in the formation of mono-DMDS adducts for other di-unsaturated lipids with one methylene-interrupted double bonds.…”