Photoionization tryptophan

D. Shemesh and R. B. Gerber. Classical trajectory simulations of photoionization dynamics of tryptophan intramolecular energy flow, hydrogen-transfer processes and conformational transitions, J. Phys. Chem. A, 110 8401-8408 (2006). 

B12. Bazin, M., Patterson, L. K., and Santus, R., Direct observation of monophotonic photoionization in tryptophan excited by 300-nm radiation. A laser photolysis study. J. Phys. Chem. 87, 189-190(1983). 

The efficient photodecarboxylation of the keto acids (77) has been studied. The reactions involve the formation of the carbanions (78). Aqueous solutions of fenofibric acid (79) at pH 7.4 show the formation of two intermediates when subjected to laser excitation. The study has indicated that the triplet state of the acid in water is of a jtji type. Photoionization is an important process in the aqueous medium. New photoreactive phenylalanine analogues (80) and (81) have been prepared. These were incorporated into position 5 of the pentapeptide, thymopentin. The resultant derivatives were photolabile and underwent decomposition on irradiation at 365 nm. Computational methods have been used to analyse the photoreactivity of the tryptophan derivative (82). The calculations were directed towards an understanding of the quenching of the fluorescence. The results indicate that hydrogen transfer alone does not quench the fluorescence, but that an aborted decarboxylation path is involved. Proton transfer... 

Tryptophan is also sensitive to oxidation, yielding mostly N -formylkynurenine and 3-hydroxykynurenine [69]. The formation of N -formylkynurenine can be activated by photoionization of tryptophan residues arising from the formation of solvated electrons [70]. The reaction is also catalyzed by the presence of metal ions [69]. Tryptophan residue can also mediate photooxidation of the disulfide bond in bovine somatotropin without being oxidized itself [70]. 

Tryptophan (Trp), tyrosine (Tyr), cystine (Cys), and phenylalanine (Phe) moieties play a determinant role regarding UV light-induced chemical alterations in many proteins. After the absorption of light by these moieties, in most cases mainly by Trp and Tyr, they undergo photoionization and participate in energy-and electron-transfer processes. This not only holds for structural proteins such as keratin and fibroin [11], but also for enzymes in aqueous media such as lysozyme, trypsin, papain, ribonuclease A, and insulin [7]. The photoionization of Trp and/or Tyr residues is the major initial photochemical event, which results in inactivation in the case of enzymes. A typical mechanism pertaining to Trp residues (see Scheme 8.3) commences with the absorption of a photon and the subsequent release of an electron. In aqueous media, the latter is rapidly solvated. By the release of a proton, the tryptophan cation radical Trp is converted to the tryptophan radical Trp. ... 

Scheme 8.4 Rupture of cystine bridges by the attachment of electrons stemming from the photoionization of tryptophan [32, 33].

Wilson KR, Jimenez-Cruz M, Nicolas C, Belau L, Leone SR, Ahmed M (2006) Thermal vaporization of biological nanoparticles fragment-free vacuum ultraviolet photoionization mass spectra of tryptophan, phenylalanine-glycine-glycine, and, beta-carotene. J Phys Chem A 110 2106... 

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