Background: Rubisco activase (Rca) maintains carbon fixation, coordinates photosynthetic pathways, and regulates darklight transitions. Results: Positive cooperativity of ATP hydrolysis is afforded by ADP binding to oligomeric assemblies in the Rca-Mg-ATP-Mg state. Conclusion: Magnesium-mediated allosteric regulation involves the coexistence of ATP-and ADP-bound states. Significance: In dark-adapted chloroplasts, wasteful ATP hydrolysis may be prevented by low magnesium.

Interest in the design and development of artificial molecular muscles has inspired scientists to pursue new stimuli-responsive systems capable of exhibiting a physical and mechanical change in a material in response to one or more external environmental cues. Over the past few decades, many different types of stimuli have been investigated as a means to actuate materials. In particular, materials that respond to reduction and oxidation of their constituent molecular components have shown great promise on account of their ability to be activated either chemically or electrochemically. Here, we introduce a novel redox-responsive mechanism of actuation in hydrogels by describing a systematic investigation into the radical-based self-assembly of a series of unimolecular viologen-based oligomeric links, present at only 5 mol % of the polymer linkers in a three-dimensional network. The actuation process results in an overall reversible contraction of a family of hydrogels, down to 35% of their original volume in the first 25 min and ultimately to 9% after a few hours, even while remaining submerged in water. The mechanism of contraction starts with a decrease in electrostatic repulsion upon chemical reduction, leading to a loss of counterions and intramolecular self-assembly of the main-chain viologen subunits. The overall mode of actuation takes place relatively quickly in comparison to hydrogels of similar size, and the rate of contraction is accelerated as higher molecular weight oligoviologen links are implemented. The contraction process ultimately leads to a 2-fold increase in elasticity of the material, and upon exposure to oxygen and water, the hydrogels quickly oxidize and regain their original size and mechanical properties, thus resulting in a reversible actuation process that is capable of lifting objects which are 5–6 times heavier than the contracted hydrogel itself.

There is a growing interest in being able to control the mechanical properties of hydrogels for applications in materials, medicine, and biology. Primarily, changes in the hydrogel’s physical properties, i.e., stiffness, toughness, etc., are achieved by modulating the network cross-linking chemistry. Common cross-linking strategies rely on (i) irreversible network bond degradation and reformation in response to an external stimulus, (ii) using dynamic covalent chemistry, or (iii) isomerization of integrated functional groups (e.g., azobenzene or spiropyran). Many of these strategies are executed using ultraviolet or visible light since the incident photons serve as an external stimulus that affords spatial and temporal control over the mechanical adaptation process. Here, we describe a different type of hydrogel cross-linking strategy that uses a redox-responsive cross-linker, incorporated in poly­(hydroxyethyl acrylate)-based hydrogels at three different weight percent loadings, which consists of two viologen subunits tethered by hexaethylene glycol and capped with styrene groups at each terminus. These dicationic viologen subunits (V2+) can be reduced to their monoradical cations (V•+) through a photoinduced electron transfer (PET) process using a visible light-absorbing photocatalyst (tris­(bipyridine)­ruthenium­(II) dichloride) embedded in the hydrogel, resulting in the intramolecular stacking of viologen radical cations, through radical–radical pairing interactions, while losing two positive charges and the corresponding counteranions from the hydrogel. It is shown how this concerted process ultimately leads to collapse of the hydrogel network and significantly (p < 0.05) increases (by nearly a factor of 2) the soft material’s stiffness, tensile strength, and percent elongation at break, all of which is easily reversed via oxidation of the viologen subunits and swelling in water. Application of this reversible PET process was demonstrated by photopatterning the same hydrogel multiple times, where the pattern was “erased” each time by turning off the blue light (∼450 nm) source and allowing for oxidation and reswelling in between patterning steps. The areas of the hydrogel that were masked exhibited lower (by 1–2 kPa) shear storage moduli (G′) than the areas that were irradiated for 1.5 h. Moreover, because the viologen subunits in the functional cross-linker are electrochromic, it is possible to visualize the regions of the hydrogel that undergo changes in mechanical properties. This visualization process was illustrated by photopatterning a larger hydrogel (∼9.5 cm on its longest side) with a photomask in the design of an array of stars.

The use of light to actuate materials is advantageous because it represents a cost-effective and operationally straightforward way to introduce energy into a stimuli-responsive system. Common strategies for photoinduced actuation of materials typically rely on light irradiation to isomerize azobenzene or spiropyran derivatives, or to induce unidirectional rotation of molecular motors incorporated into a 3D polymer network. Although interest in photoredox catalysis has risen exponentially in the past decade, there are far fewer examples where photoinduced electron transfer (PET) processes are employed to actuate materials. Here, a novel mode of actuation in a series of redox-responsive hydrogels doped with a visible-light-absorbing ruthenium-based photocatalyst is reported. The hydrogels are composed primarily of polyethylene glycol and low molar concentrations of a unimolecular electroactive polyviologen that is activated through a PET mechanism. The rate and degree of contraction of the hydrogels are measured over several hours while irradiating with blue light. Likewise, the change in mechanical properties-determined through oscillatory shear rheology experiments-is assessed as a function of polyviologen concentration. Finally, an artificial molecular muscle is fabricated using the best-performing hydrogel composition, and its ability to perform work, while irradiated, is demonstrated by lifting a small weight.

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