Probing the determinants of porosity in protein frameworks: co-crystals of cytochrome c and an octa-anionic calix[4]arene

Sulfonato-calix[n]arenes (sclx_n) are promising tools to generate crystalline protein frameworks. We report, for the first time, a lower rim functionalised octa-anionic calix[4]arene (sclx₄mc) in complex with proteins. Two crystal structures of sclx₄mc bound to yeast or horse heart cytochrome c (cytc) are described. Highly porous honeycomb or tubular assemblies were obtained with yeast or horse cytc, respectively. Related frameworks were obtained previously with sclx₈ and sclx₆ but not with sclx₄, suggesting that the ligand charge is a determining factor.