Ionization Of Phenolic Hydroxyl Groups Research Articles

Polyethylenimine-Derived Fluorescent Nonconjugated Polymer Dots with Reversible Dual-Signal pH Response and Logic Gate Operation

A kind of nonconjugated polymer dot possessing autofluorescence was synthesized by hyperbranched polyethylenimine and salicylaldehyde via environmentally friendly imine cross-linking and self-assembly in the aqueous phase. Both fluorescence and ultraviolet–visible (UV–vis) absorption of the polymer dots showed a sensitive and reversible response to pH. The mechanism of pH-dependent optical properties was investigated. The existence of two proton-responsive functional groups (phenolic hydroxyl groups and amine groups) on the polymer dot surface is crucial. The pH-dependent fluorescence is ascribed to electron transfer, which is controlled by the protonation degree of amine groups, while the pH-dependent UV–vis absorption is induced by ionization of phenolic hydroxyl groups. Based on the pH-mediated optical properties, a colorimetric and fluorescent dual-signal pH sensor and multiple logic gates were developed. The fluorescent nonconjugated polymer dots with easy preparation and sensitive response to pH are...

The pH Jump Study of Enzyme Proteins

Rapid conformational changes due to pH jump were studied kinetically at 25 degrees mainly by the stopped-flow method using liquefying alpha-amylase from Bacillus subtilis [EC 3.2.1-.1, liquefying]. First, the conformational change due to a pH jump produced by mixing with alkali was monitored as a function of time at 245 nm through the ionization of phenolic hydroxyl groups of tyrosine residues which were originally buried and finally become exposed due to the pH jump. Three distinct phases of conformational change were clearly recognized by this method by varying the final pH values. Each phase involved the exposure of an essentially definite number of tyrosine residues, whose rate constant was crucially dependent on pH. Second, these phases of conformational change were subjected to examination in terms of the optical rotation change at 411 nm and the reversibility upon reverse pH jump with respect to conformational reconstitution, as observed through the protonation ofphenolic hydroxyl groups of ionized tyrosine residues and the enzyme activity. The first phase, which occurs above pH 12.5, involves no change in the optical rotation and is reversible as observed by the above two monitoring methods. In contrast, the other two phases, which are observed above pH 12.7, are accompanied by an optical rotation change and no appreciable reversibility was detected by these methods.