Electrospray ionization mass spectrometric studies on the characteristic fragmentation of Asp/cyclen conjugates

Abstract

Rationale: Differentiation and structural characterization of Asp/cyclen conjugates by electrospray ionization tandem mass spectrometry (ESI-MS(n)) are significantly important for their biomedical application. Hence, the present study is conducted.

Methods: The fragmentations of Asp/cyclen conjugates generated by positive ion mode electrospray ionization were examined here by low-energy collision-induced dissociation (CID). ESI-MS(n) spectra of cyclen were acquired to confirm cyclen contraction products derived from the studied compounds. The fragments derived from the Asp/cyclen conjugates were proved by deuterium-exchange experiments.

Results: Asp/cyclen conjugates displayed characteristic dissociation pathways, including cleavages of amide bonds, loss of NH3 and cyclen contraction pathways. It was observed that cleavages of C-terminal amide bonds generated b2 and b2 + H2O ions from the protonated CyclenAspAspAsp and a b1 + H2O ion from the protonated CyclenAspAsp. In addition, various cyclen contraction products were also observed.

Conclusions: In ESI-MS(n) spectra of studied compounds, fragments of bn-1 + H2O or cyclic anhydride were generated due to facile mobilization of C-terminal or side-chain COOH protons. In addition, the cyclen contraction products were detected. These results might provide sufficient information for the identification of Asp/cyclen conjugates by mass spectrometry.