Thermodynamics of hydrophobic interactions: Entropic recognition of a hydrophobic moiety by poly(ethylene oxide)-zinc porphyrin conjugates

The recognition of 4-alkylpyridines by water-soluble poly(ethylene oxide)-zinc porphyrin conjugates was studied with a focus on the thermodynamic parameters of binding. Microcalorimetric studies indicated that binding of the alkyl group of the guest in water is driven by the entropic term (delta Delta H-0=Delta H-0(4-pentytpyridine)-Delta H-0(4-methylpyridine) = + 1.7 U mol(-1), delta T Delta S-0= T Delta S-0(4-pentylpyridine)T Delta S-0(4-methylpyridine) = + 11.8 U mol(-1) at 298 K), thus showing the significance of water reorganization during host-guest interaction. The enthalpy-entropy compensation temperature of binding of 4-alkylpyridines was as low as 38 K; only below this temperature could the enthalpic term be a driving force. The binding affinity was modulated by the addition of cations and by varying the degree of polymerization of poly(ethylene oxide), which suggests that guest binding is coupled with polymer conformation.