The equilibrium thermodynamic properties of a heteropolymer chain of 4 hydrophobic and 26 hydrophilic segments with Lennard-Jones interaction among segments were studied by multicanonical Monte Carlo methods in a three-dimensional off-lattice model.It is approved that the multicanonical algorithms provide us a practical way to accurately calculate thermodynamic properties of the chain at lower temperature.The collapse transition of a heteropolymer chain cooled from higher temperature is able to be separated in three steps: First,the hydrophobic segments associate together to form a hydrophobic core;then,the hydrophilic segments collapse on the surface of the core;finally,the heteropolymer chain reorganizes into its lowest energy state.
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