Various building block proteins have been designed to construct unique nanoporous materials. We have previously developed a cytochrome (cyt) Formula: see text-based trimer (cp-Formula: see text) 3 and a relevant closed cyclic trimer CL-(Formula: see text)3, where (cp-Formula: see text) 3 and CL-(Formula: see text) 3 assembled into a tetrahedron and a nanoporous structure, respectively. In this study, we aimed to reinforce the tetrahedron by introducing I8K and T14D mutations to cp-Formula: see text, which was designed to form salt bridges at the trimer interfaces. However, the X-ray crystallographic structure of the cp-Formula: see text I8K/T14D trimer ((cp-Formula: see text I8K/T14D) 3 ) revealed a nanoporous structure similar to that of CL-(Formula: see text) 3 . The cp-Formula: see text I8K/T14D variant exhibited a Soret band at approximately 407 nm, identical to that of CL-(Formula: see text) 3 , indicating a similar His/Met heme coordination structure. The X-ray structure of (cp-Formula: see text I8K/T14D) 3 showed that the introduced amino acids did not form a direct hydrogen bond with neighboring amino acids, where the packing of the trimers was stabilized by Gln-Gln hydrogen bonds analogous to those in the nanoporous structure of CL-(Formula: see text) 3 . The introduced amino acids may induce the cp-Formula: see text I8K/T14D molecules to assemble into a nanoporous structure by long-range electrostatic interactions and/or may stabilize the nanoporous structure by facilitating the construction of a hydrogen bond network with water molecules at distances of 2.2-3.5 Å at the interface of (cp-Formula: see text I8K/T14D) 3 .
Novientri et al. (Wed,) studied this question.