Protein Dimerization on a Phosphonated Calix[6]arene Disc

Complex formation between cationic cytochrome c and the water-soluble, poly-anionic p-phosphonatocalix[6]arene (pclx₆) was investigated. A crystal structure (at 1.8 Å resolution) revealed a remarkable dimeric disc of pclx₆ that acts like glue to mediate a symmetric (C₂) protein dimer. The calixarene disc has a diameter of about 1.5 nm and masks about 360 Ų of protein surface. The key protein–calixarene contacts occur via two linchpin lysines, with additional contacts provided by a small hydrophobic patch. The protein–calixarene supramolecular assemblies were observed in solution by size-exclusion chromatography with multi-angle light scattering and NMR spectroscopy. Using isothermal titration calorimetry and NMR data, an apparent K_d in the low micromolar range was determined for the charge-rich protein–calixarene complex. In contrast to p-sulfonatocalix[4]arene, the larger pclx₆ has a single, well-defined binding site that mediates the assembly of cytochrome c in solution.