Phototransformation of selected pharmaceuticals during UV treatment of drinking water

— The photophysics and photochemistry of tryptophan and its simple derivatives is comprehensively reviewed with special emphasis on excitation by near-UV radiation. Topics explicitly discussed include the origins of large Stokes shifts in the fluorescence spectra, photoionization, the puzzle of multiple tryptophan fluorescence decay time, photochemical reactions in the presence and absence of oxygen, and the possible mechanisms of these reactions. A separate section reviews the photosensitizing properties of N-formylkynurenine, an important photooxidation product of tryptophan.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1751-1097.1984.tb03890.x

The photophysics and photochemistry of the near-uv absorbing amino acids–ii. tyrosine and its simple derivatives

http://nathan.instras.com/documentDB/paper-273.pdf

Primary ultrafast events preceding the photoinduced proton transfer from pyranine to water

Proteins are an important target of photochemical damage to the eye. The wavelength of irradiation is a major determinant of the initial mechanisms. The effects of ultraviolet radiation are initiated by absorption in the aromatic amino acid residues and cystine. Photoionization of tyrosine and tryptophan residues leads to aromatic free radicals. The ejected electrons are stabilized in the aqueous medium as hydrated electrons and may be temporarily trapped by cystyl bridges. N-formylkynurenine is an important tryptophan photoproduct, which can act as an endogenous photosensitizer of near-ultraviolet radiation by generating singlet oxygen and superoxide. Singlet oxygen is produced also by exogenous sensitizers absorbing in the visible and near ultraviolet regions. The mechanisms are illustrated by porphyrins and furocoumarins, in which dark binding interactions influence the photosensitization pathways.

Photochemistry of proteins: a review.

Photodegradation of the steroid hormones 17β-estradiol (E2) and 17α-ethinylestradiol (EE2) in dilute aqueous solution

Abstract. The dependence of primary photophysical and photochemical processes, especially of electron photoejection, in phenol and related compounds in aqueous solution on excitation intensity and excitation energy is examined. Theoretical and experimental evidence is presented for the possibility of three pathways for electron ejection: (1) A monophotonic pathway via the fluorescent state, which most probably does not involve the lowest triplet state; (2) a monophotonic pathway requiring higher excitation energies, which takes place in competition with internal conversion to the fluorescent state; and (3) a consecutive biphotonic pathway in which the lowest triplet state absorbs the second photon, and which can become predominant at high intensities, e.g. under flash conditions. It is shown that this model reconciles apparently conflicting results published in the literature.

Pathways for formation of hydrated electrons from excited phenol and related compounds

— The two main primary photoprocesses (electron ejection and H-atom release) for indole, 5-methoxyindole and N-methylindole in various polar and nonpolar solvents were studied as a function of the excitation energy and were correlated with the corresponding fluorescence quantum yields. In hydrocarbon solvents, N–H bond cleavage is the main primary photoprocess from the 1Bb band of the substrates with the exception of N-methylindole. In alcohols, both processes are of negligible importance. Hydrated electrons (e−aq) are ejected from the relaxed singlet states of all three compounds in aqueous solutions with a similar yield for excitation at 280 and 254 nm (1La and 1Lb states). The yield increases when the excitation is into the 1Bb band. The quantum yields of the two primary processes from the higher excited states are generally lower than the fraction of molecules not converting to the fluorescent state. This is explained by an efficient back reaction in competition with a thermally activated radical release from an intermediate state or radical pair formed from the S2 (1Bb) state. The non-occurrence of a photoionization energy threshold is discussed.

Wavelength effects on the photoprocesses of indole and derivatives in solution

Formation of radical cations resulting from photoinduced electron ejection from methoxylated benzenes in aqueous solutions has been observed.

J. Zechner

Papers

Pathways for formation of hydrated electrons from excited phenol and related compounds

Wavelength effects on the photoprocesses of indole and derivatives in solution

Photogeneration of radical cations from aqueous methoxylated benzenes

Photoprocesses of indolic compounds in solution

Photoreactions of derivatives of 2-phenylindan-1,3-dione in polar and non-polar solvents