Synthesis and characterization of photolabile o-nitrobenzyl derivatives of urea

We present here the synthesis and characterization of four photolabile derivatives of urea in which alpha-substituted 2-nitrobenzyl groups are covalently attached to the urea nitrogen. These derivatives photolyze readily in aqueous solution to release free urea. The alpha-substituents of the 2-nitrobenzyl group strongly influence the rate of the photolysis reaction measured with transient absorption spectroscopy. Rates of photolysis at pH 7.5 and room temperature (approximately 22 degrees C) for N-(2-nitrobenzyl)urea, N-(alpha-methyl-2-nitrobenzyl)urea, N-(alpha-carboxymethyl-2-nitrobenzyl)urea, and N-(alpha-carboxy-2-nitrobenzyl)urea are, respectively, 1.7 x 10(4), 8.5 x 10(4), 4.0 x 10(4), and 1.1 x 10(5) s(-)(1). The quantum yields determined by measurement of free urea following irradiation by a single laser pulse at 308 nm were 0.81 for N-(2-nitrobenzyl)urea, 0.64 for N-(alpha-methyl-2-nitrobenzyl)urea, and 0.56 for N-(alpha-carboxy-2-nitrobenzyl)urea. The caged N-(alpha-carboxy-2-nitrobenzyl)urea is not a substrate of the enzyme urease, while the photolytically released urea is. Also, neither this caged urea nor its photolytic side products inhibit hydrolysis of free urea by urease. Thus, the alpha-carboxy-2-nitrobenzyl derivative of urea is suitable for mechanistic investigations of the enzyme urease.     

Branched Crystalline Patterns Formed Around Poly(ethylene oxide) Dots in Humidity

Branched patterns reminiscent of diffusion‐limited aggregation form around poly(ethylene oxide) dots on Au surfaces in humidity by crystallization of polymer chains in water‐assisted diffusion, involving fractal‐like structure and dense branching‐like morphology (DBM). Higher humidity leads to a slower growth of crystal lamellae. Crystal growth promotes outward diffusion. Fractal‐like lamellae are viewed flat‐on. Due to the reduction of chain availability in the diffusion field, the fractal‐like structure turns into DBM.     

Fundamental Frequency in Monolingual English, Bilingual English/Russian, and Bilingual English/Cantonese Young Adult Women

Mean F0 of nine young adult English/Russian female bilinguals and nine young adult English/Cantonese female bilinguals were examined from samples of connected speech in each language. Mean F0 were compared in each language and in English with those of a monolingual English control group of ten young adult female speakers. Acoustic measurements were analyzed with the Kay Elemetrics Multispeech program (Kay Elemetrics, Lincoln Park, NJ). The results indicate that the English/Russian bilinguals consistently had a higher mean F0 in Russian than in English. Mean F0 did not change with language switch for the English/Cantonese speakers. There were no significant differences between the groups in their English production. Clinical implications regarding norms for both monolingual and bilingual persons, as well as implications for understanding the nature of bilingualism, in particular code-switching, are discussed.     

Performance Validation for Microplate Fluorimeters

As the sophistication of instruments that make fluorimetric measurements on samples in microplates has increased, so has the need for methods to validate instrumental performance. This paper describes a solid-state validation microplate that tests multiple aspects of fluorescence performance, including signal linearity, gain, noise, sensitivity, wavelength accuracy, and polarization stability. Both the operating principles and the validation of the validation microplate are discussed.