Fig. 5: The crystal structure of 3D protein nanocage arrays.

a Assembly of ^T158HMjFer nanocages into sc packing induced by a combination of metal coordination and His-His interactions, and closeup views of π–π stacking and metal coordination. Ni²⁺ ions are shown as yellow spheres, histidines and glutamic acids are shown as magenta and cyan sticks, respectively. b The overlook view and side view of His-participated π–π stacking interactions and Ni²⁺-induced coordination between two neighboring nanocages. c Zoomed-in view of a couple of interactional subunits. d A zoomed-in view of one Ni²⁺ complex where Ni²⁺ is coordinated to four H₂O and glutamic acid residues Glu95 and Glu95′ contributed from two protein nanocages, respectively. The Ni²⁺ ions and water molecules are shown as yellow sphere and small orange spheres, respectively. The distances of H₂O-1, H₂O-2, H₂O-3 to Ni²⁺ (2.1 Å) are identical, while the distance of H₂O-4 to Ni²⁺ is 2.4 Å and the carboxylic group of Glu to Ni²⁺ is 2.5 Å. e A zoomed-in view of one Ni²⁺ complex where two water molecule ligands are substituted by two imidazole molecules. The distance of water molecules, imidazole molecules and the carboxylic group of Glu to Ni²⁺ are 2.3 Å, 2.6 Å, and 2.9 Å, respectively. The coordination of Ni²⁺ with surrounding atoms were marked with black dash lines.