Photoisomerizable unit

Three general methodologies for photoregulating such activities of biomaterials as catalytic, binding, or recognition functions have been suggested (Scheme 2). One method involves the tethering of photoisomerizable units to a protein (Scheme 2(A)). In one photoisomer state, state A, the tertiary structure of the protein is... 

Photoswitchable Biomaterial Functions through Tethering of Photoisomerizable Units to Proteins... 

Chemical modification of the biomaterial with photoisomerizable units represents one approach to controlling intermolecular affinity interactions (Scheme 2(A)). In one photoisomer state of the biomaterial, its tertiary, biologically active structure is retained and the formation of the intermolecular complex is facilitated. In the complementary photoisomer state, the bioactive binding site is distorted and the formation of the intermolecular recognition complex is switched off. The bind-... 

Tethering of photoisomerizable groups to enzymes has been used to photostimu-late the biocatalytic functions of proteins.134,351 Papain was modified by the covalent coupling of the photoisomerizable units trans-4-carboxyazobenzene (7), trans-3-car-boxyazobenzene (8), or trans-2-carboxyazobenzene (9) to the protein s lysine residues (Scheme 4). The new azobenzene-modified papains underwent reversible trans cis... 

Monolayers representing two-dimensional arrays of membrane-mimetic assemblies, consisting of azobenzene (poly-L-lysine) with 43 % loading of the photoisomerizable units, were prepared.1441 The compressed trans-azobenzene polymer mono-layer exhibited a surface pressure of 7 mN m 1, whereas photoisomerization of the monolayer to the cis-azobenzene state by UV light decreased the surface pressure to... 

Scheme 7 Electronic transduction of photo-switchable bioelectrocatalytic functions of proteins, (A) by the tethering of photoisomerizable units to the protein (R is a diffusional electron mediator that electrically contacts the redox...

The only example presently available of photochemically controllable ring motion through an electron transfer reaction in a catenane structure concerns a Cu+-based [2]catenate, which is discussed in Volume 111, Part 2, Chapter 8 [64], Examples of catenanes containing cis-trans photoisomerizable units and where ring motions can be photochemically controlled are also known [65]. 

Although the tethering of photoisomerizable units to the protein leads to photoswitchable bioelectrocatalytic properties, the OFF state of the photoisomerizable GOx exhibits residual bioelectrocatalytic activity as the structural distortion of the protein in the 4b-state is not optimized. 

The reconstitution method was suggested as a means to introduce the photoisomerizable unit into the vicinity of the biocatalytic redox-center, thereby generating a light-switchable bioelectrocatalyst that operates between fiilly switched ON and OFF states. Apo-glucose oxidase, apo-GOx, was reconstituted with the nitrospiropyran-FAD cofactor unit, (20), Fig. 3-30. 

It also was found that the direction of the photobiocatalytic switch of the nitrospiropyran-FAD-reconstituted enzyme is controlled by the electrical properties of the electron transfer mediator. With ferrocene dicarboxylic acid as a diffusional electron transfer mediator, the enzyme in the nitrospiropyran-FAD state (10a) was found to correspond to the OFF state biocatalyst, while the protonated nitromerocyanine state of the enzyme (10b) exhibits ON behavior. In the presence of the protonated l-[l-(dimethyl-amino)ethyl]ferrocene, the direction of the photobioelectrocatalytic switch is reversed. The nitrospiropyran-enzyme state (10a) is activated toward the electrocatalyzed oxidation of glucose, while the protonated nitromerocyanine enzyme state (10b) is switched OFF, and is inactive for the electrochemical oxidation of glucose. This control of the photoswitch direction of the photoisomerizable reconstituted enzyme was attributed to electrostatic interactions between the diffusional electron mediator and the photoisomerizable unit... 

Figure 4.7 Three methods for switching the activity of a biomaterial (a) tethering of photoisomerizable units to a protein (b) integrating the biomaterial within a photosensitive environment and (c) use of photoisomerizable inhibitors.

Photochemical Control by Enzyme-bound Photoisomerizable Units... 

The enzyme GOx was transformed into a photoswitchable enzyme by its chemical modification with nitrospiropy-ran photoisomerizable units [382]. The... 

Fig. 32 Cyclic voltammograms of an electrode bearing a monolayer of COx reconsitiuted with photoisomerizable dyad 28(a/b) in the presence of glucose (50 mM), ferrocene carboxylic acid (50 pM), and with the photoisomerizable units (a, c) in the spiropyran-state (28a), and (b, d) in the merocyanine-state (28b). Recorded in 0.01 M phosphate buffer, pH 7.3, scan rate 5 mV s . Inset switching behavior of the electrocatalytic current as a function of the state of the photoisomerizable group s and m represent the photoisomerizable units in the spiropyran and the merocyanine states, respectively.

Photonic control over electroactivated bio-catalytic processes can also be achieved by the use of electrode surfaces that are modified by photoisomerizable units. These interfaces, whose state controls the ability of a substrate to interact with them, are known as command surfaces [382, 386-390]. In one example, a... 

Cyt c (0.1 mM) at a mixed (29(a/b)) and pyridine monolayer-modified electrode in (a) the neutral spiropyran-state (29a) and (b) the positively charged merocyanine-state (29b), recorded at 50 mV s Inset switching behaviour of the Cyt c electrochemistry as a function of the state of the photoisomerizable group circles and squares represent the photoisomerizable units in the spiropyran and the merocyanine states, respectively (b) cyclic voltammetric response of Cyt c (0.1 mM) with COx (1 pM) at a mixed... 

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