Symbiotic interaction between rhizobia and host plants takes place inside root nodules. Besides reducing atmospheric nitrogen into ammonium that is used by the plant to grow in nitrogen deficient soils, rhizobia produce siderophores enabling iron uptake.[1,2] Siderophores are strong iron chelators due to their hydroxamate, catecholate or carboxylate functional groups.[3] In this work, the siderophores of the beta-rhizobial genus Paraburkholderia were analyzed by ultra-high-performance liquid chromatography high-resolution tandem mass spectrometry (UHPLC-HR-MS/MS). In particular, the production of the novel siderophore phymabactin by Paraburkholderia phymatum was confirmed and the structures of eleven derivatives were elucidated by MS/MS and nuclear magnetic resonance (NMR) spectroscopy. Sharing a linear tetrapeptide backbone with hydroxamate and hydroxy-carboxylate groups, phymabactins are structurally closely related to another family of siderophores called ornibactins. Interestingly, spontaneous metal complexation of phymabactin derivatives with aluminum was observed.[4] To explore this in more detail, the collision cross sections (CCS) of phymabactins, ornibactins and corresponding metal complexes were determined by trapped ion mobility spectrometry. The CCS values of aluminum-phymabactin complexes were smaller than those of iron-phymabactin complexes, while an inverse relationship was observed for ornibactin complexes (see Figure). These findings on the chemical nature of phymabactins and their metal complexes disclose many potential application fields, including chelation therapy, crop protection and bioremediation.

[1] D. Golaz, C. K. Papenfuhs, P. Belles-Sancho, L. Eberl, M. Egli, G. Pessi, NPJ Microgravity, 2024, 10, 44.

[2] P. Bellés-Sancho, C. Beukes, E. K. James, G. Pessi, Nitrogen, 2023, 4, 135-158.

[3] A. Khan, P. Singh, A. Srivastava, Microbiological Research, 2018, 212-213, 103-111.

[4] D. Golaz, L. Bürgi, M. Egli, L. Bigler, G. Pessi, Life, submitted.