|dc.description.abstract||Curcumin analogues are novel antineoplastic agents designed by structural modifications of the natural product curcumin to enhance its therapeutic effects. Various curcumin analogues displayed a significant cytotoxic effect towards different cancer cell lines including leukemia, melanoma, and colon cancer. In order to evaluate the safety, efficiency and metabolism of the new anticancer candidates, sensitive and high throughput analytical methods are needed. Thirteen curcumin analogues with the backbone structure of 3,5-bis(benzylidene)-4-piperidone were tested. The ionization behavior of curcumin analogues was investigated to reveal the possible mechanisms for the unusual formation of the positively charged [M-H]+ ions during single stage positive ion mode MALDI-MS analysis. Different ionization techniques (i.e., ESI, APCI, APPI, and MALDI) were used to evaluate this phenomenon. The results showed that curcumin analogues ionize into [M-H]+ along with the expected [M+H]+ species during MALDI and dopant free APPI-MS. In contrast, ESI, APCI and the dopant mediated APPI showed only the expected [M+H]+ peak. Our experiments revealed that photon energy triggers the ionization of the curcumin analogues even in the absence of any ionization enhancer such as matrix, solvent or dopant. Three proposed mechanisms for the formation of [M-H]+ were evaluated, two of them are probably involved in the [M-H]+ formation: (i) hydrogen transfer from the analyte radical cation and (ii) hydride abstraction.
In addition to the ionization behavior, the collision induced dissociation-tandem mass spectrometric (CID-MS/MS) fragmentation behavior of curcumin analogues was evaluated showing similar dissociation pathways that centered on the piperidone ring of the 3,5-bis(benzylidene)-4-piperidone moiety. The presence of different substitutes on that moiety resulted in specific product ions for each curcumin analogue. The fragmentation patterns were established to confirm the chemical structure of the tested compounds and identify the diagnostic product ions of each compound. Twelve common product ions were identified resulting from the breakage of various bonds within the piperidone moiety. There was a tendency for the formation of highly conjugated product ions that are stabilized via resonance. Common product ions were identified allowing for the establishment of a general MS/MS behavior for any curcumin analogue that belongs to the 3,5-bis(benzylidene)-4-piperidone structural family. The fragmentation routes and the genesis of the product ions were confirmed via MS3 and neutral loss analysis.
In summary, the ionization of curcumin analogues provided insights into the formation of unique [M-H]+ ions which were linked to photo ionization of such compounds without the need for additives, such as matrix, dopant or solvent. As such, curcumin analogues should be evaluated as MALDI matrices in the future. The CID-MS/MS analysis of curcumin analogues revealed a common fragmentation behavior of the tested compounds. It will be applied, in the future to determine metabolic by-products of the tested compounds as well as to develop targeted liquid chromatography (LC)-MS/MS methods.||en_US