Photochemical reagents

Photochemical reagents have been devised for crosslinking both soluble and particulate proteins (Chapter 5). In a recent study, Johnson et al. (1981) were able to crosslink radiolabeled glucagon to its receptor, merely by adding a bifunctional reagent that first reacted with amino groups and could subsequently be induced to form crosslinks by photolysis. 

Nitrophenyl ethers capable of undergoing photochemical nucleophilic aromatic substitution (Cornelisse et al., 1975, 1979 Comelisse and Havinga, 1975 Havinga and Comelisse, 1979) are recent additions to the list of photochemical reagents (Jelenc et al., 1978 Fig. 2.8). 

An inexpensive commercial apparatus or equipment adapted from other uses usually makes a light source suitable for most experiments with photochemical reagents. I have found the Rayonet Model RMR-400 minireactor to be very useful. The apparatus comprises a single lamp (26 cm... 

In a recent instructive example Rinke et al. (1980) used azidoaiylimidates to form RNA-protein crosslinks in E. coli ribosomes. Five photochemical reagents were tested and only one, methyI-4-azidophenylacetimidate, was found to give a useful extent of crosslinking. Ribosomes (A260 20 U/ml) were reacted with the imidoester (5 mM) in the dark, at pH 8 to 9, for 30 min at 37 °C. The ribosomes were then precipitated with ethanol and redissolved in buffer at a somewhat lower concentration (5 U/ml) before irradiation. The analysis was greatly aided by the use of biosynthetically... 

In recent years there has been considerable interest in the structure of biological membranes, and photochemical reagents capable of yielding low resolution structural information about membrane proteins have been developed. Several examples of photochemical surface-labeling reagents have appeared, and much effort has been devoted to the development of photoactivatable hydrophobic reagents for labeling from within the lipid bilayer. 

This list has been divided into two parts (i) suppliers of photochemical equipment and (ii) suppliers of photochemical reagents. The readers attention is also drawn to five useful directories which can be found in most libraries. 

Fortunately, the experiments that can be done with photochemical reagents are too varied (and unpredictable) to allow the writing of a true laboratory manual. Instead I have tried to give an account of the possible experiments (Chapter 1), a description of the reagents that can be used (Chapter 2), and a discussion, rather than detailed protocols or dogmatic assertions, of how the experiments can be done (Chapters 3 to 6). In addition, the extensive bibliography of over 400 references will provide access to useful examples in the primary literature. 

This monograph is an expansion and revision of the review I wrote earlier with my mentor Jeremy Knowles (Bayley and Knowles, 1977). Jeremy s earlier short review remains an excellent summary of the essential idea of photoaffinity labeling. Another review that will remain valuable is from Frank Westheimer s laboratory where photoaffinity labeling was invented (Chowdhry and Westheimer, 1977). Westheimer has also written a delightful short history and prospectus of the subject (Westheimer, 1980). Several additional reviews concerning special aspects of photochemical reagents are cited in this text. 

Benzophenone (Amax = 340 nm, log e = 2.5, n-ir electronic transition) can be used as a photochemical reagent and eq. 4.25 shows a radical Michael-addition reaction with benzophenone. The formed benzophenone biradical (triplet state, Tx) abstracts an electron-rich a-hydrogen atom from methyl 3-hydroxypropanoate (62) to generate an electron-rich a-hydroxy carbon-centered radical [III], then its radical adds to the electron-deficient (3-carbon of a, (3-unsaturated cyclic ketone (63) through the radical Michael addition. The electrophilic oxygen-centered radical in the benzophenone biradical abstracts an electron-rich hydrogen atom from methyl 3-hydroxypropanoate (62) [70]. So, an a-hydroxy carbon-centered radical [III] is formed, and an electron-deficient a-methoxycarbonyl carbon-centered radical [III7] is not formed. 

We would expect similar effects to show up in bimolecular reactions. A transition state in which reagent A of a reacting All pair is solvated by a strongly or rapidly adsorbing site will be different from one in which B is solvated by such a site. This means that the rates, and perhaps even the products, of a bimolecular reaction on a surface could depend on which reagent is applied first. In photochemical reactions, the result might depend on whether the photochemical reagent or a quencher is applied first. 

Neckers, D.C. (1988) Properties of Polymeric Rose Bengals - Polymers as Photochemical Reagents, in Synthesis and Separations Using Functional Polymers (eds D.C. Sherrington and P. Hodge), John Wiley Sons, Inc., New York, pp. 209-26. 

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