FIG. 4.

ESI-MS/MS spectra of Ga-chelated siderophores from aerobically grown P. stutzeri and reference substances. The fragmentation pattern of the cyclic ferrioxamine Ga-desferrioxamine E (a) differs from those of the linear ferrioxamines Ga-desferrioxamine B (d) and Ga-desferrioxamine G (c). The unknown siderophore (b) fragments in the same way as ferrioxamine E and is thus supposed to be cyclic. Fragmentation of the proton adduct was performed at a collision energy of 40 eV. Black lines indicate the ⁶⁹Ga isotope, while gray lines indicate the ⁷¹Ga isotope. The spectra obtained for Ga-desferrioxamine E and Ga-desferrioxamine G from the aerobic cultures of P. stutzeri were matched by reference substance spectra.