Microcalorimetric Determination of Thermodynamic Parameters for Ionophore−Siderophore Host−Guest Complex Formation

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Abstract
Thermodynamic parameters (ΔH, ΔS, and ΔG) were determined by microcalorimetry in wet chloroform for host−guest assembly formation involving second-sphere complexation of the siderophore ferrioxamine B by crown ether (18-crown-6, cis-dicyclohexano-18-crown-6, benzo-18-crown-6) and cryptand (2.2.2 cryptand) hosts. Similar data were also collected for the same hosts with the pentylammonium ion guest, which corresponds to the pendant pentylamine side chain of ferrioxamine B. Host−guest assembly formation constants (Ka) obtained from microcalorimetry agree with values obtained indirectly from chloroform/water extraction studies in those cases where comparable data are available. On the basis of a trend established by the pentylammonium guest, an enhanced stability relative to the crown ethers is observed for the assembly composed of ferrioxamine B and 2.2.2 cryptand that is due to entropic effects. Trends in ΔH and ΔS with changes in host and guest structure are discussed and attributed directly to host−guest complex formation, as solvation effects were determined to be insignificant (ΔCp = 0).