77Se-13C based dipolar correlation experiments to map selenium sites in microcrystalline proteins

Sulfur-containing sites in proteins are of great importance for both protein structure and function, including enzymatic catalysis, signaling pathways, and recognition of ligands and protein partners. Selenium-77 is an NMR active spin-1/2 nucleus that shares many physiochemical properties with sulfur and can be readily introduced into proteins at sulfur sites without significant perturbations to the protein structure. The sulfur-containing amino acid methionine is commonly found at protein–protein or protein–ligand binding sites. Its selenium-containing counterpart, selenomethionine, has a broad chemical shift dispersion useful for NMR-based studies of complex systems. Methods such as (¹H)-⁷⁷Se-¹³C double cross polarization or {⁷⁷Se}-¹³C REDOR could be valuable to map the local environment around selenium sites in proteins but have not been demonstrated to date. In this work, we explore these dipolar transfer mechanisms for structural characterization of the GB1 V39SeM variant of the model protein GB1 and demonstrate that ⁷⁷Se-¹³C based correlations can be used to map the local environment around selenium sites in proteins. We have found that the general detection limit is ~ 5 Å, but longer range distances up to ~ 7 Å can be observed as well. This study establishes a framework for the future characterization of selenium sites at protein–protein or protein–ligand binding interfaces.