HYDROLYSIS OF FERRIOXAMINE-B IN AQUEOUS MICELLAR SOLUTION

The association constant for the Fe(III) complex of the linear trihydroxamic acid siderophore ferrioxamine B (FeHDFB+;HDFB2-= H3N-[(CH2)5-N(O)C(O)-(CH2)2C(O)NH-)]2(CH2)s-N(O)C(O)-CH3) binding to sodium dodecylsulfate (SDS) micelles was determined by ultrafiltration and kinetic methods. Both methods are in agreement and yield an average value of the association constant K(m) = 98 M-1. Association constants for the Al(III) analogue of ferrioxamine B, AlHDFB+, and of the metal-free ligand deferriferrioxamine B, H4DFB+, were also determined by ultrafiltration and found to be K(m) = 90 and 362 M-1, respectively. Ion-exchange constants (K(ex)) are calculated from K(m) values [K(ex)(FeHDFB+/Na+) = K(ex)(AlHDFB+/Na+) = 13; K(ex)(H4DFB+/Na+) = 46] and are discussed in relation to K(ex) for alkylammonium cations. Kinetic and thermodynamic parameters for the hydrolysis of ferrioxamine B in SDS solutions (0.01-0.15 M) at 25-degrees-C have been obtained by stopped-flow and spectrophotometric titration methods at I = 0.1 M (NaClO4/HClO4, NaNO3/HNO3) over a p[H+]tot range from 4.80 to 2.05. Good agreement is found between equilibrium constants determined by spectral and kinetic methods. Comparison with the parameters previously reported for the hydrolysis of FeHDFB+ in aqueous acidic solution suggests that FeHDFB+ resides in the Stern layer of the micelle, and consequently in a region of increased H+ concentration. Once a correction is made for the higher [H+] in the Stern layer, kinetic and thermodynamic parameters obtained in the presence of SDS micelles agree well with those reported for the reaction in aqueous medium. This agreement also gives additional support to the K(m) and K(ex) values obtained by ultrafiltration and kinetics. Results reported here show that there is no micellar stabilization of the siderophore complex.