Optimization of the Lariat Ether Carboxylic Acid Host Structure for Ferrioxamine B: Demonstration of a Second Coordination Shell Chelate Effect

Publisher Website
Google Scholar
Abstract
Host−guest supramolecular assembly formation constants involving the second-sphere complexation of the siderophore ferrioxamine B (FeHDFB+) by a lariat ether carboxylic acid host (Ln+2COOH) in wet chloroform were obtained from liquid−liquid extractions at pH values above and below the host pKa (∼5.3). The host−guest formation constants, Ka, determined at pH = 3.2 for the assemblies {FeHDFB+,Ln+2COOH,ClO4-} (n = 4, 7, 10, 15) in wet chloroform are similar to those of the parent crown ether, benzo-18-crown-6. At pH = 9.3, the lariat ethers are ionized, and this results in a more stable assembly, {FeHDFB+,Ln+2COO-}, as measured by the host−guest formation constant, Kapp. This enhanced stability is shown to be a function of the lariat ether sidearm chain length (n = 4, 7, 10, 15) and is corroborated by molecular modeling calculations. Additionally, molecular modeling and extraction data demonstrate that there is an optimum lariat ether sidearm chain length with respect to host−guest assembly stability as measured by Kapp. We attribute the enhanced stability effect of the ionized lariat ether in the host−guest assembly {FeHDFB+,Ln+2COO-}, relative to {FeHDFB+,Ln+2COOH,ClO4-} or to {FeHDFB+,B18C6,ClO4-}, to a second coordination shell chelate effect.

This publication has 21 references indexed in Scilit: