Lattice simulations of protein crystal formation

A new algorithm is presented for the lattice simulation of protein crystal growth. The algorithm allows the calculation of the size distribution of microcrystals in the volume and timescale of experiments and within the framework of the previously-published microscopic model [A.M. Kierzek, W.M. Wolf, P. Zielenkiewicz, Biophys. J. 73 (1997) 571-580]. Simulations for the tetragonal lysozyme crystal show that there are two critical sizes in the development of ordered phase. The first one corresponds to the size of the smallest stable complex which, in the case of the tetragonal lysozyme crystal, is the particular tetramer. In a volume of 5 mul the tetramer appears in the millisecond timescale. The second critical radius of approximately 100 monomers is only reached by a few of all the smallest stable complexes formed in the solution. The model predicts that out of 10(7) tetramers which appear in solution, only eight reach the size of 100 monomers within 8 h. After exceeding the second critical radius the microcrystals grow to the size of 10(4) monomers in the minute timescale and are thus assumed to quickly lead to macroscopic crystals. The predicted number of crystals formed during 8 h of nucleation is in qualitative agreement with arrested nucleation experiments.