Determination of Phospholipid Regiochemistry by Ag(I) Adduction and Tandem Mass Spectrometry

Collision-activated dissociation (CAD) and infrared multiphoton dissociation (IRMPD) of Ag-adducted phospholipids were investigated as structural tools. Previously, determination of the acyl chains at the two phospholipid esterification sites has been performed based on the R1COO-/R2COO- ratio in negative ion mode CAD tandem mass spectrometry. However, the observed product ion ratio is dependent on the extent of unsaturation of the fatty acyl group at sn-2 as well as on the total chain length. Similarly, in positive ion mode CAD with/without alkaline or alkaline earth metal adduction, the ratio of product ions resulting from either R1COOH or R2COOH neutral losses is dependent on the nature of the phospholipid polar headgroup. Ag+ ion chromatography, in which silver ions are part of the stationary phase, can provide information on double bond number/distribution as well as double bond configuration (cis/trans) because of interaction between Ag+ ions and olefinic electrons of fatty acids and lipids. We hypothesized that interactions between double bonds and Ag+ may be utilized to also reveal phospholipid esterification site information in tandem mass spectrometry. CAD and IRMPD of Ag-adducted phospholipids with unsaturated fatty acids (RxCOOH, x = 1 or 2) provided characteristic product ions, [RxCOOH + Ag](+), and their neutral losses. The characteristic product ions and their abundances do not depend on the type of polar headgroup or the number of double bonds of unsaturated acyl chains. Tandem mass spectrometry of Cu-adducted phospholipids was also performed for comparison based on the Lewis acid and base properties of Cu+ and phospholipid double bonds, respectively.