Photochemical activation functions

There have been few studies to date of the functionality and stability of AP-trapped photosynthetic reaction centers. Rhodobacter sphaeroides reaction centers were shown to remain intact following trapping with AP A8-75 (a more highly charged analog of A8-35), but neither their functionality nor their stability over time were studied[5]. Synechocystis PCC 6803 PS1 reaction centers trapped with A8-35 and deposited on a gold electrode have been shown to be electrochemically active, but their long-term stability has not been studied[12]. The photochemical activity of A8-35-trapped pea PS2 reaction centers, measured at room temperature by the accumulation of the pheophytin free radical upon illumination, was found to be intermediate between that in chaps and in P-DM solutions [A. Zehetner H. Scheer, personal communication ref. 13],... 

Several groups have investigated three-component systems encompassing both chemical amplification and dissolution inhibition. As stated earlier, Smith and Bonham (63) reported resist materials composed of a binder resin (novolac), a nonpolymeric compound containing acid-labile functional groups such as acetals, and a trihalomethyl-substituted 5-triazine acid photogenerator. The acid-labile compound acts as a novolac dissolution inhibitor in a manner analogous to the action of DNQ in conventional positive resists. However, in this case, the inhibitor is not photochemically active. Instead,... 

Porphyrin synthesis and functionalization based on the chemistry of Mannich bases, briefly mentioned in previous chapters, are recalled here. As far as porphyrin synthesis is concerned, studies of biomimetic models of photochemically active reaction centers are worth noting. The synthetic procedure involves amino group replacement of the pyrrole bis-Mannich base with formation of the tetrapyrrole ring of porphyrin (see 360, Chap. 11). 

Initial reports on the borylation of alkanes using isolated transition-metal-boryl complexes date back to 1995, when Hartwig showed that Cp Re(CO)2(Bpin)2 converts pentane to 1-borylpentane with high regioselectivity. " The catalytic C-H borylation of alkanes with Cp Re(CO)3 using photochemical activation was demonstrated soon thereafter (equation 25). Also, an efficient thermal process that involves the use of rhodium catalysts has since been developed (equation 26). It is interesting to note that this methodology is not restricted to small molecules, but has recently been exploited for the direct side-chain functionalization of polyolefins. ... 

Photolysis of o-Nitrobenzyl Derivatives - The cleavage of o-nitrobenzyl derivatives is one of the relatively few classes of photofragmentations in which the two fragments can both bear complex functionality. The protection of alcohols as o-nitrobenzyl ethers is well known, and provides a photochemical method of deprotection, but other applications include the design of photoclea-vable polymers and photochemically active links for molecules synthesized on polymer beads. 

A detailed discussion of the binding of specific amino acids to the various cofactors will be deferred to later chapters. The concept of subunits DI and D2 of PS II being the structural and functional analogues of the L- and M-subunits of photosynthetic bacteria has clearly gained ample support since it was first proposed and has been reinforced by the timely preparation of a photochemically active PS-II reaction-center particle composed of DI, D2 and Cyt b559 by Nanba and Satoh in 1987. 

Malkin used two methods to assay the photochemical activity of both the intact PS 1-200 complex and the complex that had lost one phylloquinone. One method measured oxygen uptake by using methyl violo-gen as the acceptor photoreduced by photosystem I, supported by plastocyanin and reduced DCIP as the electron donors. The other method measured the amount of FeS-A photoreduced at 15 K by EPR spectroscopy. It turned out that both the extracted and unextracted samples gave nearly identical results, namely -500 p,M Oj/mg Chl /i, and the EPR spectra showed nearly identical amounts of FeS-A reduced on an equivalent chlorophyll basis. These results thus indicated that there is one nonfunctional phylloquinone that is easily removed by extraction with an organic solvent and one tightly bound, functional phylloquinone that is presumably present in a highly hydrophobic environment and which was subsequently identified as an intermediary electron carrier in the acceptor chain of photosystem I. 

Finally, in 1979 Florton reported the synthesis of 3-amino-2,3-dideoxyhexu-ronic acids. In six steps, methyl a-D-maimopyranoside was converted to a highly functionalized 3-acetamido-6-azido-2,3,6-trideoxy derivative. Photochemical activation of the azide provided an imine, which was subsequently hydrolyzed to the aldehyde. Bromine was used to oxidize the aldehyde to the acid, which was esterified with methyl iodide (Fig. 9). 

Some molecular components can adopt several different states as a function of several inputs [28], These compounds are of particular interest for their potential as processing devices. Flavylium salts, for instance, interconvert between a bewildering array of isomers as a result of pH, photonic and thermal inputs [29]. A more elegant example is the interconversion of 5, 6, 7-tri-tert-butyl-1, 4-anthraquinone (11) between itself and three isomeric states (12, 13, 14) by photochemical and electrochemical means (Fig. 6) [30]. This system could be considered a dyad of an electrochemically active quinone and a photochemically active tri-ferf-butyl-substituted polyacene, but, as the conjugation of the entire system is important for its properties, it is treated as a single unit. Each state has a different absorbance spectrum, and the spectral changes for each transformation were measured (Fig. 6). Unfortunately, the conditions required for the electrochemical and photochemical transformations differ, so a single sample cannot be readily interconverted between all four states. 

Fig. 2-18. Photodissociation coefficients for several photochemically active atmospheric trace components as a function of altitude. Data for H20 are from Park (1974) for an overhead sun, for N20 and HNOj from Johnston and Podolske (1978) for global average conditions, for CF2C12 from Rowland and Molina (1975) for an overhead sun, and for H202, N205, HN02, and N02 from lsaksen et al. (1977) for 60° solar zenith angle.

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