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Two-Step Mechanism of Macromolecular Nucleation and Crystallization: Field Theory and Simulations

机译：大分子成核和结晶的两步机制：场理论与模拟

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We developed a phase-field method of simulations of crystallization of macromolecules from solutions, which provides a good combination of the efficiency, thermodynamic consistency, and predictive capability. The method allowed us to address some of the open questions of protein crystallization. The phase diagram of the solution contains low-density and high-density liquid and solid phases with the low-density liquid being the initial solution and the high-density solid being the final crystal. We observed three steps of the transformation process where the first step was homogeneous nucleation of the intermediate high-density liquid droplets, the second step was heterogeneous nucleation of the crystals on the droplet’s surface, and the third step was dissolution of some of the crystallites and dense liquid droplets back into the liquid state of low density. We found the thermodynamic criterion of the two-step nucleation, which says that there is a gap of the overall initial solution concentrations below the liquidus where the one-step process is thermodynamically impossible. Beyond the gap, we observed a one-step transformation, which ended up with the formation of a phase morphologically similar to a gel. At large values of the liquid interaction coefficient and moderate values of the overall concentration, we observed the transformation scenario, which was reminiscent of the amorphous phase formation. The simulation results allowed us to suggest a resolution of the George-Wilson problem as the lack of dense liquid droplets for weak macromolecular attraction and ease of the amorphous phase formation for the strong attraction.
机译：我们开发了一种模拟溶液中大分子结晶的相场方法，它将效率、热力学一致性和预测能力很好地结合起来。该方法使我们能够解决一些蛋白质结晶的开放性问题。溶液的相图包括低密度和高密度的液相和固相，其中低密度的液体是初始溶液，高密度的固体是最终晶体。我们观察到转变过程的三个步骤，第一步是中高密度液滴的均匀成核，第二步是液滴表面晶体的非均匀成核，第三步是部分微晶和致密液滴溶解回低密度的液态。我们发现了两步成核的热力学判据，即在热力学上不可能进行一步成核的液相线以下，溶液的总初始浓度存在一个间隙。在间隙之外，我们观察到了一步转化，最终形成了一个形态类似于凝胶的相。在大的液体相互作用系数和中等的总浓度值下，我们观察到了转变情况，这让人想起了非晶相的形成。模拟结果使我们能够提出解决George Wilson问题的方法，即缺乏稠密的液滴来实现弱的大分子吸引，而易于形成非晶相来实现强吸引。

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