Blue light-induced low mechanical stability of ruthenium-based coordination bonds: an AFM-based single-molecule force spectroscopy study

Abstract

Metal complexes containing coordination bonds play a prominent role in many essential biological systems in living organisms. Examples of such complexes include hemoglobin containing iron, chlorophyll containing magnesium, and vitamin B₁₂ containing cobalt. Although the thermodynamic and other collective properties of metal complexes are well established, their mechanical stability remains minimally explored. Single-molecule force spectroscopy has been used to determine the structural and mechanical properties of chemical bonds; however, it has been minimally utilized in the field of coordination chemistry. Thus, here, we select a unique molecule of interest, HA-Ru^II, {HA = hyaluronan and Ru^II = [(bpy)₂Ru(4-pyNH₂)₂](PF₆)₂} and subject it to single-molecule force spectroscopy analysis to directly study its bond-rupture process. The molecule is excited by blue-light irradiation, and surprisingly, this whole process could be reversed without applying any external energy, such as heat or solvent exposure. Our results demonstrate the reversibility of the luminescent Ru^II complex to its original state, a phenomenon that can be further applied to other coordination compounds.

Fig. 1. (A) The synthetic scheme of the HA–Ru^II complex. (B) Schematic representations of the HA–Ru^II complex, the scheme of single-molecule AFM experiments using the “multi-fishhook” approach. The Ru^II complex was conjugated to a hyaluronan polymer through amide bonds. In a typical simple experiment, the cantilever tip was brought into contact with the polymeric molecule, resulting in stretching, indicated by force-extension curves with sawtooth-like appearance. The individual force peaks in the curves correspond to the mechanical unfolding of individual domains, and the last peak corresponds to the stretching of the fully unfolded polymer chain and its subsequent detachment from either the AFM tip or glass substrate. Therefore, the last peak of each trace was excluded in the data analysis.

Fig. 2. (A and C) Two representative sawtooth-like force-extension traces at a pulling speed of 1000 nm s⁻¹ for the HA–Ru^II complex without (in black) and with continuous blue-light illumination (in blue), respectively, in a Tris buffer (containing 100 mM Tris, 50 mM NaCl, pH 7.2). The height of these peaks directly corresponds to the rupture force. Each peak corresponds to a rupture event between Ru^II and 4-iminopyridine. Red lines correspond to worm-like chain (WLC) fitting to the rupture events using the same persistence length of ∼0.4 nm, indicating single-molecule pulling events. The last peak corresponds to the stretching of the fully unfolded polymer chain and its subsequent detachment from either the AFM tip or the glass substrate. (B and D) The rupture-force histogram obtained at a pulling speed of 1000 nm s⁻¹ without (in black, n = 649) and with continuous blue-light illumination (in blue, n = 457), respectively.

Fig. 3. Single-molecule force spectroscopy experiments of the HA–Ru^II complex in the reversible condition. (A) Experimental series in the neutral condition. (B) Series during continuous blue-light illumination. (C) Series after turning off the blue light and proceeding with the experiment. All the experiments were performed with the same tip and sample traces at a pulling speed of 1000 nm s⁻¹ in a Tris buffer (containing 100 mM Tris, 50 mM NaCl, pH 7.2).

Fig. 4. (A) Plot of the rupture forces at different loading rates for the HA–Ru^II complex without (black) and with continuous blue-light illumination (blue), respectively, in a Tris buffer (containing 100 mM Tris, 50 mM NaCl, pH 7.2). The lines correspond to the fitting of the Bell–Evans model to the experimental data. The R² values (rupture force vs. logarithmic loading rate) for the HA–Ru^II complex are 0.932 and 0.922 without and with continuous blue-light illumination, respectively.

Fig. 5. The optimized geometry of the ground-state [Ru(4-Ampy)₂]²⁺.

Fig. 6. Natural transition orbitals for S₀ → S₅ of [Ru(4-Ampy)₂]²⁺.