Universality and identity ordering in heteropolymer coil-globule transition

Abstract

The coil-globule transition of an energy polydisperse chain, a model heteropolymer system where the number of monomer species is as large as the total number of monomers, is studied by means of computer simulations. In this study, we systematically explore the consequences of having different functional form and variance of the energy distribution on the coil-globule transition in general. In particular, considering Gaussian (G) and uniform (U) distributions, the effect of varying polydispersity index, $\delta$, on the transition temperature $\theta^\ast$, chain size, internal structure and spatial organization of monomers in the globule, and kinetics of the folding are addressed. It is found that the transition temperature of the model heteropolymer is lower than that of the homopolymer counterpart, and $\theta^\ast$ increases with $\delta$ (both G and U) and $\theta^\ast({\rm U}) < \theta^\ast({\rm G})$ consistently. The results of our study suggest that $\theta^\ast$ is governed by the most probable value (rather than the width) of the pair-wise energy distribution. Interestingly, the nature of the collapse transition turns out to be universal, i.e., when scaled properly (irrespective of the functional form and variance) all the swelling curves fall on a master curve and it is well described by the same scaling form of the homopolymer counterpart. However, following quenching, the transition from coil to globule is relatively fast for heteropolymer (with no significant difference between G and U systems, and no significant $\delta$ dependence within the considered range). On the other hand, internal organization in the collapsed state, quantified through mean contact probability, show distinct scaling regimes. Also we observe segregation of monomers based on their identities which is more pronounced in the case of uniform distribution.