Evaluation of ion activation strategies and mechanisms for the gas-phase fragmentation of sulfoquinovosyldiacylglycerol lipids from Rhodobacter sphaeroides

Sulfoquinovosyldiacylglycerol (SQDG) lipids, found in plants and photosynthetic bacteria, can substitute for phospholipids under phosphate limiting conditions. Here, various low-energy ion activation strategies have been evaluated for the identification and characterization of deprotonated SQDG lipids from a crude membrane lipid extract of Rhodobacter sphaeroides, using collision-induced dissociation-tandem mass spectrometry (CID-MS/MS) in either a triple quadrupole mass spectrometer or in a hybrid quadrupole ion trap-multipole mass spectrometer coupled with high resolution/accurate mass analysis capabilities. In the triple quadrupole instrument, using energy resolved CID-MS/MS experiments, the SQDG head group specific product ion at m/z 225 (C6H9O7S−), rather than m/z 81 (SO3H−), was determined to provide the greatest sensitivity for SQDG lipid detection, and is therefore the preferred ‘fingerprint’ ion for the identification of this lipid class from within complex lipid mixtures when using precursor ion scan mode MS/MS experiments. A comparison of conventional ion trap CID-MS/MS and -MSn, with ‘low Q’ CID-MS/MS, pulsed Q dissociation (PQD)-MS/MS and higher energy collision induced dissociation (HCD)-MS/MS performed in an LTQ Orbitrap Velos mass spectrometer, revealed that HCD-MS/MS coupled with high resolution/accurate mass analysis represents the most sensitive, and perhaps most importantly the most specific strategy, for ion trap based identification and characterization of SQDG lipids, due to the ability to readily distinguish the SQDG head group specific product ion at m/z 225.0069 from other products that may be present at the same nominal m/z value. Finally, the mechanisms responsible for formation of each of the major product ions observed by low-energy CID-MS/MS of deprotonated SQDG lipids were elucidated using uniform H/D exchange, HCD-MS/MS and high resolution mass analysis. Formation of the m/z 225 ‘fingerprint’ ion occurs via a charge-remote cis-elimination reaction, likely involving transfer of a hydrogen from the hydroxyl group located on the C2 position of the sugar ring.

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► The SQDG m/z 225 product ion provides the greatest sensitivity for lipid detection in triple quad. mass spectrometers.
► High res./accur. mass HCD-MS/MS is the most sensitive/specific strategy for SQDG lipid identification and characterization.
► High res./accur. mass MS/MS and H/D exchange facilitates elucidation of mechanisms for deprotonated lipid ion fragmentation.