Subzero-Temperature NMR Observation of Protein Dynamics and Ligand Binding Enabled by Encapsulation Within Coordination Cages

Lowering temperatures provides an effective strategy for probing rapid protein dynamics. However, conventional nuclear magnetic resonance (NMR) measurements are usually limited to temperatures above 0°C due to water freezing and increasing rotational correlation times. In this study, we report subzero-temperature NMR analysis enabled by encapsulating a protein within spherical coordination cages. Spatial confinement within the metallo-cages stabilizes proteins by preventing aggregation, enabling multi-dimensional NMR measurements of a 20 kDa protein at temperatures as low as -30°C in low-viscosity acetone-water mixtures. Below 0°C, the NMR spectra of the protein revealed markedly slowed dynamics, resolving two distinct conformations as split signals. Furthermore, the subzero-temperature analysis enabled detection of exchangeable hydroxy protons that are unobservable at ambient temperatures. Saturation transfer difference (STD) NMR revealed binding of carbohydrate hydroxy groups to lysozyme, owing to the large apertures of the cages, which permit ligand diffusion while the cages stabilize proteins under subzero conditions. This cage confinement thus facilitates the study of rapid protein dynamics and ligand binding beyond the limits of conventional solution NMR.