Difference between ``proteinlike'' and ``nonproteinlike'' heteropolymers

Based on a simple two-dimensional (2D) hydrophobic-polar (H-P) lattice model, properties of amino acid chains are studied by enumeration and Monte-Carlo simulation methods. Among them some chains with large average energy gap (E_g¯) are thought to be ``proteinlike'' while the others are ``nonproteinlike.'' The large E_g¯ between the low excited conformations and the native conformation guarantees not only the thermodynamic stability of protein but also its fast-folding property. The phase transition from molten globule to the native conformation for the ``proteinlike'' polymer is found to be of first order, while that for the ``nonproteinlike'' polymer is not. Some properties of chains as a function of E_g¯ shows that the transition from ``nonproteinlike'' to ``proteinlike'' heteropolymers is continuous. The simulation of folding at different temperature indicates that the main reason why some polymers fold slowly to its native conformation is their low folding temperature which makes the effective energy barrier (E_b/T_f) much higher than ``proteinlike'' chains.