Molecular dynamics simulation of adrenocorticotropin (1-10) peptide in a solvated dodecylphosphocholine micelle

Adrenocorticotropin (ACTH) (1-10), an adrenocorticotropin hormone fragment, has been studied by molecular dynamics (MD) simulation in an NPT ensemble in an explicit dodecylphosphocholine (DPC) micelle. Two starting configurations of the peptide/micelle system, corresponding to the insertion and surface-binding modes, were used. A common equilibrated configuration, in which the peptide lies parallel to the micellar surface, was reached from both simulations. In the initial part of the simulations, distance restraints derived from NMR nuclear Overhauser enhancements were incorporated before the peptide reached an equilibrium configuration with respect to the micelle. Analyses of the trajectories from the subsequent free (unrestrained) MD simulation showed that ACTH (1-10) does not conform strictly to a helical structure. The loss of the helical structure is due to decreased intramolecular hydrogen bonding accompanied by an increase of hydrogen bonding between the amide protons of the peptide and the micellar head groups. However, the extent of the latter interaction is less pronounced than in the negatively charged SDS micelle. The final structure enhances the amphipathic nature of the peptide, facilitating better interactions at the water-hydrophobic interface. The primary hydrophobic interactions with the micelle came from the side chains of Met4, Phe7, and Trp9. All peptide bonds were either hydrated or were involved in intramolecular hydrogen bonding. The interactions with the DPC micelle, the conformation of the bound peptide, and the dynamics of the peptide, as revealed by the time correlation functions of the N-H bonds, were compared with those of the ACTH (1-10)/SDS system studied previously by MD simulations.