FOLDING KINETICS OF PROTEINLIKE HETEROPOLYMERS

Using a simple three-dimensional lattice copolymer model and Monte Car lo dynamics, we study the collapse and folding of proteinlike heteropo lymers. The polymers are 27 monomers long and consist of two monomer t ypes. Although these chains are too long for exhaustive enumeration of all conformations, it is possible to enumerate all the maximally comp act conformations, which are 3 X3X3 cubes. This allows us to select se quences that have a unique global minimum. We then explore the kinetic s of collapse and folding and examine what features determine the vari ous rates. The folding time has a plateau over a broad range of temper atures and diverges at both high and low temperatures. The folding tim e depends on sequence and is related to the amount of energetic frustr ation in the native state. The collapse times of the chains are sequen ce independent and are a few orders of magnitude faster than the foldi ng times, indicating a two-phase folding process. Below a certain temp erature the chains exhibit glasslike behavior, characterized by a slow ing down of time scales and loss of self-averaging behavior. We explic itly define the glass transition temperature (T,), and by comparing it to the folding temperature (T-f), we find two classes of sequences: g ood folders with T-f>T-g and non-folders with T-f<T-g.