Mass spectrometry imaging (MSI) enables the spatial distributions of molecules possessing different mass‐to‐charge ratios to be mapped within complex environments revealing regional changes at the molecular level. Even at high mass resolving power, however, these images often reflect the summed distribution of multiple isomeric molecules, each potentially possessing a unique distribution coinciding with distinct biological function(s) and metabolic origin. Herein, this chemical ambiguity is addressed through an innovative combination of ozone‐induced dissociation reactions with MSI, enabling the differential imaging of isomeric lipid molecules directly from biological tissues. For the first time, we demonstrate both double bond‐ and sn‐positional isomeric lipids exhibit distinct spatial locations within tissue. This MSI approach enables researchers to unravel local lipid molecular complexity based on both exact elemental composition and isomeric structure directly from tissues.

Canonical fatty acid metabolism describes specific enzyme-substrate interactions that result in products with well-defined chain lengths, degree(s), and positions of unsaturation. Deep profiling of lipids across a range of prostate cancer cell lines reveals a variety of fatty acids with unusual site(s) of unsaturation that are not described by canonical pathways. The structure and abundance of these unusual lipids correlate with changes in desaturase expression and are strong indicators of cellular phenotype. Gene silencing and stable isotope tracing demonstrate that direct Δ6 and Δ8 desaturation of 14:0 (myristic), 16:0 (palmitic), and 18:0 (stearic) acids by FADS2 generate new families of unsaturated fatty acids (including n-8, n-10, and n-12) that have rarely-if ever-been reported in human-derived cells. Isomer-resolved lipidomics reveals the selective incorporation of these unusual fatty acids into complex structural lipids and identifies their presence in cancer tissues, indicating functional roles in membrane structure and signaling.

Potenital pathways for the deactivation of hindered amine light stabilisers (HALS) have been investigated by observing reactions of model compounds—based on 4-substituted derivatives of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)—with hydroxyl radicals. In these reactions, dilute aqueous suspensions of photocatalytic nanoparticulate titanium dioxide were irradiated with UV light in the presence of water-soluble TEMPO derivatives. Electron spin resonance (ESR) and electrospray ionisation mass-spectrometry (ESI-MS) data were acquired to provide complementary structural elucidation of the odd- and even-electron products of these reactions and both techniques show evidence for the formation of 4-oxo-TEMPO (TEMPONE). TEMPONE formation from the 4-substituted TEMPO compounds is proposed to be initiated by hydrogen abstraction at the 4-position by hydroxyl radical. High-level ab initio calculations reveal a thermodynamic preference for abstraction of this hydrogen but computed activation barriers indicate that, although viable, it is less favoured than hydrogen abstraction from elsewhere on the TEMPO scaffold. If a radical is formed at the 4-position however, calculations elucidate two reaction pathways leading to TEMPONE following combination with either a second hydroxyl radical or dioxygen. An alternate mechanism for conversion of TEMPOL to TEMPONEvia an alkoxyl radical intermediate is also considered and found to be competitive with the other pathways. ESI-MS analysis also shows an increased abundance of analogous 4-substituted piperidines during the course of irradiation, suggesting competitive modification at the 1-position to produce a secondary amine. This modification is confirmed by characteristic fragmentation patterns of the ionised piperidines obtained by tandem mass spectrometry. The conclusions describe how reaction at the 4-position could be responsible for the gradual depletion of HALS in pigmented surface coatings and secondly, that modification at nitrogen to form the corresponding secondary amine species may play a greater role in the stabilisation mechanisms of HALS than previously considered.