Light induced activation

The first examples for photocatalyzed ROMP (PROMP) using ruthenium initiators were introduced by Miihlebach et al. [33] in 1995. The catalysts systems were based on three different types r 6-arene half-sandwich complexes 51, sandwich 

The disadvantages of these PROMP catalyst systems are the overall moderate activity and the long periods (generally between 1 and 3 days) required to obtain reasonable yields. Another subject that must be mentioned is the need for a protic polar solvent (EtOH, HjO) in order to dissolve the cationic active species. While in some applications the use of these solvents may be viewed as advantageous, it 

Buchmeiser argued that for practical applications the complexes must be stable and completely latent in the presence of the monomer at room temperature and switch to active state only at elevated temperatures or by UV irradiation. In order 

Another important demonstration of indirect activation by irradiation was reported by Frechet et al. [49]. In this approach, the Grubbs second-generation ruthenium catalyst (5wt% solution in toluene) was encapsulated in carbon nanotubes (CNTs) and dispersed in neat DCPD for weeks without any noticeable increase in viscosity. IR laser irradiation resulted in the rupture of the capsules and concomitant polymerization of DCPD, leading to intense gelling within minutes. 

---

Presently photodynamic therapy (PDT) is considered as a perspective way for therapy of different diseases, including cancer. PDT is a method based on the local light-induced activation of photosensitizers able to accumulate selectively in energy-deficient cells (malignant or dysplastic ones) not influencing the remaining normal cells of the body (Pass, 1993). Upon the action of irradiation of certain wavelength characteristic for individual photosensitizer, photochemical reaction occurs... 

Table 2 summarizes different possible applications of photoswitchable biomaterials, while detailing the nature of the biomaterial, the area of application, and, when possible, specific examples. Reversible light-induced activation and deactivation of redox proteins (enzymes) corresponds to write - read - erase functions. The photonic activation of the biomaterial corresponds to the write function, whereas the amperometric transduction of the recorded optical information represents the read function of the systems. Switching off of the redox functions of the proteins erases the stored photonic information and regenerates the photosensory biomaterial. These integrated, photoswitchable redox enzyme electrode assemblies mimic logic functions of computers, and may be considered as first step into the era of biocomputers. 

Photoswitchable enzymes could have an important role in controlling biochemical transformations in bioreactors. Various biotechnological processes generate an inhibitor, or alter the environmental conditions (pH, for example) of the reaction medium. Photochemical activation of enzymes that adjust environmental conditions or deplete the inhibitor to a low concentration may maintain the bioreactor at optimal performance. More specifically, integration of the photoswitchable biocataly-tic matrix with a sensory electrode might yield a feedback mechanism in which the sensor element triggers the light-induced activation/deactivation of the photosensitive biocatalyst. 

Figure 32.25. Visual Sigual Transductiou. The light-induced activation of rhodopsin leads to the hydrolysis of cGMP, which in turn leads to ion channel closing and the initiation of an action potential.

FIG. 10.3 Light-induced activation of second-order NLO properties based on aggregation of photomerocyanine (right). This product is obtained from photochromic nitro-BIPS type spiropyran (left) (from ref. 53). 

There is another large class of receptors whose occupancy by an agonisf leads to inhibition of adenylate cyclase. These include the tt2 adrenergic receptors, receptors for acetylcholine, adenosine, prostaglandin E2 (Chapter 21), somatostatin, and some receptors for dopamine. Their responses are mediafed by inhibitory proteins Gj, which closely resemble Gg in their sizes, amino acid sequences, and heterotrimeric structures, but which inhibit adenylate cyclase when activated. A clear distinction between the Gg and Gj proteins is evident in the fact that Gg is irreversibly activated by the action of cholera toxin, while G loses its ability to respond to occupied receptors when modified by the action of Pertussis toxin (Box 11-A). A specialized heterotrimeric G protein known as transducin mediates the light-induced activation of a cyclic GMP phosphodiesterase in the retina " (see Chapter 23). Its a subunit is designated aj. The related gustducin is found in taste buds. ... 

Four molecules of IPP, one of which is an isomer produced by IPP isomerase (IPPI) are combined to produce the twenty-carbon isoprenoid, GGPP (geranylgeranyl pyrophosphate). This step is mediated by GGPP synthase (GGPPS). Light-induced activation of IPPI has been associated with the phytochrome-... 

Light-induced activation of second-order NLO properties... 

The mechanism of photopolymerization in many application, generally involves the light-induced activation of a sensitizer material, such as an aromatic ketone, for example, Michler s ketone or a polyaromatic, to its singlet state, followed by crossover to form its triplet-excited state, followed by energy transfer to an initiator. The activated initiator usually splits into two radicals, one or both of which react witir tire monomer causing polymerization to occur. Norrish type I and type II reaction mechanisms are examples of this science [3]. Acrylates and methacrylates are commonly the monomers of choice because of their high polymerization rate, low cost, and acceptable toxicity (Figure 7.5). Additionally, acrylates and methacrylates... 

The work done in Ziegler s laboratory has shown that irradiation of green leaves and isolated chloroplasts causes a rapid and reversible increase (up to 4,5-fold) in the activity of the NADP-dependent GAld3P dehydrogenase. The in vivo light-induced activation of the enzyme is closely connected with the operation of the non-cyclic electron flow of photosynthesis, as shown by experiments with inhibitors. NADPH causes a specific activation (up to about 10-fold) of the inactive dark-enzyme , and ATP is also a similarly eflFective activating agent in vitro. Anderson has achieved the conversion of one electrophoretic-... 

Muller, B. (1970) On the mechanism of the light-induced activation of the NADP-dependent glyceral-dehyde phosphate dehydrogenase. Biochim. Biophys. Acta, 205,102-109. 

Fu Y, Sun D, Chen Y, Huang R, Ding Z, Fu X, Li Z (2012) An amine-functionalized titanium metal-organic framework photocatalyst with visible-light-induced activity for CO2 reduction. 

---