New Lanthanide Chelating Tags for PCS NMR Spectroscopy with Reduction Stable, Rigid Linkers for Fast and Irreversible Conjugation to Proteins.

Lanthanide chelating tags (LCTs) have been used with great success for determining structures and interactions of proteins and other biological macromolecules. Recently LCTs have also been used for in-cell NMR spectroscopy, but the bottleneck especially for demanding applications like pseudocontact shift (PCS) NMR is the sparse availability of suitable tags that allow for site-selective, rigid, irreversible, fast, and quantitative conjugation of chelated paramagnetic lanthanide ions to proteins via reduction stable bonds. We report here several such tags and focus on a new pyridine thiazole derivate of DOTA, that combines high affinity, rigidity, and selectivity with unprecedented tagging properties. The conjugation to the cysteine thiol of the protein results in a reductively stable thioether bond and proceeds virtually quantitatively in less than 30 min at 100 μM protein concentration, ambient temperature, and neutral pH. Upon conjugation of the new tag to two single cysteine mutants of ubiquitin and a single cysteine mutant of human carbonic anhydrase type II (30 kDa) only one stereoisomer is formed (square antiprismatic coordination, Λ(δδδδ)) and large to very large pseudocontact shifts as well as large residual dipolar couplings (RDCs) are observed by NMR spectroscopy. The PCS and RDC show excellent agreement with the solid state structure of the proteins. We believe that the pyridine thiazole moiety reported here has the potential to serve as a thiole reactive group in various conjugation applications; furthermore, its terbium complex shows strong photoluminescence upon irradiation and may thus serve as a donor group for Förster resonance energy transfer spectroscopy.