The role of non-specific interactions in a patchy model of protein crystallization

We use a coarse-grained model for generic proteins to investigate the formation of structures with P212121 symmetry, the most prevalent space group of protein crystals. To account for the string directionality of protein-protein interactions that has been suggested by previous studies, we represent proteins as spherical particles that are covered by a large number of small, attractive "patches" that are randomly distributed on the protein surface. Attractive interactions between two proteins can then involve several pairs of patches interacting simultaneously. Our results suggest that the unit cell with the lowest energy is not necessarily the one that grows fastest. Rather, growth is favoured if 1) new particles can attach with enough bonds to the growth front and 2) particles that attach in crystallographically inequivalent positions bind to the surface with similar strength [corrected]. We subsequently study the impact of interactions that are not part of crystalline contacts and find that when these non-specific interactions are few and weaker than the crystal contacts, both nucleation and growth are successful. If the proportion of non-specific interactions is increased, crystal growth is still possible in a small range of model temperature.