Synthesis, Structure, and Molecular Dynamics of Gallium Complexes of Schizokinen and the Amphiphilic Siderophore Acinetoferrin

Abstract

A new general synthesis of the citrate-based siderophores acinetoferrin (Af) and schizokinen (Sz) and their analogues is described. The molecular structure of gallium schizokinen, GaSz, was determined by combined ¹H NMR, Hartree−Fock ab initio calculations, DFT, and empirical modeling of vicinal proton NMR spin−spin couplings. The metal-coordination geometry of GaSz was determined from NOE contacts to be cis-cis with respect to the two chelating hydroxamates. One diaminopropane adopts a single chairlike conformation while another is a mixture of two ring pucker arrangements. Both amide hydrogens are internally hydrogen bonded to metal-ligating oxygen atoms. The acyl methyl groups are directed away from each other with an average planar angle of ca. 130°. The kinetics of GaSz racemization were followed by selective, double spin−echo inversion−recovery ¹H NMR spectroscopy over the temperature range of 10−45 °C. The racemization proceeds by a multistep mechanism that is proton independent between pD 5 and 12 (k₀ = 1.47 (0.15 s^-1)) and acid catalyzed below pD 4 (k₁ = 2.25 (0.15) × 10⁴ M^-1 s^-1). The activation parameters found for the two sequential steps of the proton independent pathway were ΔH^⧧ = 25 ± 3 kcal M^-1, ΔS^⧧ = 25 ± 7 cal M^-1 K^-1 and ΔH^⧧ = 17.1 ± 0.2 kcal M^-1, ΔS^⧧ = 0.3 ± 2.7 cal M^-1 K^-1. The first step of the proton-independent mechanism was assigned to the dissociation of the carboxyl group. The second step was assigned to complex racemization. The proton-assisted step was assigned to a complete dissociation of the α-hydroxy carboxyl group at pD < 4. The ab initio modeling of gallium acinetoferrin, GaAf, and analogues derived from the structure of GaSz has shown that the pendant trans-octenoyl fragments are oriented in opposite directions with the average planar angle of ca. 130°. This arrangement prevents GaAf from adopting a phospholipid-like structural motif. Significantly, iron siderophore complex FeAf was found to be disruptive to phospholipid vesicles and is considerably more hydrophilic than Af, with an eight-fold smaller partition coefficient.