Photoactivation amines

Sulfonamides are very difficult to hydrolyze. However, a photoactivated reductive method for desulfonylation has been developed.240 Sodium borohydride is used in conjunction with 1,2- or 1,4-dimethoxybenzene or 1,5-dimethoxynaphthalene. The photoexcited aromatic serves as an electron donor toward the sulfonyl group, which then fragments to give the deprotected amine. The NaBH4 reduces the radical cation and the sulfonyl radical. 

Figure 4.21 BASED can react with molecules after photoactivation to form crosslinks with nucleophilic groups, primarily amines. Exposure of its phenyl azide groups to UV light causes nitrene formation and ring expansion to the dehydroazepine intermediate. This group is highly reactive with amines. The cross-bridge of BASED is cleavable using a disulfide reducing agent.

Figure 5.16 Photoactivation of a phenyl azide group with UV light results in the formation of a short-lived nitrene. Nitrenes may undergo a number of reactions, including insertion into active carbon-hydrogen or nitrogen-hydrogen bonds and addition to points of unsaturation in carbon chains. The most likely route of reaction, however, is to ring-expand to a dehydroazepine intermediate. This group is highly reactive toward nucleophiles, especially amines.

Figure 5.17 NHS-ASA reacts with amine-containing compounds to form stable amide linkages. Photoactivation with UV light results in ring expansion to a dehydroazepine intermediate, which can react with amines to form covalent bonds.

Figure 5.18 SASD is a photoreactive crosslinker that can be used to modify amine-containing compounds through its NHS ester end and subsequently photoactivated to initiate coupling with nucleophiles (after ring expansion to an intermediate dehydroazepine derivative). The crosslinks may be selectively cleaved at the internal disulfide group using DTT.

Figure 5.22 The NHS ester of ANB-NOS reacts with amines to form amide bonds. Subsequent photoactivation of the complex with UV light causes phenyl azide ring expansion and reaction with neighboring amines.

Figure 5.24 SADP reacts with amines via its NHS ester end to produce amide bonds. The modified molecule then may be photoactivated to create a nucleophile-reactive dehydroazepine intermediate able to covalently couple with amine-containing compounds.

Figure 5.29 pNPDP reacts with amine-containing compounds by its p-nitrophenyl ester group to form amide bonds. After photoactivation of the diazo derivative with UV light, a Wolff rearrangement occurs to a highly reactive ketene intermediate. This group can couple to nucleophiles such as amines. 

Figure 5.35 ABH reacts with aldehyde-containing compounds through its hydrazide end to form hydrazone linkages. Glycoconjugates may be labeled by this reaction after oxidation with sodium periodate to form aldehyde groups. Subsequent photoactivation with UV light causes transformation of the phenyl azide to a nitrene. The nitrene undergoes rapid ring expansion to a dehydroazepine that can couple to nucleophiles, such as amines.

Figure 5.37 APG can be used to label specifically arginine residues in proteins, producing stable, cyclic Schiff base-like bonds with the side-chain guanidino groups. Photoactivation with UV light then causes ring expansion of the phenyl azide group, initiating covalent bond formation with amines.

Figure 6.1 The Wedekind trifunctional crosslinker can react with amine groups via its p-nitrophenyl ester to form amide bond linkages. The phenyl azide group then can be photoactivated with UV light to generate covalent bond formation with a second molecule. The biotin side chain provides binding capability with avidin or streptavidin probes.

Figure 28.12 Sulfo-SBED first is used to label a bait protein through reaction of the sulfo-NHS ester with available amine groups on the protein, yielding an amide bond linkage. This labeled bait protein then is added to a sample containing proteins that potentially could interact with the bait. After an incubation period, the sample is exposed to UV light to photoactivate the phenyl azide group. This reaction causes any interacting prey proteins to be crosslinked with the bait protein, forming a complex containing a biotin affinity tag.

Photoactivation of Mg(amine)+ complexes has been the subject of several studies. The photofragmentation pathways are dependent on the structure of the amine. For... 

Photoactivation of auric triarylmethanecyanide dyes with amine activator... 

For solution phase photopolymerization, photosensitive solutions must be prepared immediately before use. In film-based compositions, stability and sensitivity are lost in 2-5 days (compare Tables 6, 10). The lack of stability is directly related to the nature of the activator, although not in a way that necessarily parallels photosensitivity. The photoactivated reaction, dye-activator redox chemistry, is a go/no-go process and does not contribute to the dark reaction. Rather, dark reactions are in general a consequence of the basicity of most common activators, for example, amines, sulfinates, or enolates. Deprotonation of active methylene groups on some dyes (e.g.,... 

Another example concerning the reduction of carbonyl compounds also relates to the salt effect theme. Shaefer and Peters (1980), Simon Peters (1981,1982,1983,1984), Rudzki et al. (1985), and Goodman and Peters (1986) described photoreductions of aromatic ketones by amines. In this case, the addition of excess NaC104 results in considerable retardation, even prevention, of final product formation. The two fundamental steps in this photoreduction consist of rapid electron transfer from the amine to the photoactivated ketone (in its triplet state), followed by the slow transfer of proton from the amine cation radical to the carbonyl anion radical ... 

The system benzophenone—triethylamine produces ketyl and amine radicals by electron transfer between the photoactivated benzophenone and the amine in the ground state. The generated radicals initiate the polymerization of methyl methacrylate [74]. 

Charge transfer (CT) complexes, different from aromatic ketone/amine systems, such as quinoline-bromine, pyridine-bromine, tetrahydrofuran-bromine etc. have also been reported to behave as initiators of vinyl polymerization, particularly under photoactivation [65-68]. 

Fig. 2.12. (A) Scheme showing photoactivation of fluorescence for the DCDHF-azide 1 to produce the long-wavelength emissive molecule 2. (B) Absorption spectra of 1 converting to 2 upon 407 nm irradiation. Inset emission due to pumping at 594 nm. For details, see [159]. (C) A covalently bound dimer of Cy3 and Cy5 with a reactive functionality [135]. (D) Superresolution image of the stalks of living C. cres-centus cells (yellow) acquired using the molecule in (C) bound to cell-surface amines. For details, see [135]...

Figure 5 Starting from natural mRNA, a cDNA library (A blue) is produced and like ribosomal display, the cDNA is transcribed into mRNA (B) with no stop codons. The 3 -end of each mRNA molecule is ligated to a short synthetic DNA linker (C) and sometimes a polyethyleneglycol spacer, which terminates with a puramycin molecule (small red sphere). The ligation is stabilized by the addition of psoralen (green clamp), which is photoactivated to covalently join both strands. Addition of crude polysomes or purified ribosomes (D) results in translation of the mRNA into protein, but the ribosome stalls at the mRNA-DNA junction. Since there are no stop codons, release factors cannot function and instead the puromycin enters the A-site of the ribosome (A). Because puramycin is an analog of tyrosyl-tRNA, the peptidyl transferase subunit catalyzes amide bond formation between the puromycin amine and the peptide carboxyl terminus, but is unable to hydrolyze the amide link (which should be an ester in tyrosyl-tRNA) to release the dimethyladenosine. The ribosome is dissociated to release the mRNA-protein fusion (E), which is protected with complementary cDNA using RT-PCR (F). The mRNA library can then be selected against an immobilized natural product probe (G), nonbinding library members washed away and the bound mRNA (H) released with SDS. PCR amplification of the cDNA provides a sublibrary (A) for another round of selection or for analysis/ sequencing.

---