Thermodynamics of partitioning of substance P in isotropic bicelles

The temperature dependence of the partition of a neuropeptide, substance P (SP), in isotropic (q = 0.5) bicelles was investigated by using pulsed field gradient NMR diffusion technique. The partition coefficient decreases as the temperature is increased from 295 to 325 K, indicating a favorable (negative) enthalpy change upon partitioning of the peptide. Thermodynamic analysis of the data shows that the partitioning of SP at 300 K is driven by the enthalpic term (DeltaH) with the value of - 4.03 kcal mol(-1), while it is opposed by the entropic term (-TDeltaS) by approximately 1.28 kcal mol(-1) with a small negative change in heat capacity (DeltaC(p)). The enthalpy-driven process for the partition of SP in bicelles is the same as in dodecylphosphocholine (DPC) micelles, however, the negative entropy change in bicelles of flat bilayer surface is in sharp contrast with the positive entropy change in DPC micelles of highly curved surface, indicating that the curvature of the membrane surface might play a significant role in the partitioning of peptides.