2D monte carlo simulation of patchy particles association and protein crystal polymorph selection

Abstract

Typically, protein crystals inherit the polymorphic form selected by nuclei arising in the solution. However, a transition of a polymorphic form may also occur at a later crystal growth stage. Unfortunately, due to the molecular-scale processes involved, the earliest stages of protein crystal nucleation and polymorph selection remain poorly understood. This paper attempts to elucidate the polymorph selection and crystal growth process in proteins (and colloidal crystals) using 2D Monte Carlo simulations and a computational model with short-range attraction for ‘protein-like’ patchy particles (PPs) of a specific patch geometry, bond width and strength. A relatively narrow temperature range is established whereby parts of the PPs monomers arrange initially in a rapidly growing unstable rhombohedral lattice (Rh). Stable trimers form simultaneously from the monomers remaining in the solution and monomers released from the Rh lattice. These trimers serve as building blocks of a more stable Kagome trihexagonal lattice (TriHex), which appears after a prolonged simulation time. The step-by-step scenario of this polymorphic transition and the specific role of PPs’ geometric and interaction anisotropies are discussed in detail.