Metal Ion-Terpyridine-Functionalized L-Tyrosinamide Aptamers: Nucleoapzymes for Oxygen Insertion into CH Bonds and the Transformation of L-Tyrosinamide into Amidodopachrome

A series of metal ion-terpyridine-modified L-tyrosinamide aptamers (M^{n +} = Cu²⁺ or Fe³⁺) act as enzyme-mimicking catalysts (nucleoapzymes) for oxygen-insertion into CH bonds and the transformation of L-tyrosinamide into amidodopachrome. The reaction proceeds in the presence of H₂O₂ and coadded L-ascorbic acid. In one series of experiments, the catalyzed oxidation of L-tyrosinamide to amidodopachrome by a set of nucleoapzymes consisting of Fe³⁺- or Cu²⁺-terpyridine complexes tethered directly or through a 4 × thymidine (4 × T) bridge, to the 5′- or 3′-end of the 49-mer L-tyrosinamide aptamer or to a shorter 23-mer L-tyrosinamide aptamer is examined. All nucleoapzymes reveal catalytic Michaelis–Menten enzyme-like activities and the separated Fe³⁺- or Cu²⁺-terpyridine and L-tyrosinamide aptamer units show only minute catalytic properties. The catalytic activities of the nucleoapzymes are attributed to the concentration of the L-tyrosinamide substrate by the aptamer units in proximity to the catalytic sites (K_d = (14 ± 0.1) × 10⁻⁶ m for all 49-mer catalysts and K_d = (2.5 ± 0.1) × 10⁻⁶ m and K_d = (0.8 ± 0.04) × 10⁻⁶ m for the 23-mer catalysts). Electron spin resonance experiments reveal that •OH radicals and ascorbate radicals participate in the transformation of tyrosine derivatives to catechol products. An autocatalytic feedback mechanism for the amplified generation of the two radicals is suggested.