Artificial tripeptides containing three pendant ligands, hydroxyquinoline (hq) and/or bipyridine (bpy), were synthesized with three different sequences, hq-hq-hq, hq-bpy-hq and bpy-hq-bpy. Separate reaction of these tripeptides with Zn(II) led to formation of Zn(II) coordinative cross-links to give tripeptide duplexes with metal complexes. The stoichiometry of binding was examined using spectrophotometric absorbance and emission titrations; titrations monitoring formation of the Zn3(hq-tripeptide)2 duplex had an equilibrium constant larger than the monomeric analogue, and a Hill coefficient >1, indicating positive cooperativity. NMR spectroscopy and mass spectrometry confirmed the identity of products. Comparison of the photophysical behaviors of these duplexes indicated sequence-dependent properties. A decrease of emission intensity was observed in Zn tripeptide duplexes containing bpy compared to the hq-hq-hq tripeptide. Dexter energy transfer between metal centers is hypothesized to be the fluorescence quenching mechanism, and is supported by variable-temperature and solvent studies.The stabilities of tripeptides and duplexes were investigated using pH-dependent spectrophotometric titrations.

Zn(II) cross-linked artificial tripeptides with pendant heterofunctional ligands

245th National Meeting (2013)

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