Surface morphology and kinetic properties in rapid growth of EAFP protein crystals investigated by atomic force microscopy

The whole process of EAFP protein monoclinic crystal growth with an extremely fast rate has been observed by atomic force microscopy. The results showed that the patterns of the growth images in rapidly growing crystals are complicated. The two-dimensional multi-layered stacks of growth steps are characteristic of higher supersaturation and the growth of steps proceeds in a manner of strong anisotropic spiral dislocations dominantly under lower supersaturation conditions. The complex dislocation sources, including multiple dislocation and multi-interacting single dislocation sources, the constant step-split and the propagation of trooped steps were observed on the {100} surfaces of growing EAFP crystals. The step height of each layer generated either by two-dimensional nucleation at higher supersaturation or by screw dislocation at lower supersaturation is about 2-­3 nm, which corresponds to the length of the crystallographic unit cell. Although the rate of advancement for each growth step is similar to that of other protein crystal growth, the unique way of the propagation distinct with the trooped steps, by which a bundle of steps are strapped together, would be responsible for the rapid growth of EAFP crystals. All features show a possible mechanism by which the fast growth of EAFP crystals could be attained. The structural basis of the growth mechanism is also discussed.