Dimeric and trimeric homo- and heteroleptic hydroxamic acid macrocycles formed using mixed-ligand Fe(III)-based...

Macrocyclic hydroxamic acids coordinate Fe(III) with high affinity as part of siderophore-mediated bacterial iron acquisition. Trimeric hydroxamic acid macrocycles, such as desferrioxamine E (DFOE), are prevalent in nature, with fewer dimeric macrocycles identified, including putrebactin (pbH₂), avaroferrin (avH₂), bisucaberin (bsH₂) and alcaligin (alH₂). This work used metal-templated synthesis (MTS) to pre-assemble complexes between one equivalent of Fe(III) and two equivalents of 4-((4-aminobutyl)(hydroxy)amino)-4-oxobutanoic acid (BBH) or 4-((5-aminopentyl)(hydroxy)amino)-4-oxobutanoic acid (PBH). Following peptide coupling, the respective Fe(III) complexes of pbH₂ or bsH₂ were formed, which analysed by LC-MS under acidic pH as [Fe(pb)]⁺ ([M]⁺, m/z_obs 426.1) or [Fe(bs)]⁺ ([M]⁺, m/z_obs 454.2). The mixed-ligand 1:1:1 Fe(III):BBH:PBH system furnished [Fe(pb)]⁺ and [Fe(bs)]⁺, together with chimeric [Fe(av)]⁺ ([M]⁺, m/z_obs 440.2). The deviation from the expected 1:2:1 distribution of [Fe(pb)]⁺:[Fe(av)]⁺:[Fe(bs)]⁺ to 1:3.2:1.6 suggested the MTS-mediated formation of dimeric macrocycles could be influenced by steric effects in the pre-complex and/or cavity size, as governed by the monomer. 21-Membered avH₂ defined the lower boundary of the optimal architecture. Mixed-ligand MTS between Fe(III):PBH-d₄:ret-PBH at 1:1.5:1.5, where ret-PBH=3-(6-amino-N-hydroxyhexanamido)propanoic acid, gave four Fe(III)-loaded trimeric hydroxamic acid macrocycles in a distribution of 1.0:3.0:2.9:1.1 that closely matched the expected distribution 1:3:3:1 for a system without any kinetic and/or thermodynamic bias. Apo-macrocycles pbH₂, avH₂ and bsH₂ were produced upon incubation with diethylenetriaminepentaacetic acid (DTPA) and co-eluted with a biosynthetic mixture of the native macrocycles. The work has demonstrated the utility of single- and mixed-ligand MTS for producing a variety of homo- and heteroleptic dimeric hydroxamic acid macrocycles as Fe(III) complexes and free ligands.