Helix transition in di-block polyampholyte

Molecular dynamics simulations are applied to study the structural organization of a bead-monomer model of di-block polyampholyte. Di-block polyampholyte is formed of two consecutive blocks of equal length, each containing charges of one sign only. The ground state of the model studied has been found to be a double-stranded helix. The folding (freezing) into the ground state is not an all-or-none process but proceeds via an intermediate molten-globule phase. In the intermediate phase, the molecule is subject to considerable structural fluctuations and its size is small compared with both the high-temperature and ground-state conformations. Also at the stage of the coil-to-globule transition the twist of the native helix is fixed. In the globular state below the collapse transition, the chain cannot change its twist spontaneously. By applying finite-size analysis to the helicity distribution function at the freezing temperature we find that the molten-globule-helix transition is of first order.