Monomer-dimer control and crystal engineering in TASPs.

We have reported a template assembled synthetic protein (cavitein Q4) as an unexpected dimer in the solid state and as a monomer-dimer equilibrium in solution. We have since reported an ability to bias a cavitein's monomer-dimer equilibrium in solution by sequence design involving histidine metal chelation or disulfide incorporation. However, little remains known about the forces contributing to dimeric cavitein crystal nucleation and lattice stabilization. We, therefore, designed glutamine variants to probe factors involved in dimeric cavitein crystallization. It was found that a key glutamate hydrogen-bonding interaction between dimers is integral to crystal formation and stabilization. Additionally, we obtained a crystal structure of a cavitein (Q4-E3H) designed to bias the dimeric structure via histidine metal coordination. The resolved structure indicates a histidine cluster interaction that likely accounts for the biased dimeric form observed in solution.