Studies of the aqueous formation of highly blue fluorescent $Zn^{2+}$-Schiff base complexes at physiological pH

Abstract

Schiff bases with the azomethine (-C=NR) group are a condensation product of a primary amine and a carbonyl group in which $H_2O$ is eliminated. These groups are easily able to chelate many types of metal ions. Hence, a range of metal complexations have been assessed and a great number of metal Schiff base complexes have been reported. Some of them have shown for stereo- / enantio-selective catalytic activity or antibacterial or anticancer effects. They also show a range of characteristics in terms of their geometry, magnetic and optical properties. Thus, research on metal-Schiff base complexes has wide-ranging implications in the fields of organic, inorganic, and biochemistry, and pharmacology fields. Here, based on an aqueous blue fluorescent dimeric $Zn^{2+}$-Schiff base complex characterized in previous work by us, we focused on in situ $Zn^{2+}$-Schiff base complexation using $Zn^{2+}$, aromatic aldehydes as a fluo-rophore, and amino acids in physiological aqueous media, with the solutions assayed through fluorescence measurements. These studies can provide useful and basic information on the design and synthesis of efficient $Zn^{2+}$ sensors with a good yield as well as allow for a better understanding of the formation of reversible Schiff base formation through self-assembly and their interactions with metal ions in biological systems. In Chapter 1, aqueous in situ $Zn^{2+}$-Schiff base formation was studied by fluorescence at the 50 μM level using salicylaldehyde and a homologous series of D/L-amino acids [$H_2NCH(COOH)(CH_2)_nNH_2$ (n = 1 - 4)]. MALDI-TOF MS spectrometry and UV-vis absorption, fluorescence and $^1H$ NMR spectroscopy were used in characterization. The three components ($Zn^{2+}$, salicylaldehyde and $H_2NCH(COOH)(CH_2)_nNH_2$) were each titrated separately at different molar concentration ratios (0-10 equiv) and reaction times (> ~28 h) at a pH of 7.4 and RT (25~30℃). All trials generally showed fluorescence ($λ_{em}$ = 450-460 nm; $...