Time resolved fluorescence spectroscopy of quercetin and morin complexes with Al³⁺

Abstract

The association process of Al³⁺ with quercetin and morin in methanol was studied by electronic absorption and emission spectroscopies. The number of species in solution with different absorption spectra were determined by the method of Rank analysis of the absorbance matrix, and the stoichiometries of the complexes were evaluated using the Job method. The number of fluorescent species in solution were calculated from the Rank analysis method of the time resolved emission spectra (TRES), and compared with a global analysis of the decay surface using a proper multi-exponential decay model. The association of Al³⁺ with morin gives rise to two complexes with 1:1 and 2:1 (morin: Al³⁺) stoichiometries, but in both species the association of the cation involves the carbonyl and 3-hydroxyl groups of the pyrone ring. The fluorescence decay surface of this system is biexponential and the lifetimes of the 1:1 and 2:1 complexes are 4.3 and 2.0 ns, respectively. The association of Al³⁺ with quercetin forms preferentially two complexes with 1:1 and 1:2 (quercetin: Al³⁺) stoichiometries where the first cation binds to the site of the pyrone ring but the second one is bound to the cathecol group of the molecule. However, the multichelation character of the quercetin ligand allows larger aggregates to be formed, thereby the species Al₂Q₃ is also detected in methanol. The lifetime of the 1:1 complex is about 2.7 ns, while for 1:2 and 3:2 complexes the lifetimes measured are 3.5 and 1.8 ns, respectively.

Introduction

Quercetin and morin are phenolic compounds derived from hydroxyl substitutions on the flavone chromophore. They complexate with metal cations to form stable products which in several cases are highly fluorescent, a property which has been explored in analytical methods of metal and ligand identification [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. For instance, quercetin and morin are well known reagents for determination of aluminum(III) traces in water and in biological samples. Besides these analytical applications, the chelation of mono and polyhydroxy flavones with cations is an important factor in their bioactivity as carriers and regulators of metal concentration. Free or complexated, these compounds are also anti-oxidants and free radical scavengers in biological systems [5], [11], [12], [13].

The enhancement of the fluorescence signal upon chelation of flavones with a nonparamagnetic metal is related to the inhibition of the excited state intramolecular proton transfer (ESPT) process [14], [15], [16], [17], [18], [19], [20], [21] between hydroxyl and 4-keto groups of the cromone ring. The ESPT mechanism, which occurs in several hydroxyl substituted flavones, give rises to a fast excited state equilibrium between the normal and tautomeric forms, and therefore to dual fluorescence usually with low emission quantum yields at room temperature.

Most of the studies of quercetin and morin association with metals have focused attention on the investigation of the stoichiometry of the complexes and determination of possible sites of binding [4], [5], [6], [7], [8], [9], [10]. In the present work, these points are also addressed, but are discussed together with the electronic excited state properties of quercetin and morin complexes with aluminum (III) in methanol solution.

The number of species in solution with different absorption spectra is determined by the Rank analysis method of the absorbance matrix, and the stoichiometry of the complexes is evaluated using the Job method. The number of fluorescent species in solution is defined by the Rank analysis method of the time resolved emission spectra (TRES) matrix. The lifetime of the complexes in solution is then determined from a global analysis of the decay surface using a proper multi-exponential decay model.