Molecular dynamics simulation of alpha-melanocyte stimulating hormone in a water-membrane model interface.

The conformation of the tridecapeptide alpha-melanocyte stimulating hormone in the presence of a double water-membrane interface was studied by molecular dynamics simulation, using the computational package THOR. In this program the solvent is represented by a continuous medium with dielectric constant epsilon, and the interface between different media is simulated by a surface of discontinuity of the dielectric constant. The electrostatic image method was used to write down the terms, added to the force field, that describe the polarisation effects induced in the interface by the atomic charges. The program was further improved by the introduction of a second surface, parallel to the first one, to mimic the membrane. A conformational search using the software Prelude was employed to find an initial geometry for the peptide in water. The molecular dynamics simulation performed during 10 ns showed that the peptide structure is flexible in water, without stabilisation of any preferential conformation. In the presence of the model membrane, the peptide moved to the medium representing the interior of the membrane. Inside the low dielectric constant medium, the structure of the peptide showed a turn in the central sequence of amino acids and a packed conformation remained stabilised during more than 7.0 ns of simulation.