Fig. 4.

Tandem mass analysis of uroporphomethene I. The 835-Da base peak at 30 min was further analyzed by tandem mass spectroscopy (SI Methods). Product ions with masses of 791, 747, 703, and 659 Da represent the loss of one or more carboxyl groups from the octacarboxylic uroporphomethene. The smaller product ions represent additional breakdown products of the tetrapyrrole macrocycle that are consistent with tri- and dipyrroles. Neutral loss of 197 or 209 Da from m/z 791 via elimination of one pyrrole ring by cleavage of bonds at one of two positions yields m/z 594 and 582 (tripyrroles) with additional breakdown via loss of CO₂. Neutral loss of 195 or 209 from m/z 582 yields m/z 387 and 373, which also decompose by loss of CO₂. Products of heterolytic cleavage of m/z 835 are also detectable at m/z 419, 431, and 448/387.