Examination of structurally selective derivatization of vitamin D₃ analogues by electrospray mass spectrometry

The structural specificity of vitamin D derivatization by PTAD (4-phenyl-1,2,4-triazoline-3,5-dione) was probed using synthetic analogues and ion trap mass spectrometry. EB 1089, a vitamin D₃ analogue which contains a second site for Diels-Alder cycloaddition on its side-chain, allowed the examination of derivatization modes and comparisons of ion fragment structures. The origins of a PTAD-vitamin D₃ ion fragment, commonly used in metabolite characterization and quantitation of vitamin D₃ analogues (m/z 314), were established; ion trap mass spectrometry revealed that the PTAD comprises a portion of this diagnostic fragment, and is not lost by a retro-Diels-Alder step. Furthermore, the unique structure of the EB 1089 side-chain also permits facile determination of its side-chain metabolism. Use of PTAD derivatization and detection of metabolite-specific ion fragments identify hydroxylation at the end of the EB 1089 sidechain. It is believed that the results from these studies provide a clearer understanding of the mass spectrometry of triazolinedione derivatives, not only in the specific case of EB 1089, but also in their application to other vitamin D compounds.