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, ,3-dihydroxybenzamidomethyl)-2,4,6-triethylbenzene (E MECAM) and 3-dihydroxybenzamidomethyl)-2,4,6-trimethylbenzene (MMECAM) , have been synthesized, and structurally and thermodynamically charac terized. By introducing three alkyl groups at the 2, 4 and 6 positions of the supporting aryl ring of the first generation analog (MECAM) th e new analogs are expected to arrange two triplets of substituent grou ps alternately above and below the scaffolding aryl ring due to steric interactions of the substituents, and consequently stabilize a predis posed ligand conformation. A single crystal structure of EMECAM confir ms the expected ligand conformation. While the three binding groups or ient in the same direction from the supporting aryl ring, the catechol ate binding units point outward due to intramolecular hydrogen bond in teractions. Variable temperature proton NMR results, recorded in d(6)- acetone and d(4)-methanol from 200 to 300 K, are consistent with the l igand conformation observed in the solid state. Thermodynamic characte rization, 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 stabil ity constants of the ferric EMECAM and MMECAM complexes were determine d 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 an d its structure was crystallographically characterized to investigate the remaining 100-fold difference in iron binding efficiency between E MECAM and enterobactin. The difference in stability is attributed to t he strain in the Fe(In) complex of EMECAM caused by the suboptimal ori entation of the CH2-N-amide bond vectors of EMECAM. This study quantit ates the predisposition of enterobactin for metal binding relative to EMECAM, and demonstrates the significance of the supporting scaffold c onformation and size of enterobactin with respect to its superior meta l binding eficiency. Crystal data: EMECAM.2 acetone conforms to the tr iclinic 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 36 33 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 ort ho-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(FeH2 L)) are 5.52(5) and 4.5(2) for EMECAM, and 5.5(1) and 4.6(3) for MMECA M. Cyclic voltammetry for Fe(III) complexes recorded in aqueous soluti on gave E-1/2 values (versus NHE): EMECAM, -1.07 V; MMECAM, -1.08 V. C orresponding values for V(V) complexes recorded in DMF gave E-1/2 valu es (versus NHE): EMECAM, 0.25 V; MMECAM, 0.24 V. (C) 1997 Elsevier Sci ence S.A.