Coordination of the six-mer peptide Gly-His-Pro-His-His-Gly to Cu II and Pd IIN-methyliminodiacetate complexes as IMAC chelation site models

Abstract

The coordination of a six-mer peptide Gly-His-Pro-His-His-Gly (GHPHHG) has been studied in its coordination behavior toward [Pd II(mida)(D 2O)] by 1H NMR, and toward [Cu II(mida)(H 2O) 2] by electronic spectroscopy (mida 2−= N-methyliminodiacetate), as models of immobilized metal ion affinity chromatography (IMAC) binding sites for proteins, peptides, and peptide affinity tags. The results are compared to two previously studied sequences used as high affinity peptide tags in the IMAC method, SPHHGG and HHHHHH, which have been observed to bind the model complexes using the terminal amino or histidyl donor of the first amino acid unit, followed by a skipping of the second amino acid unit and coordination of histidyl donors from both the third and fourth amino acid units. For the GHPHHG peptide at pH values near 8.53, it has been observed herein that the `high-binding' run of H *PH *H * (stars denoting attached groups) in the interior of the peptide is used to provide the three-point contact to the [M II(mida)] component. This is similar to the S *PH *H * coordination of SPHHGG, and the H *HH *H * coordination of HHHHHH. Models predict that steric factors for the six-mer peptide favor the use of the tautomeric coordination modes of the histidyl groups such that the histidyl methylene linkage is placed near the coordinating N donor for the first and fourth amino acid units and remote for the middle third amino acid unit, termed herein as the `4-5-4' tautomer forms. However, evidence from the absence of C-4H protons in the NMR spectrum suggests broadening due to the occurrence of rapid tautomerism between these sites. The proline in GHPHHG provides the structural rigidity to enhance the H *PH *H * binding mode, and this also shows up in the absence of peptide-ionized coordination to the Cu II(mida) model at pH<10, as for SPHHGG, but it occurs for HHHHHH having an available amido hydrogen on the second amino acid along the coordination sequence in the `skipped amino acid position' (both GHPHHG and SPHHGG do not and thus avoid peptide-ionized coordination).