Selective recognition of copper(II) by a water-soluble, emitter-receptor conjugate containing a ruthenium chromophore, a lysine bridge, and a cyclen unit

The amino acid derivative [H-Lys(Ru(bipy)(2)m)-cyclenH(2)](PF6)(5) (7; Lys= lysine, bipy= 2,2'-bipyridyl, m = 4-carbonyl-4'-methyl-2,2'-bipyridyl, cyclen = 1,4,7,10-tetraazacyclododecane) has been synthesized. A modular approach was taken which involves only standard amide-coupling methods well-known from peptide synthesis. Compound 7 is readily soluble in water. It contains a luminescent ruthenium chromophore and a cyclen ligand which serves as a binding site for metal ions. The emission of 7 is pH-independent but efficiently quenched by CU2+ ions in a pH range of 6-7. Copper(II) binding is reversible upon protonation of the ligand at pH values below 6. In contrast, no significant spectral changes are observed with Zn2+ and Ni2+, respectively. Thus, 7 selectively recognizes copper(II) in aqueous solution under slightly acidic to neutral conditions. Unfortunately, applications at higher pH values are limited by metal-promoted hydrolysis of 7 under mildly basic conditions.