Molecular Mechanisms of Hematin Crystallization from Organic Solvent

Abstract

Here, we employ n-octanol to elucidate the fundamental processes of hematin crystallization from an organic solvent, identify the operational mechanisms of growth, and determine the respective control parameters. The values of the enthalpy, entropy, and free energy of the crystal−solution equilibrium suggest that octanol may structure around the hematin solute molecules and along the crystal interface. Time-resolved in situ atomic force microscopy demonstrates that hematin crystal growth strictly follows classical layer growth mechanisms. Steps propagate by the attachment of solute molecules, described by a first-order chemical rate law. The molecules reach the steps via adsorption on the crystal surface, followed by surface diffusion, and the kinetic barriers of this pathway offer additional crystallization control strategies. Solute incorporation into steps from the adjacent lower and upper terraces is strongly asymmetric, with the lower terrace contributing the major solute amount. These findings provide a foundation for the rational design of hematin crystals that may find applications utilizing their high magnetic and optical anisotropy.

Early Onset of Kinetic Roughening due to a Finite Step Width in Hematin Crystallization

“…1(c)] [24,37]. The smooth surface agrees with the predictions of the above criteria for the interface structure of crystals near equilibrium with the solution.…”

“…It is not a consequence of limited imaging resolution, since the pixel size on the images in Figs. 1(c)-(f) is about 3 nm, close to the hematin molecular size of 1.2 nm [37]. Moreover, AFM tip curvature artifacts do not constrain the resolution for objects with surface height variations <5 nm [43,44], and the highest observed height variation in the profiles used to calculate R q is 4.5 nm.…”

“…The hematin crystal structure [38] in Supplemental Material [26] implies that the coordination number of the hematin molecules in the crystal is Z ¼ 8. The mean ψ ¼ 2jΔH Criteria for the onset of kinetic roughening include vanishing R c (critical radius of the 2D nucleus) and the corresponding barrier for 2D nucleation, ΔG Ã , at high supersaturation [9,10,[35][36][37]. According to the classical nucleation theory (CNT), applied to 2D islands on a crystal substrate [39], island nuclei form as a result of fluctuations of the concentration of molecules on the surface.…”

“…2(b)] [5]. Trains of parallel steps have not been observed on growing hematin crystals [24,37]. Irrespective of the roughening mechanism, it is generally accepted that, owing to a consistently increasing 2D nucleation rate in the regime of rough growth, the interface roughness will increase monotonically with supersaturation [6,13,22].…”

“…1(c)-1(f) and 2(c)-2(g)] with the R c ðΔμÞ dependence. For the highly anisotropic hematin crystals [37,38], we define R c as an azimuthally averaged half-width of the 2D nuclei and determine it from observations of the size and shape fluctuations of newly nucleated islands, as illustrated in Figs. 3(a)-3(f) [24].…”