Abstract
A molecular dynamics simulation has been performed on a synthetic membrane-spanning ion channel, consisting of four alpha-helical peptides, each of which is composed of the amino acids leucine (L) and serine (S), with the sequence Ac-(LSLLLSL)3-CONH2. This four-helix bundle has been shown experimentally to act as a proton-conducting channel in a membrane environment. In the present simulation, the channel was initially assembled as a parallel bundle in the octane portion of a phase-separated water/octane system, which provided a membrane-mimetic environment. An explicit reversible multiple-time-step integrator was used to generate a dynamical trajectory, a few nanoseconds in duration for this composite system on a parallel computer, under ambient conditions. After more than 1 ns, the four helices were found to adopt an associated dimer state with twofold symmetry, which evolved into a coiled-coil tetrameric structure with a left-handed twist. In the coiled-coil state, the polar serine side chains interact to form a layered structure with the core of the bundle filled with H2O. The dipoles of these H2O molecules tended to align opposite the net dipole of the peptide bundle. The calculated dipole relaxation function of the pore H2O molecules exhibits two reorientation times. One is approximately 3.2 ps, and the other is approximately 100 times longer. The diffusion coefficient of the pore H2O is about one-third of the bulk H2O value. The total dipole moment and the inertia tensor of the peptide bundle have been calculated and reveal slow (300 ps) collective oscillatory motions. Our results, which are based on a simple united atom force-field model, suggest that the function of this synthetic ion channel is likely inextricably coupled to its dynamical behavior.