Selective phosphatidylcholine double bond fragmentation and localisation using Paternò-Büchi reactions and ultraviolet photodissociation.

The effect of double bond functionalisation for selective double bond localisation by ultraviolet photodissociation of phosphatidylcholines is investigated. Paternò-Büchi reactions in nanoESI emitter tips enable attachment of acetophenone to double bonds of unsaturated phosphatidylcholines after 100 s of 254 nm light irradiation with about 50-80% reaction yield. Functionalized phosphatidylcholines dissociate upon 266 nm irradiation yielding double bond selective fragment ions in contrast to results for ultraviolet photodissociation of unmodified lipids. Ultraviolet photodissociation of Paternò-Büchi modified lipids results in a selectivity increase of up to 2.2 towards double bond localisation compared collision-induced dissociation experiments. Double bond localisation is also possible with ultraviolet photodissociation when alkali metal ion attachment to Paternò-Büchi modified phosphatidylcholines occurs in contrast to classic collision-induced dissociation experiments. The developed methodology is used to differentiate lipid double bond isomers and applied to phosphatidylcholines from egg yolk to identify 15 phosphatidylcholines. Results from this study demonstrate that locally depositing energy in close vicinity to cleavable bonds via ultraviolet photodissociation can result in increased dissociation selectivity. This method can help to disentangle contributions from different structural elements in complex tandem mass spectra of lipids and aid to the structural characterization of phospholipids in a "top-down" approach.