The Photochemistry of N-Arylsulfonyl Amino Acids and Peptides

Moore, Sharon Ann (2001). The Photochemistry of N-Arylsulfonyl Amino Acids and Peptides. PhD thesis The Open University.



The photochemistry of arylsulfonamides has received continuing interest for the last four decades for three main reasons: 1) arylsulfonamides offer convenient acceptor properties for studying photo-induced electron transfer (PIET.) processes in chromophorically modified peptide models, 2) arylsulfonamides have potential as photoremovable protecting groups in organic synthesis, but this has so far remained largely unrealized, 3) arylsulfonamide pharmaceuticals are generally photolabile and photostability is an important consideration in the development of new drugs.
We have studied a series of p-toluenesulfonyl amino acid derivatives and undertaken comprehensive product analysis to elucidate the photoproducts and identify common pathways of photodegradation. We found that the tosyl a-amino acids formed toluenesulfinic acid as a major photoproduct in conjunction with ammonia, carbon dioxide and a carbonyl compound. Upon changing the carboxylate to an ester or an amide, a Cɑ-H abstraction became the dominant process to give a carbonyl compound, toluenesulfinic acid and ammonia. These were also the major photoproducts with a tosyl ß-amino acid. A mechanism is proposed that involves an initial electron transfer (ET) from the carboxyl function to the sulfonyl moiety to give a biradical intermediate that could react to produce most of the observed products.
Some differences in product distribution were found with different amino acid side-chains, although bulky aliphatic or sulfur containing side-chains produced little variation. A tyrosine derivative underwent side-chain cleavage, implying electron transfer from the side-chain rather than the carboxyl function. Peptide bond cleavages occurred in the majority of amide derivatives, which can also be explained by an electron transfer mechanism. The results showed that S-N cleavage to form the free amino acid moiety occurs in very poor yield due to a complex array of competing photoreactions.
A photoyellowing was seen in many of the amide compounds which may be due to photochromism, a process that is consistent with our photo-induced electron transfer hypothesis.

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