Photoreactive Compounds

Three major classes of photoreactive compounds have been used to label specific sites of proteins nitrenes, carbenes, and free radicals. Carbenes, generated by photolysis of diazo derivatives, are highly reactive and relatively nonselective in their target sites. Nitrenes, formed by photoactivation of azido derivatives, are less reactive than carbenes and more electrophilic, exhibiting a preference 

Some proteins specific for GTP will accept 8-N3-GTP, but not 8-N3-ATP, as a specific photoprobe. Tubulin provides an example in the dark, 8-N3-GTP substitutes for GTP in promoting tubulin polymerization, whereas photolysis leads to incorporation into the j8 subunit of the heterodimeric protein (224, 225). Protection against photoincorporation of 8-N3-GTP was provided specifically by GTP but not ATP, indicating that the decreased incorporation could not be attributed to the trivial effect of light absorption by the added nucleotides. Photoaffinity labeling of the G and G protein subunits of retinal rod outer segments was also effected by 8-N3-GTP (226). 

5-Azido-2 -deoxyuridine 5 -triphosphate (5-N3-dUTP) has been enzymically converted by E. coli DNA polymerase (in the dark) to a photoreactive DNA which was then used to photolabel the lac repressor (230). Alternatively, when 5-N3-dUTP was irradiated in the presence of DNA polymerase, it functioned as a photoaffinity label of the enzyme (231). The syntheses of several photoreactive 3 -arylazido derivatives of ATP (232) as well as of 9-(3 -azido-3 -deoxy-/3-D-xylofuranosyl)adenosine and -guanosine 5 -triphosphates (233) have been described. 

Despite the fact that they can be much more reactive than nitrenes, carbene derivatives are less frequently cited in reports on photoaffinity labels. Two such derivatives of thyroid hormone have been described in which the amino group of the alanine side chain of 3,5,3 -triiodo-L-thyronine or 3,5,3, 5 -tetraiodo-L-thyronine was derivatized with 2-diazo-3,3,3-trifluoropropionate (246). Irradiation at 254 nm of these compounds with several cell lines yielded low incorporation of reagent (0.5-13.5%) into two forms of thyroid hormone receptor proteins. Low incorporation was also characteristic of early reports of photoaffinity labeling (e.g., 247) in which 0 -ethyl-2-diazomalonyl-cAMP and A, C)2. (ji(ethyl-2-diazomalonyl)-cAMP were used to generate carbenes which modify rabbit muscle phosphofructokinase. Similarly, only 3-14% of the potential sites of lactate dehydrogenase were labeled by the carbene derived from 3-(3//-diazirino)pyridine adenine dinucleotide (248). 

---

Figure 11.15 The photoreactive compound psoralen-PEC>3-biotin can intercalate into double stranded DNA or RNA segments and covalently link to thymine bases via a photoreaction process.

Consider the elementary design of a photoacoustic calorimeter, shown in figure 13.3. The cell contains the sample, which is, for instance, a dilute solution of a photoreactive species. When the laser pulse travels throughout the cell, part of its energy is absorbed by the photoreactive compound and initiates the process of interest, while the remaining excess laser energy is deposited in the solution... 

Other coumarin-related compounds include furans, found in moldy sweet potatoes, and the furan coumarin complexes (furanocoumarins) found in parsnip leaves (psoralens). They are photoreactive compounds (primary photosensitizers) that exacerbate sunburn to psoralen exposed skin. 

Kearsley has used a simple numerical description for the overlap of the reactive orbitals with which to rationalize the reactivity of photoreactive compounds [80], More often than not, the relative orientation of the reacting partners are such... 

Fig. II Examples of MS-ffiendly photoreactive compounds used in mixed photoaffinity labeling. CsA cyclosporin A...

For photoreactive compounds such as PAHs, reaction in surface hlms can be their main degradative loss process in an urban system. Kwamena et al. (2004) and Poschl et al. (2001) found under experimental conditions that reactions of PAHs on organic-coated hlms exceeded those on atmospheric particulate matter (PM), the reason for which was not clear. Using these experimentally derived reaction rates in MUM, Kwamena et al. (2007)... 

As already mentioned above, spectral sensitization may also become indispensable when the light absorption properties of a potentially photoreactive compound do not permit direct excitation in the desired wavelength region. By application of sensitizers with adjusted excited state properties, it is, for example, possible to induce photochemical reactions of otherwise colorless compounds with visible light. Another important application in photochemistry is the sensitized population of excited state levels, which are not easily reached by direct absorption of light due to the limitations of quantum chemical selection rules. This phenomenon has been extensively exploited in mechanistic and synthetic organic photochemistry, where enhanced yields of triplet state population could be achieved in various dye-photosensitized processes (98). 

Rose bengal is a photoreactive compound.With excitation light it generates singlet oxygen, which may be responsible for its ability to kill microorganisms such as bacteria and viruses. 

Prochiral molecules may be subject to spontaneous resolution and can crystallize in chiral space groups [177], When such monocrystals, having the chirality of the crystal as the only source of asymmetry, are irradiated, chiral products with high percentages of ee can be formed. 2 + 2 Photocycloadditions [178] and di-7c-methane rearrangements [179] have thus led to spectacular results. Unfortunately, spontaneous resolution of photoreactive compounds in the solid state are very rare. 

Another way to induce chirality in crystals is to start with optically active materials made of achiral, photoreactive compounds containing polar groups, such as salts of carboxylic acids and optically active amines. These salts are forced to crystallize in chiral space groups and high ee values have been reported for such photoreactions [180]. 

Hindered amine light stabilizers (HALS), which represent more recent developments in photoreactive compounds and are referred to as scavengers, absorb light above 250 nm and therefore do not act as UV absorbers or quenchers. 

In this context, we have recognised that such supramolecular approach could be exploited in order to improve the yield of any photoreaction until near quantitative yield, based on the combination of multi-steps number grinding-irradiation cycles of the resulting mixture (product, remaining photoreactive compound and template). 

Novolacs are thermoplastic materials, that need to be transformed by crosslinking reactions to get insoluble, nonmelting, thermally stable thermosets. Different crosslinking agents have been used for thermal curing, for instance l,3,5,7-tetiaazatricyclo[3.3.3. P Jdecane (hexamethylenetetramine), oxiranes (epoxides), diisocyanates, urea, melamine, urea methanal resins, melamine metfaanal resins, and imide precursors. Furthermore, photoreactive compounds crosslink novolacs with methacrylate and methacryloylchloride resins, respeetively by itradialion [1]. 

Polymerization and polymer grafting to substrate surface is initiated by electron beam, plasma, gamma ray, and ultraviolet irradiations in the presence of monomer (and photoreactive compounds). 

Vinylarenes can undergo several reactions on irradiation depending on the substitution involved. Sometimes the photoreactive compound is bichromophoric, with the possibility of interaction between the vinyl group and another substituent. In some instances, supramolecular techniques can be applicable. An example of this is shown in Scheme 4. The excited singlet styrene moiety could react with the tertiary- or... 

---