Kinks and Kink Density

Under all conditions used in the experiments reported here, the crystals of ferritin and apoferritin as seen in the optical microscope attached to the atomic force microscope had the typical octahedral shapes with sharp edges. Accordingly, the AFM images in Fig. 4, 6-9 and all figures below indicate growth by layer generation and spreading to cover the whole facet.

Fig. 7. (A) Molecular structure of growth step on ferritin crystal at protein concentration of 70 mg/mL, corresponding to supersaturation o = A|j./kBT = 0.7, (C/Ce - 1) = 1. Dark area: lower layer; light gray: advancing upper layer. (B) Distribution of molecules between kinks on steps located approx 0.5 mm apart, obtained from images similar to (A) at same (C/Ce - 1) = 1. (From ref. 31.)

Fig. 7. (A) Molecular structure of growth step on ferritin crystal at protein concentration of 70 mg/mL, corresponding to supersaturation o = A|j./kBT = 0.7, (C/Ce - 1) = 1. Dark area: lower layer; light gray: advancing upper layer. (B) Distribution of molecules between kinks on steps located approx 0.5 mm apart, obtained from images similar to (A) at same (C/Ce - 1) = 1. (From ref. 31.)

The molecular structures of a (111) apoferritin face and of a growth step are shown in Fig. 6. The kinks at a step are the sites where incoming molecules attach. A molecule attached at a kink has half of the neighbors that a molecule in the crystal bulk has; in the f.c.c. lattice of ferritin and apoferritin, the half-number equals six, with three molecules belonging to the underlying layer, and three molecules from the step (42,43,48). Thus, the kink density is a fundamental variable that determines the ability of the crystal to incorporate solute molecules and grow (49,50).

From Fig. 6 and approx 15 other similar images, we determine the kink density along a step by counting the molecules between two kinks, nk, also called kink length (51) and plot their distribution in Fig. 6B-D for apoferritin, and in Fig. 7B for ferritin. Note that kink density is affected by the presence of surface point defects, such as vacancies or vacancy clusters, seen in Fig. 6A, as well as impurity cluster adsorbed on the surface (37). These features act as stoppers: straight step segments as long as eight molecules form and the step propagation is locally delayed (25). Hence, for the statistics in Figs. 6 and 7, we did not consider step segments around such stoppers. We obtain nk = 3.5 for both proteins. Comparing Fig. 6A-C, we see that the nk distributions are nearly the same near equilibrium, as well as at very high supersaturations.

Was this article helpful?

0 0

Post a comment