Iridium(III) polypyridine artificial metalloenzymes with tunable photophysical properties: a new platform for visible light photocatalysis in aqueous solution

Abstract

Artificial metalloenzymes (ArMs) can combine the unique features of both metal complexes and enzymes by incorporating a cofactor within a protein scaffold. Herein, we describe a panel of ArMs constructed by covalently linking Ir(III) polypyridyl complexes into a prolyl oligopeptidase scaffold. Spectroscopic methods were used to examine how properties of the resulting ArMs are influenced by structural variation of the cyclometalated ligands and the protein scaffold. Visible light photocatalysis by these hybrid catalysts was also examined, leading to the finding that they catalyze inter/intra-molecular [2 + 2] photocycloaddition in aqueous solution. Low but reproducible enantioselectivity was observed using a cofactor that undergoes partial kinetic resolution upon bioconjugation within the ArM active site, showing the importance of scaffold/cofactor interactions for enabling selective ArM photocatalysis. Further evidence of the importance of cofactor/scaffold interactions was provided by analyzing native POP peptidase catalysis by the ArMs. Together, these studies show how Ir(III)-based ArMs constitute a promising starting point for ongoing studies to control the stereoselectivity of EnT reactions by engineering substrate binding/activation motifs in POP.

Fig. 1

(a) Chemical structures of Ir(iii) cofactors 1–5 and scheme for the formation of POP-Z53–1–5; (b) Schemes for visible-light-induced photocycloaddition reactions catalyzed by Ir(iii)-ArMs.

Fig. 2

(a) Normalized UV-vis absorption and photoluminescence (PL) spectra and (b) PL decay characteristics of 3 (50 μM) in 10% CH₃CN/H₂O and POP-Z53–3 (50 μM) in H₂O at room temperature. (c) Emission spectra of POP-Z53–1–5 (50 μM) in H₂O at room temperature. (d) Circular dichroism spectra of POP-Z53–2 (Δ/Λ/rac) in aqueous solution (10 or 50 μM).

Fig. 3

Kinetic analysis for the hydrolysis of Z-Ala-Pro-pNA by POP-Z53/POP-Z53–2/POP-Z53–5 (10 nM) at 85 °C with or without different amounts of (S)-1-Boc-2-cyanopyrrolidine. Using the Michaelis–Menten equation, a plot of initial rates (μM/sec) versus the substrate concentration (μM) was fitted.

Scheme 1

(a) Intermolecular [2 + 2] photocycloaddition of cinnamoyl imidazole (6) and 4-methoxy styrene (7). (b) Intramolecular [2 + 2] photocycloaddition of 3-(3-buten-1-yloxy)-2(1H)-quinolinone (9). Yields and diastereomer ratios (d.r.) were determined by UHPLC analysis using 1,3,5-trimethoxybenzene (TMB) or phenol as an internal standard. Reactions were performed in triplicate and are reported as averages with standard deviations. Enantiomeric excess was determined by using chiral ultra-performance liquid chromatography (UPLC) analysis.