Tuning protein frameworks via auxiliary supramolecular interactions

Protein crystals with their precise, periodic array of functional building blocks have potential applications in biomaterials, sensing, and catalysis. This paper describes how a highly porous crystalline framework of a cationic redox protein and an anionic macrocycle can be modulated by a small cationic effector. Ternary composites of protein (∼13 kDa), calix[8]arene (∼1.5 kDa), and effector (∼0.2 kDa) formed distinct crystalline architectures, dependent on the effector concentration and the crystallization technique. A combination of X-ray crystallography and density functional theory (DFT) calculations was used to decipher the framework variations, which appear to be dependent on a calixarene conformation change mediated by the effector. This “switch” calixarene was observed in three states, each of which is associated with a different interaction network. Two structures obtained by co-crystallization with the effector contained an additional protein “pillar”, resulting in framework duplication and decreased porosity. These results suggest how protein assembly can be engineered by supramolecular host–guest interactions.This research was supported by NUI Galway, the SYSPROD project (AXELERA Pôle de Compétitivité) and Science Foundation Ireland (Grant No. 13/CDA/2168 to PBC). We thank SOLEIL synchrotron (Paris) for beam time allocation, and the staff at beamline PROXIMA-2A for their assistance with data collection.peer-reviewed2020-09-0