Microcalorimetric determination of thermodynamic parameters for ionophore-siderophore host-guest complex formation

Thermodynamic parameters (Delta H, Delta S, and Delta G) were determined by microcalorimetry in wet chloroform for host-guest assembly formation invol ving second-sphere complexation of the siderophore ferrioxamine B by crown ether (18-crown-6, cis-dicyclohexano-18-crown-6, benzo-18-crown-6) and cryp tand (2.2.2 cryptand) hosts. Similar data were also collected for the same hosts with the pentylammonium ion guest, which corresponds to the pendant p entylamine side chain of ferrioxamine B. Host-guest assembly formation cons tants (K-a) obtained from microcalorimetry agree with values obtained indir ectly from chloroform/water extraction studies in those cases where compara ble data are available. On the basis of a trend established by the pentylam monium guest, an enhanced stability relative to the crown ethers is observe d for the assembly composed of ferrioxamine B and 2.2.2 cryptand that is du e to entropic effects. Trends in Delta H and Delta S with changes in host a nd guest structure are discussed and attributed directly to host-guest comp lex formation, as solvation effects were determined to be insignificant (De lta C-p = 0).