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.
Highlights
Crystal polymorphism [1], which is the ability of a substance of the same chemical composition to exist in more than one crystal structure, is decisive for the therapeutic function of drug formulations.crystal polymorphism is of particular interest to the pharmaceutical industry
These trimers serve as building blocks of a more stable Kagome trihexagonal lattice (TriHex), which appears after a prolonged simulation time
Being a the simulations showed that the TriHex phase appeared first on the circumference of the already grown catalytic-like surface for the formation and growth of this more stable TriHex phase, the unstable rhombohedral lattice (Rh)
Summary
Crystal polymorphism [1], which is the ability of a substance of the same chemical composition to exist in more than one crystal structure, is decisive for the therapeutic function of drug formulations. They make protein crystal nucleation a self-assembly process with high precision This process produces stable clusters formed as a result of selective and directional interactions among biological macromolecules. There is a large quantity of literature on crystals formed of patchy particles (and such polymorphs); for instance, it has been shown that the rational design of patch shape and symmetry can drive patchy colloids to crystallize in a single, selected morphology by structurally suppressing the undesired competing crystal and favoring the self-assembly of the desired structure [27] This is the central idea of Romano and Sciortino, who studied the crystallization of the so-called “triblock Janus” colloids with two triangular patches oriented in a staggered geometry and eclipsed patch arrangement. Patchy colloidal particles, which include binary mixtures as well as attraction and repulsion between the patches, were studied by Doppelbauer et al [31]; using optimization techniques based on genetic algorithms, the authors identified a broad variety of highly non-trivial ordered structures
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