Photoisomerizables

I. Willner, R. Blonder, and A. Dagan, Application of photoisomerizable antigenic monolayer electrodes as reversible amperometric immunosensors. J. Am. Chem. Soc. 116, 9365-9366 (1994). 

I. Willner and B. Willner, Electronic transduction of photostimulated binding interactions at photoisomerizable monolayer electrodes novel approaches for optobioelectronic systems and reversible immunosensor devices. Biotechnol. Prog. 15, 991-1002 (1999). 

An interesting example of a system whose components become mechanically bound upon surface immobilization is rotaxane trans-21 (Fig. 13.24), consisting of a ferrocene-functionalized /3-cyclodextrin (fi-CD) macrocycle threaded on a molecule containing a photoisomerizable azobenzene unity and a long alkyl chain.33 A monolayer of trans-21 was self-assembled on a gold electrode. Therefore, the ring... 

Several attempts have been made to construct stimuli-responsive polymer solution and gel systems which undergo isothermal phase transitions by external stimulation, such as photons or chemicals. Aqueous solutions of poly(A -isopropylacrylamide) having photoisomerizable chromophores or host molecules in the pendant groups showed reversible phase separations by photoirradiation or by the addition of specific metal or ammonium ions. The gels made of the polymers also underwent photostimulated or chemical-induced volume phase transitions. 

So far, various kinds of polymers which change their conformation reversibly by photoirradiation have been reported [1-6]. The polymers contain pendant or backbone photoisomerizable chromophores, and the molecular property changes, such as geometrical structure or dipole moment changes, control the conformation. The polymers change their conformation in proportion to the number of photoisomerized chromophores. Thus, when the polymers contain more photoisomerizable chromophores and absorb more photons, the conformation changes more. Physical and chemical properties associated with the conformation changes also vary with the number of absorbed photons. 

The concept of photostimulated phase separation can be applied to construct chemical-induced phase transition systems, which change the conformation reversibly in response to special chemicals. For the systems, host molecules are used as the receptor groups instead of photoisomerizable chromophores. Host molecules, such as crown ethers or cyclodextrins, are known to change the property by capturing guest chemicals in their cavity [16]. We employed benzo[l 8]crown-6 as the receptor molecule and incorporated it into the pendant groups of PNIPAM. 

Intermediate 10 turned out to be easily photoisomerizable yielding the stable acylsi-lane 11. According to CCSD(T)/6-31G(d,p)//MP2/6-31G(d,p) calculations23 carbene 10 originates from the initial complex 9. 

Silyldisilene, which could be photoisomerizable into the likewise unknown cyclotrisilane, has also not yet been observed69. 

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... 

Scheme2 Methods forthe reversible photoactivation/deacti-vation of biomaterials by (A) tethering of photoisomerizable groups onto the biomaterial, (B) immobilization ofthe biomaterial in a photoisomerizable matrix, (C) the application of a photoisomerizable inhibitor (or photoisomerizable cofactor).

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-... 

Scheme 3 Synthesis of photoisomerizable Concanavalin A by the chemical linkage of photoisomerizable thiophene fulgide or nitrospiropyran residues to the protein.

Tab. 1 Association constants of 5 to the photoisomerizable Concanavalin A systems (3)-Con.A and (4)-Con.A. as a function of the degree of loading.

Fig. 3 Reversible photostimulated binding and dissociation of the photoisomerizable hapten 6 to and from the Z1 HOI monoclonal antibody, respectively. Vand U indicate irradiation with visible and UV light, respectively. Visible...

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... 

Scheme 4 Synthesis and photoisomerizable properties of azobenzene-functionalized papain.

The physical and chemical properties of photoisomerizable molecular films or photoisomerizable polymers are controlled by light. Photochemical control of the formation of liquid crystal phases, or sol-gel transitions,137 381 of polymers containing photoisomerizable components demonstrates signal regulation of the structure and properties of microscopic and macroscopic phases. Physicochemical properties of photoisomerizable membrane-mimetic assemblies such as liposomes,1241 mono-... 

B) Schematic presentation of the a-helix/random-coil transition of the photoisomerizable polymer 12. 

Fig. 6 Energy-minimized structures of a photoisomerizable cyclic azobenzene polypeptide.

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... 

Fig. 7 Schematic structural transformation of the photoisomerizable polyglutamic acid 16.

Scheme 6 Photoswitching of a-chymotrypsin through its immobilization in photoisomerizable polymers.

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