Research Spotlight: Localization of Cyclopropane Modifications in Bacterial Lipids via 213 nm Ultraviolet Photodissociation Mass Spectrometry

Posted on January 21, 2021

Mass Spectrometry

Structural analysis of complex lipids typically relies on tandem mass spectrometry (MS/MS) or collision-based activation methods such as collision-induced dissociation (CID) and higher-energy collisional dissociation (HCD). Previously, these methods had not been used to probe subtle features of glycerophospholipids such as sn-stereochemistry and unsaturation elements including cyclopropane modification. A simple cyclopropane modification may seem insignificant but has a large impact on biological activity. Cyclopropane fatty acids (CFAs) are generated by the addition of a methylene group across the double bonds of unsaturated fatty acids (UFAs) of bacterial phospholipids. This conversion from UFA to CFA results in retention of stereochemistry regardless of cis or trans stereochemistry in the UFA. This is extremely common in Escherichia coli (E. coli) and other Gram-positive and Gram-negative bacteria in response to harsh environmental conditions. Mycobacterium tuberculosis (M. tuberculosis) contains cyclopropyl fatty acids called mycolic acids (MAs). MAs found in mycobacterium have been linked to critical roles in their pathogenesis. Structural analysis of cyclopropyl lipids is extremely important due to the biological impact that these molecules have.

Many methods previously used for structural characterization of cyclopropyl lipids are unable to provide information on the location and type of the unsaturation elements contained in the lipids. UVPD-MS was used to elucidate some of the more subtle aspects of cyclopropyl lipids in this work. UVPD-MS was able to produce a diagnostic pair of fragment ions spaced 14 Da apart. This pair of fragment ions correspond to the dual cross-ring C-C cleavage of each cyclopropane ring. This method was able to generate this key diagnostic fragmentation in both positively and negatively charged lipids and across all lipid classes. In the case of MAs, UVPD was able to generate multiple pairs of diagnostic ion pairs. This enabled multiple cyclopropane rings to be localized within a single lipid. The UVPD-MS method employed by the Brodbelt research group enables the structural analysis and characterization of subtle lipid features that are known to play vital roles in bacterial pathogenesis.

Once again we would like to say thank you to Molly for submitting this fascinating research, and also for choosing to be a loyal Avanti customer when her research calls for the highest purity lipids!

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