Dissecting the general physicochemical properties of noncovalent interactions involving tyrosine side chain as a second-shell ligand in biomolecular metal-binding site mimetics: an experimental study combining fluorescence, 13C NMR spectroscopy and ESI mass spectrometry.

Detailed physicochemical features inherent in the dynamic cation-pi interactions of aromatic amino acid side chains in the secondary coordination spheres around metal ions were extracted and mapped by intrinsic tyrosine fluorescence titration experiments with two homologous, artificially engineered metal-binding scaffolds which mimic metal-binding sites in metalloproteins. A newly formulated method for the treatment of fluorescence titration data allows straightforward assessment of both the magnitudes and properties of metal-chelation-assisted cation-aromatic interactions (K2) underlying a proposed two-step metallosupramolecular association process. The unprecedented linear platform-motif correlations between the two contrasting scaffolds in their changes in tyrosine fluorescence on binding of 3d metal cations help to elucidate the properties of general cation-arene recognition corresponding to the metal-responsive characteristics of the second-shell Tyr residue surrounding the metal-binding sites in the supramolecular context, and thereby define a new noncovalent design principle for metal-ion recognition in aqueous solution.