Coordination of the six-mer peptide Gly-His-Pro-His-His-Gly to Cu-II and (PdN)-N-II-methyliminodiacetate complexes as IMAC chelation site models

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) (D2O)] by H-1 NMR, and toward [Cu-II(mida)(H2O)(2)] by electronic spectroscopy (mida(2-) = N-methyliminodiacetate), as models of immobilized metal ion affinity chr omatography (IMAC) binding sites for proteins, peptides, and peptide affini ty 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 th ird 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* (st ars denoting attached groups) in the interior of the peptide is used to pro vide 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 HHH HHH. Models predict that steric factors for the six-mer peptide favor the u se of the tautomeric coordination modes of the histidyl groups such that th e histidyl methylene Linkage is placed near the coordinating N donor for th e first and fourth amino acid units and remote for the middle third amino a cid unit, termed herein as the '4-5-4' tautomer forms. However, evidence fr om 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 h aving an available amido hydrogen on the second amino acid along the coordi nation sequence in the 'skipped amino acid position' (both GHPHHG and SPHHG G do not and thus avoid peptide-ionized coordination). (C) 1999 Elsevier Sc ience Inc. All rights reserved.