Photolabile protection for amino acids: studies on the release from novel benzoquinolone cages.

The synthesis of a novel fused nitrogen heterocycle, benzoquinolone, for evaluation as a photocleavable protecting group is described for the first time by coupling to model amino acids (alanine, phenylalanine and glutamic acid). Conversion of the phenylalanine ester conjugate to the thionated derivative was accomplished by reaction with Lawesson's reagent. Photocleavage studies of the carbonyl and thiocarbonyl benzoquinolone conjugates in various solvents and at different wavelengths (300, 350 and 419 nm) showed that the most interesting result was obtained at 419 nm for the thioconjugate, revealing that the presence of the thiocarbonyl group clearly improved the photolysis rates, giving practicable irradiations times for the release of the amino acids (less than 1 min).

Effect of Diamond Bur Grit Size on Composite Repair.

PURPOSE: This study investigated the effect of diamond bur grit size on the repair bond strength of fresh and aged resin composites. MATERIALS AND METHODS: Blocks of microhybrid composite (Opallis, FGM) were stored in distilled water at 37°C for 24 h (fresh composite) or subjected to 5000 thermal cycles (aged composite). The surfaces were roughened using diamond-coated, flame-shaped carbide burs with medium grit (#3168), fine grit (#3168F), or extra-fine grit (#3168FF). The control group underwent no surface treatment. Surface roughness, water contact angle, and surface topography by scanning electron microscopy (SEM) were evaluated (n = 3). Samples were restored with resin composite and sectioned into beam-shaped specimens, which were subjected to microtensile bond testing. Failure modes were classified using a stereomicroscope. Data were statistically analyzed using the Student- Newman-Keuls test and two-way ANOVA, with significance set at p < 0.05. RESULTS: Higher surface roughness was observed for groups treated with the medium- and fine-grit burs; aged composites were rougher than fresh composites. The water contact angle formed on the aged composite was lower than that on the fresh composite. The highest repair bond strength was observed for the fine-grit bur group, and the lowest was recorded for control. Interfacial failures were more predominant. SEM images showed that the surfaces treated with fine- and extra-fine-grit burs had a more irregular topography. CONCLUSION: Surface roughening of fresh or aged resin composites with diamond burs improved retention of the repair material. Fine-grit burs generally performed better than medium- and extra-fine-grit burs.

A photoactivable amino acid based on a novel functional coumarin-6-yl-alanine.

A novel fluorescent amino acid, L-4-chloromethylcoumarin-6-yl-alanine, was obtained from tyrosine by a Pechmann reaction. The assembly of the heterocyclic ring at the tyrosine side chain could be achieved before or after incorporation of tyrosine into a dipeptide, and amino acid and dipeptide ester conjugates were obtained by coupling to a model N-protected alanine. The behaviour of one of the fluorescent conjugates towards irradiation was studied in a photochemical reactor at different wavelengths (254, 300, 350 and 419 nm). The photoreaction course in methanol/HEPES buffer solution (80:20) was followed by HPLC/UV monitoring. It was found that the novel unnatural amino acid could act as a fluorescent label, due to its fluorescence properties, and, more importantly, as a photoactivable unit, due to the short irradiation times necessary to cleave the ester bond between the model amino acid and the coumarin-6-yl-alanine.

Light-induced cleavage of model phenylalanine conjugates based on coumarins and quinolones.

In order to evaluate the application of quinolone as a new photocleavable protecting group, in comparison with coumarin, a series of model phenylalanine conjugates were prepared by reaction with chloromethylated O and N heterocycles. The photophysical properties of the resulting ester conjugates were evaluated as well as the photosensitivity under irradiation at 250, 300, 350, and 419 nm. The results obtained showed that the quinolone conjugates were readily photolysed, with complete release of the amino acid in short irradiation times and could be considered a new addition to the family of photocleavable protecting groups for the carboxylic acid function of amino acids.