Kynurenines (KNs) are natural UV filters of the human eye lens, protecting the eye tissues from solar UV radiation. Key points of their effective protection are the intramolecular charge transfer (ICT) in the excited state and the fast dissipation of absorbed light energy into heat via the intermolecular H-bonds. Herein we report that the introduction of an unsaturated double bond in the amino acid side chain, operating as an ICT-enhancing electron donor group, drastically accelerates the internal conversion (IC) due to a conical intersection (CI) between the potential energy surfaces of the excited and ground states. Our photophysical study of a deaminated KN (carboxyketoalkene, CKA), an intrinsic product of KN thermal decomposition, demonstrates an unusually fast excited state decay in a broad range of solvents of different polarity and proticity. The detailed analysis of interactions in the excited state by different computational techniques revealed that the changes in molecular structure – the twist of the carbonyl group from the plane of the aromatic ring followed by the formation of two mutually orthogonal conjugated substructures – are responsible for the CI of the excited and ground state potential energy surfaces. Intermolecular H-bonds facilitate the transition to the CI, but do not play a crucial role in the fast decay of the excited state. An extremely fast and efficient IC in CKA opens the way for the design of new types of organic UV filters and their applications in material science, cosmetics and medicine.

The photophysics and photochemistry of kynurenic acid (KNA) and kynurenine yellow (KNY) in neutral aqueous solutions were investigated using time-resolved optical spectroscopy. Both molecules have similar quinoline-like structures, the only difference being the absence of conjugation in the nitrogen containing cycle in KNY. The main channel of S₁ excited state decay in the case of partially-unconjugated KNY is the solvent assisted S₁ → S₀ radiationless transition via intermolecular hydrogen bonds (Φ_IC = 0.96), whereas, in the case of fully-conjugated KNA, it is intersystem crossing to the triplet state (Φ_T = 0.82). The major intermediate products of the singlet excited KNY deactivation are the triplet state (Φ_T = 0.022) and, most probably, the enol form (Φ_enol = 0.012), which decay with the formation of 2,3-dihydro-4-hydroxyquinoline and 4-hydroxyquinoline, respectively. The results obtained show that KNA and KNY, which are products of the decomposition of the UV filter kynurenine, are significantly more photoactive and less photostable than the parent molecule.

PURPOSE. To compare the photochemical properties of UV filter molecules present in the human lens (kynurenine, KN; 3-hy- droxykynurenine, 3OHKN; 3-hydroxykynurenine O-gluco- side, 3OHKG; 4-(2-aminophenyl)-4-oxobutanoic acid, AHA; and glutathionyl-kynurenine, GSH-KN) with the use of the following parameters: excited singlet lifetime, fluorescence quantum yield, triplet quantum yield, photodecomposition quantum yield.

METHODS. The excited singlet lifetimes were measured with the use of fluorescence upconversion (time resolution, 210 fs) and pump-probe transient absorption (time resolution, 200 fs) methods. The fluorescence quantum yields were determined relative to an aqueous solution of quinine bisulfate. The triplet quantum yields were measured with the use of nanosecond laser flash photolysis. The photodecomposition quantum yields were determined by steady state photolysis followed by the high-performance liquid chromatography analysis.

RESULTS. The secondary UV filters—AHA and GSH-KN are better photosensitizers than the primary ones -KN, 3OHKN and 3OHKG: the singlet state lifetimes of the secondary UV filters are longer, and the quantum yields of fluorescence and triplet state formation are higher.

CONCLUSIONS. With aging, the ratio primary/secondary UV filters in the human lens decreases from approximately 10:1 to 2:1. The obtained results demonstrate that the quality of the secondary UV filters is inferior compared to the primary ones, which may result in a higher susceptibility of old lenses to UV light. That might be an important factor for the development of the age-related cataract.

The photophysics and photochemistry of kynurenine (KN) covalently bound to the amino acids lysine, cysteine, and histidine, the antioxidant glutathione, and the protein lysozyme have been studied by optical spectroscopy with femto- and nanosecond time resolution. The fluorescence quantum yield of the adducts of KN to amino acids is approximately 2 times higher than that of the free KN in solution; KN attached to protein exhibits a 7-fold increase in the fluorescence quantum yield. The S1 state dynamics of KN-modified lysozyme reveals a multiphasic decay with a broad dispersion of time constants from 1 ps to 2 ns. An increase of the triplet yield of KN bound to lysozyme is also observed; the triplet state undergoes fast intramolecular decay. The obtained results reveal an increase of the photochemical activity of KN after its covalent attachment to amino acids and proteins, which may contribute to the development of oxidative stress in the human lensessthe main causative factor for the cataract onset.

The properties of xanthurenic acid (XAN) in ground and photoexcited states have been studied using steady-state and time-resolved optical methods as well as quantum chemistry calculations. In neutral aqueous solution and in alcohols, XAN is present in a single tautomeric form (keto form), whereas in aprotic solvents and probably in basic aqueous solutions, more than one tautomeric form is present. UV irradiation of aqueous and alcoholic solutions of XAN results in a very rapid solvent-assisted tautomerization to the enol form, the later undergoes solvent-assisted transformation back to the keto form. The photolysis of XAN in aprotic solvents gives rise to the formation of numerous intermediate forms of XAN in both triplet and ground states. Under intense laser irradiation, XAN undergoes biphotonic ionization, the precursor for ionization being the excited singlet state.

The excited-state dynamics of kynurenine (KN) has been examined in various solvents by femtosecond-resolved optical spectroscopy. The lifetime of the S1 state of KN amounts to 30 ps in aqueous solutions, increases by more than 1 order of magnitude in alcohols, and exceeds 1 ns in aprotic solvents such as DMSO and DMF, internal conversion (IC) being shown to be the main deactivation channel. The IC rate constant is pH independent but increases with temperature with an activation energy of about 7 kJ/mol in all solvents studied. The dependence on the solvent proticity together with the observation of a substantial isotope effect indicates that hydrogen bonds are involved in the rapid nonradiative deactivation of KN in water. These results give new insight into the efficiency of KN as a UV filter and its role in cataractogenesis.