Synthesis, structure, and molecular dynamics of gallium complexes of schizokinen and the amphiphilic siderophore acinetoferrin

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 transoctenoyl 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.