Studies towards dynamic kinetic resolution of 4-hydroxy-2-methylcyclopent-2-en-1-one and its E - O -trityloxime

Design of metal-mediated protein assemblies via hydroxamic acid functionalities

The self-assembly of proteins into sophisticated multicomponent assemblies is a hallmark of all living systems and has spawned extensive efforts in the construction of novel synthetic protein architectures with emergent functional properties. Protein assemblies in nature are formed via selective association of multiple protein surfaces through intricate noncovalent protein-protein interactions, a challenging task to accurately replicate in the de novo design of multiprotein systems. In this protocol, we describe the application of metal-coordinating hydroxamate (HA) motifs to direct the metal-mediated assembly of polyhedral protein architectures and 3D crystalline protein-metal-organic frameworks (protein-MOFs). This strategy has been implemented using an asymmetric cytochrome cb₅₆₂ monomer through selective, concurrent association of Fe³⁺ and Zn²⁺ ions to form polyhedral cages. Furthermore, the use of ditopic HA linkers as bridging ligands with metal-binding protein nodes has allowed the construction of crystalline 3D protein-MOF lattices. The protocol is divided into two major sections: (1) the development of a Cys-reactive HA molecule for protein derivatization and self-assembly of protein-HA conjugates into polyhedral cages and (2) the synthesis of ditopic HA bridging ligands for the construction of ferritin-based protein-MOFs using symmetric metal-binding protein nodes. Protein cages are analyzed using analytical ultracentrifugation, transmission electron microscopy and single-crystal X-ray diffraction techniques. HA-mediated protein-MOFs are formed in sitting-drop vapor diffusion crystallization trays and are probed via single-crystal X-ray diffraction and multi-crystal small-angle X-ray scattering measurements. Ligand synthesis, construction of HA-mediated assemblies, and post-assembly analysis as described in this protocol can be performed by a graduate-level researcher within 6 weeks.

Constructing protein polyhedra via orthogonal chemical interactions

A large fraction of proteins naturally exist as symmetrical homooligomers or homopolymers¹. The emergent structural and functional properties of such protein assemblies have inspired extensive efforts in biomolecular design^2-5. As synthesized by ribosomes, proteins are inherently asymmetric. Thus, they must acquire multiple surface patches that selectively associate to generate different symmetry elements needed to form higher-order architectures^1,6 – a daunting task for protein design. Here we introduce an inorganic chemical approach to address this outstanding problem, whereby multiple modes of protein-protein interactions and symmetry are simultaneously achieved by selective, “one-pot” coordination of soft and hard metal ions. We show that a monomeric protein (protomer) appropriately modified with biologically inspired hydroxamate groups and Zn-binding motifs assembles through concurrent Fe³⁺ and Zn²⁺ coordination into discrete dodecameric and hexameric cages. Closely resembling natural polyhedral protein architectures^7,8 and unique among designed systems^9-13, our artificial cages possess tightly packed shells devoid of large apertures, yet they can assemble and disassemble in response to diverse stimuli owing to their heterobimetallic construction on minimal interprotein-bonding footprints. With stoichiometries ranging from [2 Fe:9 Zn:6 protomer] to [8 Fe:21 Zn:12 protomer], these protein cages represent some of the compositionally most complex protein assemblies–or inorganic coordination complexes–obtained by design.