Binding of Stimuli-Responsive Ruthenium Aqua Complexes with 9-Ethylguanine

Abstract

Stimuli-responsive ruthenium complexes proximal- and distal-[Ru(C₁₀tpy)(C₁₀pyqu) OH₂]²⁺ (proximal-1 and distal-1; C₁₀tpy = 4'-decyloxy-2,2':6',2″-terpyridine and C₁₀pyqu = 2-[2'-(6'-decyloxy)-pyridyl]quinoline) were experimentally studied for adduct formation with a model DNA base. At 303 K, proximal-1 exhibited 1:1 adduct formation with 9-ethylguanine (9-EtG) to yield proximal-[Ru(C₁₀tpy)(C₁₀pyqu)(9-EtG)]²⁺ (proximal-RuEtG). Rotation of the guanine ligand on the ruthenium center was sterically hindered by the presence of an adjacent quinoline moiety at 303 K. Results from ¹H NMR measurements indicated that photoirradiation of a proximal-RuEtG solution caused photoisomerization to distal-RuEtG, whereas heating of proximal-RuEtG caused ligand substitution to proximal-1. The distal isomer of the aqua complex, distal-1, was observed to slowly revert to proximal-1 at 303 K. In the presence of 9-EtG, distal-1 underwent thermal back-isomerization to proximal-1 and adduct formation to distal-RuEtG. Kinetic analysis of ¹H NMR measurements showed that adduct formation between proximal-1 and 9-EtG was 8-fold faster than that between distal-1 and 9-EtG. This difference may be attributed to intramolecular hydrogen bonding and steric repulsion between the aqua ligand and the pendant moiety of the bidentate ligand..

Scheme 1. Photoisomerization and Thermal Back-Isomerization between proximal-1 and distal-1

Scheme 2. Top: Synthetic Route (4 Steps) of C₁₀pyqu Starting from 2-Acetylpyridine,

Middle: 2-step syntheses of C₁₀pyqu in this work starting from 6-bromo-2-acetylpyridine. Bottom: synthetic route for Ru complexes. Adapted in part with permission from M. Hirahara (2016). Visible-light-induced morphological changes of giant vesicles by photoisomerization of a ruthenium aqua complex. Chem. Eur. J.2016, 22, 2590–2594. Copyright 2016 Wiley-VCH GmbH.

Figure 1

(A) Absorption spectral changes of proximal-1 (29 μM) in the presence of 9-EtG (570 μM, 20 equiv) at 303 K. Solvent: H₂O/CH₃OH/2,2,2-trifluoroethanol (TFE) = 70:50:2, v/v/v. (B) Absorbance changes at 550 nm (black dot) during the reaction and fitting curve (solid line) based on the pseudo-first-order reaction model. (C) Residual plots for the fitting curve. (D) Concentration dependency of the observed rate constants (k_obs) for the ligand substitution reaction of proximal-1 with 9-EtG at 303 K.

Figure 2

ESI-MS spectra of [Ru(C₁₀tpy)(C₁₀pq)(9-EtG)]²⁺. Conditions: proximal-1 (0.1 mM) and 9-ethylguanine (2 mM) in mixed solvents (H₂O/methanol/TFE = 70/50/2, v/v/v). The sample solution was left at room temperature in the dark for 24 h to complete the ligand substitution reaction. Peaks at 381.20 and 739.40 m/z correspond to the ionized species of [(9-EtG)₂ + Na]⁺ and [(9-EtG)₄ + Na]⁺, respectively. Inset: observed spectra at 516 m/z and calculated isotope pattern of [Ru(C₁₀tpy)(C₁₀pq)(9-EtG)]²⁺.

Figure 3

¹H NMR spectra of (A) proximal-1 (B) 10 min after mixing proximal-1 (1 mM) with 9-EtG (5 mM) and (C) proximal-1 and 9-EtG after incubation for 1 day. Peaks associated with proximal-1 at 6.64 ppm and proximal-RuEtG at 6.93 ppm are colored gray and red, respectively. In (C), peaks corresponding to aromatic protons of C₁₀tpy of proximal-RuEtG are highlighted with yellow. Solvent: D₂O/CD₃OD/2,2,2-trifluoroethanol (TFE) = 70:50:2, v/v/v. The singlet at 7.8 ppm corresponds to the C–H proton of the free 9-EtG.

Figure 4

Kinetic traces of proximal-1 (1 mM) during the substitution reaction with 5 mM 9-EtG at 303 K. Red: proximal-RuEtG; black: proximal-1. The concentrations of the complexes were calculated by integrating peaks at 6.92 ppm (proximal-RuEtG) and 6.63–6.70 ppm (proximal-1 and proximal-RuEtG).

Figure 5

(A) Absorption spectral changes of proximal- and distal-1 (28.3 μM) in the presence of 9-EtG (566 μM, 20 equiv) at 303 K. (B) Kinetic profile (black dot) during the reaction and fitting curve (red line) based on the double exponential curve. (C) Residual plots for the fitting curve. Solvent: D₂O/CD₃OD/TFE = 70:50:2, v/v/v.

Figure 6

¹H NMR spectral changes during the substitution reaction of proximal-1 and distal-1 with 9-EtG in the dark at 303 K. (A) Proximal-1 and distal-1 in the photostationary state. (B) Proximal-1 and distal-1 (1 mM) after 15 min of mixing with 9-EtG (5 mM). (C) Proximal-1 and distal-1 after 4 h of mixing with 9-EtG. (D) Proximal-RuEtG. Solvent: D₂O/CD₃OD/TFE = 70:50:2, v/v/v.

Figure 7

Kinetic traces of proximal-1 and distal-1 (total concentration: 1 mM) during the substitution reaction with 9-EtG (5 mM) at 303 K. Red: proximal-RuEtG; blue: distal-RuEtG; magenta: distal-1; black: proximal-1. The concentration of each complex was calculated based on the integration of peaks at 8.90 ppm (distal-RuEtG), 8.80 ppm (proximal-1), 6.92 ppm (distal-1 and proximal-RuEtG), and 6.63–6.70 ppm (proximal-1, proximal-RuEtG and distal-RuEtG). The solid lines are simulated curves according to first-order kinetics based on ligand substitution (proximal-1 to proximal-RuEtG, distal-1 to distal-RuEtG) and thermal isomerization (distal-1 to proximal-1) reactions.

Scheme 3. Binding Reactions with 9-EtG for the Stimuli-Responsive Ruthenium Complexes proximal-1 and distal-1

The insets show steric repulsion between the aqua ligand and quinoline moiety in proximal-1 and intramolecular hydrogen bonding between the aqua ligand and alkoxy unit in distal-1.

Figure 8

¹H NMR spectra of (A) proximal-RuEtG, (B) proximal- and distal-RuEtG, and (C) proximal-RuEtG after visible light irradiation for 16 h. Representative peaks of distal-RuEtG are highlighted as yellow. Solvent: D₂O/CD₃OD/TFE = 70:50:2, v/v/v.