Interpretation of the differential UV–visible absorbance spectra of metal-NOM complexes based on the quantum chemical simulations for the model compound esculetin

Highlights

• Different absorbance spectra features of ten metal/esculetin complexes are different.

• Peaks in DAS of esculetin and NOM induced by metal ions binding are similar.

• Peaks in DAS are generated by electron transitions from esculetin to metal.

• Peak intensity of spectra correlate with covalent index of metals (${({\chi }_m)}^2{r}_c$).

• Spectra properties relate to electron density at interaction of metal/esculetin.

Abstract

In this study, the model compound esculetin that has functional groups typical for natural organic matter (NOM) was used to ascertain the nature of the characteristic bands in the differential UV–visible absorbance spectra (DAS) associated with the formation of metal-NOM complexes. The binding of ten different metal ions (Cu(II), Ni(II), Co(II), Fe(III), Cr(III), Al(III), Zn(II), Ca(II), Mg(II) and Pb(II)) with esculetin generate four bands in the DAS. These bands are similar to those present in the DAS of metal-NOM complexes. The UV–visible absorbance spectra of the metal-esculetin systems were calculated using time-dependent density functional theory (TD-DFT). The TD-DFT results demonstrate that the prominent features of the DAS of esculetin are primarily associated with the electron transitions between the molecular orbitals near the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the metal-esculetin complex. Charge decomposition analysis (CDA) results demonstrated that these electron transitions originate from the esculetin fragment to the Zn(II) fragment in the complex. Covalent indexes [(χ_m)²r_c] of the metal ions were found to be correlated with the metal-specific features of the DAS of metal-esculetin systems. The strength of the linear correlations between the quantitative parameters of the electron density of the bond critical points (BCP) is indicative of the strength of the metal-esculetin interactions.

Introduction

Natural organic matter (NOM) forms strong complexes with many metal ions. As a result, it tends to control their speciation, mobility and bioavailability (Benedetti et al., 1995; Keller et al., 2010; Redman et al., 2002; Schmitt et al., 2003; Yan et al., 2017b). Thus, a better understanding of the structures and properties of the metal-NOM complexes improves capabilities of the models employed to predict and control the behavior of metal ions in natural and engineered ecosystems.

Numerous methods have been used to probe metal-DOM interactions, including sensitive ion-selective electrodes (ISE) (Christl et al., 2001; Yang and Berg 2009), voltammetry (Cheng and Allen 2006), fluorescence excitation-emission matrix (EEM) (Al-Reasi et al., 2011; Guo et al., 2012; Hur and Lee 2011; Xu et al., 2013), X-ray reflectivity (XR) (Lee et al., 2011) and nuclear magnetic resonance spectroscopy (NMR) (Christl et al., 2001; Hertkorn et al., 2004; Leenheer et al., 1998). These methods tend to require using relatively high concentrations of NOM and the target metal and their results are difficult to elucidate thoroughly because of extreme heterogeneity and complexity of NOM.

Recent research demonstrates that the differential UV–visible absorbance spectroscopy (DAS) is a powerful tool to characterize metal-NOM complexation and quantify the amount of bound metals at environmentally-relevant concentration (David G. Kinniburgh et al., 1996; Gao et al., 2015; Kinniburgh et al., 1999; Lu et al., 2017; Yan et al., 2014; Yan et al., 2016). The DAS of Suwannee River fulvic acid (SRFA) binding with ten metal ions, (e.g. Ca(II), Mg(II), Fe(III), Al(III), Cu(II), Cd(II), Cr(III), Eu(III), and Th(IV)) have been shown to comprise discrete Gaussian bands located at 200, 240, 276, 316, 385, and 547 nm (denoted as A₀, A₁, A₂, A₃, A₄, and A₅, respectively) (Yan and Korshin 2014). The shape and intensity of DAS are metal-specific and the intrinsic mechanisms responsible for these and other features of DAS remain unclear.

With the recent progress in quantum chemistry (QC) and in particular in time-dependent density functional theory (TD-DFT), it is possible to predict the UV–visible absorbance spectra of metal ions binding with a variety of ligands (Yan et al., 2017a; Zhang et al., 2019), determine the structures of these complexes by comparison of the calculated and experimental spectra and interpret the features of the absorbance bands based on frontier molecular orbitals theory (FMO). However, at present it is impossible to model the absorbance spectra of NOM because of its polydispersity, presence of diverse functional moieties and its largely unknown structure (Croue et al., 2000). This complication can be partially circumvented via the use of model compounds, for instance flavonoids which have been demonstrated to be excellent model compounds to study NOM and their interactions with halogens and metal ions (Lu et al., 2004; Zhang et al., 2019). TD-DFT approached has been successfully studied to examine metal-flavonoid interactions (Alvarez-Diduk et al., 2013; Moncomble and Cornard 2014; Primikyri et al., 2015; Yan et al., 2017a). Much research has focused on esculetin, a lactone with structural commonalities with flavonoids, and its antioxidant, biochemical properties and complexation affinity to metal ions (Le Person et al., 2014; Shinde et al., 2018; Turkekul et al., 2018; Wu et al., 2007).

In this study, esculetin (Fig. 1) was used to examine important features of DAS of metal-NOM complexes. The structures of metal ion-esculetin complexes were optimized and their calculated DAS were compared with the experimental DAS data. The metal-specific features in the examined DAS were shown to be quantifiable by applicable covalent indexes [(χ_m)²r_c] and elucidated based on FMO, charge decomposition analysis (CDA) and topological analysis of the electron density of bond critical points (BCP) of the metal-esculetin complexes.