Poster Presentation Royal Australian Chemical Institute National Congress 2026

Ga(III) complexes from the chemoenzymatic synthesis of macrocyclic hydroxamic acid chelators using length-modulated substrates (#419)

Joseph Wang 1 , Callum A Rosser 1 , Todd E Markham 1 , Rachel Codd 1
  1. School of Medicinal Sciences , The University of Sydney, Sydney, NSW, Australia

Metal-radionuclide pharmaceutical agents for imaging and therapy are rising in the clinic with increased access to a diverse range radiometals, thereby necessitating the development of more selective and stable chelators.1 Nature already provides a diverse range of chelators known as siderophores whose biosynthetic pathways have been previously elucidated2 and could be exploited to generate pools of novel chelators. Desferrioxamine B (DFOB) is one such siderophore, currently used clinically3 and whose biosynthetic DesABCD gene cluster is characterised4. Of particular interest is the last-in-line enzyme DesD which oligomerises and subsequently macrocyclises hydroxamic acid monomers of N-hydroxy-N-succinyl cadaverine (HSC) to form DFOG1 (3 HSC units) and its macrocyclic counterpart DFOE. In contrast to the efficient DesD-mediated end-to-end macrocyclisaton of the linear trimer DFOG1 to form DFOE, the linear dimer bisucaberin B is poorly processed by DesD, with only modest production of its cognate dihydroxamic acid macrocycle bisucaberin.

This suggested the strain in the positioning of the HSC units in the DesD active site necessary for ring closure of bisucaberin B might be alleviated by including flexible regions within the HSC termini. Research towards generating macrocyclic chelators is useful, since macrocyclic chelators are more favourable for chelation compared to their linear counterparts.5 The insertion of a variable length polyethylene glycol (PEG) internal units between two flanking HSC termini was undertaken to evaluate the substrate limits of DesD for cyclisation to form dihydroxamic acid macrocyles with variable cavity sizes.

A set of non-native linear HSC oligomers containing variable length PEG internal units was synthesised on solid-phase, with the substrates next incubated with DesD and cofactors. The combinatorial pool of chemoenzymatically synthesised chelators was characterised by LC-MS/MS and then screened for function by chelation to gallium(III). A bi-model distribution was observed whereby the macrocycles containing the shortest and longest PEG inserts were produced in highest relative concentration together with the Ga(III) complexes, with macrocycles containing intermediate-length PEG inserts formed at lower concentrations.

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