Enhanced iron(III) chelation through ligand predisposition: syntheses, structures and stability of tris-catecholate enterobactin analogs

To investigate the design features of enterobactin with respect to its extraordinary iron binding efficiency, two second generation analogs of enterobactin, 1,3,5-tris(2,3-dihydroxybenzamidomethyl)-2,4,6-triethylbenzene (EMECAM) and 1,3,5-tris(2,3-dihydroxybenzamidomethyl)-2,4,6-trimethylbenzene (MMECAM), have been synthesized, and structurally and thermodynamically characterized. By introducing three alkyl groups at the 2, 4 and 6 positions of the supporting aryl ring of the first generation analog (MECAM) the new analogs are expected to arrange two triplets of substituent groups alternately above and below the scaffolding aryl ring due to steric interactions of the substituents, and consequently stabilize a predisposed ligand conformation. A single crystal structure of EMECAM confirms the expected ligand conformation. While the three binding groups orient in the same direction from the supporting aryl ring, the catecholate binding units point outward due to intramolecular hydrogen bond interactions. Variable temperature proton NMR results, recorded in d(6)-acetone and d(4)-methanol from 200 to 300 K, are consistent with the ligand conformation observed in the solid state. Thermodynamic characterization, including spectrophotometric titrations of both ligands and their iron(In) complexes, demonstrates that iron(III) chelation by the second generation analogs has been enhanced substantially. The stability constants of the ferric EMECAM and MMECAM complexes were determined to be 10(47.1(3)) and 10(45.8(5)) respectively, about 10(4) and 10(3) higher than that of MECAM. A V(IV) complex of EMECAM was prepared and its structure was crystallographically characterized to investigate the remaining 100-fold difference in iron binding efficiency between EMECAM and enterobactin. The difference in stability is attributed to the strain in the Fe(In) complex of EMECAM caused by the suboptimal orientation of the CH2-N-amide bond vectors of EMECAM. This study quantitates the predisposition of enterobactin for metal binding relative to EMECAM, and demonstrates the significance of the supporting scaffold conformation and size of enterobactin with respect to its superior metal binding eficiency. Crystal data: EMECAM.2 acetone conforms to the triclinic space group <P(1)over bar> with a = 11.202(1), b = 11.394(2), c = 17.527(2), alpha = 74.04(4), beta = 82.14(2), rho = 66.56(1), V = 1972(1), Z = 2. For 3743 reflections with F-o(2) > 3 sigma(F-o(2)) the final R(R-w) = 0.046(0.047). Crystals of K-2[V((EMECAM)]. 2DMF . Et2O conform to the monoclinic space group P2(1)/n with a = 12.756(3), b = 31.890(4), c = 13.212(3), beta = 99.38(2), V = 5302(2), Z = 4. For 3633 reflections with F-o(2) > 3 sigma(F-o(2)) the final R(R-w) = 0.046(0.052). Solution thermodynamic measurements were by potentiometric and spectrophotometric titrations. The protonation constants of three ortho-hydroxyl oxygen atoms (logK(4), logK(5), logK(6)) are 8.4(1), 7.4(2) and 6.4(1) for EMECAM, and 8.5(2), 7.4(3) and 6.2(2) for MMECAM. The protonation constants of the Fe(III) complexes (logK(FeHL), logK(FeH2L)) are 5.52(5) and 4.5(2) for EMECAM, and 5.5(1) and 4.6(3) for MMECAM. Cyclic voltammetry for Fe(III) complexes recorded in aqueous solution gave E-1/2 values (versus NHE): EMECAM, -1.07 V; MMECAM, -1.08 V. Corresponding values for V(V) complexes recorded in DMF gave E-1/2 values (versus NHE): EMECAM, 0.25 V; MMECAM, 0.24 V. (C) 1997 Elsevier Science S.A.