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The donut-shaped cucurbit[n]urils (Qn, n = 6-8) are rigid macrocyclic receptors with widespread use in protein recognition. To date, most applications have centred on the encapsulation of N-terminal aromatic residues by Q7 or Q8. Less attention has been placed on Q6, which can recognize lysine side chains due to its high affinity for alkylamines. In this work, we investigated protein-Q6 complexation by using NMR spectroscopy. Attempts to crystallize protein-Q6 complexes were thwarted by the crystallization of Q6. We studied four proteins that vary in size, net charge, and lysine content. In addition to Q6 interactions with specific Lys or dimethylated Lys residues, we report striking evidence for N-terminal recognition. High affinity (micromolar) binding occurred with the N-terminal Met-Lys motif present in one of the four model proteins. Engineering this feature into another model protein yielded a similar high affinity site. We also present evidence for Q8 binding at this N-terminal feature. These data expand the cucurbituril toolkit for protein sensing.

F420H2-oxidase (FprA) catalyses the four-electron reduction of O2 to 2H2O using the reduced form of F420 as electron donor. The hydrophobic O2-channel detected by Kr-derivatization and the concerted movement of a gating loop could contribute to prevent unwanted side-reaction between the catalytic intermediates and solvents, therefore preventing reactive oxygen species formation.

A crystalline biohybrid with a 4 : 1 protein : cucurbituril mass ratio is presented. This result was achieved by engineering additional cucurbit[7]uril (Q7) binding sites into a β-propeller protein. In contrast to the parent protein, Q7-controlled assembly of the engineered variant occurred in solution, as evidenced by NMR and SAXS measurements.

Co-crystallization of a 2 kDa tether-free sulfonated foldamer and the 13 kDa lysine-rich cytochrome c yielded a remarkable biohybrid assembly with chiral resolution of the foldamer helix handedness. In the crystal a ∼5 nm foldamer stack was surrounded by eight molecules of protein. NMR and CD experiments suggest interesting differences in the solution state recognition processes.

Macromolecular crystallography suffers from two major issues: getting well-diffracting crystals and solving the phase problem inherent to large macromolecules. Here, we describe the first example of a lanthanide complex family named "crystallophore" (Xo4), which contributes to tackling both bottlenecks. This terbium complex, Tb-Xo4, is an appealing agent for biocrystallography, combining the exceptional phasing power of the Tb(iii) heavy atom with powerful nucleating properties, providing ready-to-use crystals for structure determination. Furthermore, protein/Tb-Xo4 co-crystals can be easily detected and discriminated from other crystalline by-products using luminescence. We demonstrate the potential of this additive for the crystallisation and structure determination of eight proteins, two of whose structures were unknown.