Eye lens color: formation and function.

Aromatic amino acids are photooxidized by near-ultraviolet light to colored products that are bound very tightly to protein amino groups. The resulting colored proteins absorb near-ultraviolet light more strongly and are rendered more hydrophobic than the untreated compounds, and they fluoresce at 440 nanometers when excited at 360 nanometers. Coloration in the lenses of diurnally active animals (including man) may be caused by this reaction, and senile cataracts may result. Such changes in many other proteins (as in the skin and retina) could lead to more serious consequences.

UV-induced photoproducts of para-aminobenzoid acid.

Cis- and trans- 4,4'-azodibenzoic acid (AZBA), two photoproducts of para-aminobenzoic acid (PABA), formed by the action of 313 nm light, have been characterized on the basis of UV and mass spectral data. The relative dose of UV light required for azodibenzoic acid formation is comparable to that obtained from a day of sunbathing. The same type of photochemical reaction could lead to the formation of PABA-protein conjugates. These photoproducts may be responsible for the known photosensitizing properties of PABA and perhaps its cytotoxic and mutagenic effects.

Stability of PABA after UV irradiation in vivo and in vitro.

Sunscreens are widely used for the prevention of acute and chronic sun damage. One of the most widely used sunscreens is para-aminobenzoic acid (PABA). It has been reported that PABA decomposes on exposure to air and light as well as exposure to high doses of UVB. In the present study we found that PABA was stable during long-term storage. PABA seems to be stable after irradiation of physiological doses of UVA and UVB in vitro. However, the in vivo studies demonstrate that significantly less PABA could be extracted from UVA-irradiated sites compared to controls.

Photomodulation of conformational states. I. Mono- and bicyclic peptides with (4-amino)phenylazobenzoic acid as backbone constituent.

The thioredoxin reductase active-site fragment H-Ala-Cys-Ala-Thr-Cys-Asp-Gly-Phe-OH [134-141], which is known for its high tendency to assume an almost identical conformation as in the intact enzyme, was backbone cyclized with the photoresponsive (4-amino)phenylazobenzoic acid (APB) to produce a monocyclic and disulfide-bridged bicyclic APB-peptide. Light-induced reversible cis/trans isomerization occurs at identical extents in both the linear and the two cyclic forms. Nuclear magnetic resonance conformational analysis clearly revealed that in the bicyclic APB-peptide both as a trans- and cis-azo-isomer the constraints imparted by the bicyclic structure do not allow the molecule to relax into a defined low energy conformation, thus making the molecule a frustrated system that flip-flops between multiple conformational states. Conversely, the monocyclic APB peptide folds into a well-defined lowest energy structure as a trans-azo-isomer, which upon photoisomerization to the cis-azo configuration relaxes into a less restricted conformational space. First femtosecond spectroscopic analysis of the dynamics of the photoreaction confirm a fast first phase on the femtosecond time scale related to the cis/trans isomerization of the azobenzene moiety followed by a slower phase in the picosecond time scale that involves an adjustment of the peptide backbone. Due to the well- defined photoresponsive two-state transition of this monocyclic peptide molecule, it represents a model system well suited for studying the ultrafast dynamics of conformational transitions by time-resolved spectroscopy.

Photomodulation of conformational states. II. Mono- and bicyclic peptides with (4-aminomethyl)phenylazobenzoic acid as backbone constituent.

It has been reported that backbone cyclization of octapeptides with the photoresponsive (4-aminomethyl)phenylazobenzoic acid imparts sufficient restraints to induce and stabilize ordered conformations of the peptide backbone in both the cis- and trans-azo-isomers (L. Ulysse, J. Cubillos, and J. Chmielewski, Journal of the American Chemical Society, 1995, Vol. 117, pp. 8466-8467). Correspondingly, the active-site octapeptide fragment H-Ala-Cys-Ala-Thr-Cys-Asp-Gly-Phe-OH [134-141] of thioredoxin reductase, with its high preference for a 3(10)-helix turn conformation centered on the Thr-Cys sequence, was backbone cyclized with this azobenzene moiety in the attempt to design a photoresponsive system where the conformational states of the peptide backbone are dictated by the configuration of the azobenzene and can be further modulated by the disulfide bridge. Nuclear magnetic resonance conformational analysis of the monocyclic compound clearly revealed the presence of two conformational families in both the cis- and trans-azo configuration. Of the higher populated conformational families, the structure of the trans-isomer seems like a pretzel-like folding, while the cis-isomer relaxes into a significantly less defined conformational state that does not exhibit any regular structural elements. Further restrictions imparted by disulfide bridging of the peptide moiety leads to an even better defined conformation for the trans-azo-isomer, whereas the cis-isomer can be described as a frustrated system without pronounced energy minima and thus with little conformational preferences. Our findings would suggest that this photoresponsive peptide template may not be of general usefulness for light-induced conformational transitions between two well-defined conformational states at least under the experimental conditions employed, even in the bicyclic form. However, trans --> cis isomerization of the bicyclic peptide is accompanied by a switch from a well-defined conformation to an ensemble of possible conformations.