High-affinity adaptors for switchable recognition of histidine-tagged proteins.

We aspired to create chemical recognition units, which bind oligohistidine tags with high affinity and stability, as tools for selectively attaching spectroscopic probes and other functional elements to recombinant proteins. Several supramolecular entities containing 2-4 nitrilotriacetic acid (NTA) moieties were synthesized, which additionally contained an amino group, to which fluorescein was coupled as a sensitive reporter probe. These multivalent chelator heads (MCH) (termed bis-, tris-, and tetrakis-NTA) were characterized with respect to their interaction with hexahistidine (H6)- and decahistidine (H10)-tagged targets. Substantially increased binding stability with increasing number of NTA moieties was observed by analytical size exclusion chromatography. The binding enthalpies as determined by isothermal titration calorimetry increased nearly additively with the number of possible coordinative bonds between chelator heads and tags. Yet, a substantial excess of histidines in the oligohistidine tag was required for obtaining fully additive binding enthalpies. Dissociation kinetics of MCH/oligohistidine complexes measured by fluorescence dequenching showed an increase in stability by 4 orders of magnitude compared to that of mono-NTA, and subnanomolar affinity was reached for tris-NTA. The gain in free energy with increasing multivalency was accompanied by an increasing loss of entropy, which was ascribed to the high flexibility of the binding partners. Numerous applications of these MCHs for noncovalent, high affinity, yet reversible tethering of spectroscopic probes and other functional elements to the recombinant proteins can be envisioned.