Mass spectral study of derivatives of the four bilirubin-IX isomers.

The beta, gamma and delta isomers of bilirubin-IX show a decreased stability as compared with the IXalpha isomer; characteristic mass spectra are obtained only for the tetrakis-(trimethylsilyl) derivatives of the IXalpha and IXgamma isomers. Hydrogenation of the vinyl substituents increases the thermal stability of the bilirubins and gives rise to a characteristic mass spectrum for the tetrakis-(trimethylsilyl) derivative of meso-bilirubin-IXdelta. The ethyl anthranilate azopigments derived from the four bilirubins yield characteristic mass spectra, except for the two unstable divinyl substituted azodipyrroles (mol. wt. 416), derived from bilirubin-IXbeta and IXdelta. The corresponding mol. wt. 420 azopigments derived from the hydrogenated bilirubins are thermally stable. Elucidation of mass spectral fragmentation pathways is facilitated by the varying positions of the substituents and by deuterium labelling, and permits the assignment of structures to the various isomeric azodipyrroles.