Anisotropic diffusion of point defects in a two-dimensional crystal of streptavidin observedby high-speed atomic force microscopy

Abstract

The diffusion of individual point defects in a two-dimensional streptavidin crystal formedon biotin-containing supported lipid bilayers was observed by high-speed atomicforce microscopy. The two-dimensional diffusion of monovacancy defects exhibitedanisotropy correlated with the two crystallographic axes in the orthorhombicC 222 crystal; in the 2Dplane, one axis (the a-axis) is comprised of contiguous biotin-bound subunit pairs whereas the other axis (theb-axis) is comprised of contiguous biotin-unbound subunit pairs. The diffusivity along theb-axis is approximately 2.4 times larger than that along thea-axis. This anisotropy is ascribed to the difference in the association free energybetween the biotin-bound subunit–subunit interaction and the biotin-unboundsubunit–subunit interaction. The preferred intermolecular contact occurs betweenthe biotin-unbound subunits. The difference in the intermolecular bindingenergy between the two types of subunit pair is estimated to be approximately0.52 kcal mol−1. Another observed dynamic behavior of point defects was fusion of two pointdefects into a larger defect, which occurred much more frequently than the fissionof a point defect into smaller defects. The diffusivity of point defects increasedwith increasing defect size. The fusion and the higher diffusivity of larger defectsare suggested to be involved in the mechanism for the formation of defect-freecrystals.