Abstract
It is widely accepted that many phase transitions do not follow nucleation pathways as envisaged by the classical nucleation theory. Many substances can traverse intermediate states before arriving at the stable phase. The apparent ubiquity of multi-step nucleation has made the inverse question relevant: does multistep nucleation always dominate single-step pathways? Here we provide an explicit example of the classical nucleation mechanism for a system known to exhibit the characteristics of multi-step nucleation. Molecular resolution atomic force microscopy imaging of the two-dimensional nucleation of the protein glucose isomerase demonstrates that the interior of subcritical clusters is in the same state as the crystalline bulk phase. Our data show that despite having all the characteristics typically associated with rich phase behaviour, glucose isomerase 2D crystals are formed classically. These observations illustrate the resurfacing importance of the classical nucleation theory by re-validating some of the key assumptions that have been recently questioned.
Highlights
It is widely accepted that many phase transitions do not follow nucleation pathways as envisaged by the classical nucleation theory
Given the apparent ubiquity of the multistep nucleation (MSN) mechanism as suggested by theory[9] and numerous experimental examples[8,10,11,13,15,21,22], the inverse question has become relevant: how common is direct, singlestep nucleation of crystalline phases and is there a competition between the two mechanisms in a given system or is each substance predisposed to one and only one nucleation pathway? One of the goals of this contribution is to provide a wellcharacterized example of crystallization that does nucleate classically in a system known to exhibit the characteristics of the two-step mechanism in other circumstances[28]
Given recent non-classical observations for the 3D case[28], it is striking to see how well glucose isomerase 2D crystallization fits into the Classical Nucleation Theory (CNT) straitjacket
Summary
It is widely accepted that many phase transitions do not follow nucleation pathways as envisaged by the classical nucleation theory. Our data show that despite having all the characteristics typically associated with rich phase behaviour, glucose isomerase 2D crystals are formed classically These observations illustrate the resurfacing importance of the classical nucleation theory by re-validating some of the key assumptions that have been recently questioned. State-of-the-art techniques utilized for small molecules, such as in situ TEM, are well-suited for monitoring the succession of microscopic phases, that is, mapping the nucleation pathways[29] They lack the lateral resolution required to provide a detailed structural characterization of subcritical clusters. For the specific system we scrutinized, glucose isomerase 2D crystallization on mica, and we find no evidence that such MSN scenarios might exist despite actively searching phase space for it
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