Zinc Binding to Fulvic acids: Assessing the Impact of pH, Metal Concentrations and Chemical Properties of Fulvic Acids on the Mechanism and Stability of Formed Soluble Complexes

• Main Authors: Patrycja Boguta, Zofia Sokołowska
• Format: Article
• Language: English
• Published: MDPI AG 2020-03-01
• Series: Molecules
• Subjects: fulvic acids; zinc soluble complexes; micronutrient organic complexes; metal binding to humic substances
• Online Access: https://www.mdpi.com/1420-3049/25/6/1297

The aim of the study was defined as a complementary analysis of molecular interactions between zinc (Zn) and fulvic acids (FAs) at a broad pH range (3&#8722;7), different metal concentrations (0&#8722;50 mg dm^&#8722;3) and chemical properties of FAs and their impact on the Zn binding mechanism, stability, and efficiency. The results showed that the complexation reaction prevailed at pH 6 and 7, whereas protons exchange dominated interactions at pH 3. Stability constant of the complexes increased along with pH (logK increased from ~3.8 to 4.2). Complexation was preferred by less-humidified structures of lower molecular mass containing more oxygen groups. The number of fluorophores available for Zn(II) increased from pH 3 to 7 by ~44%. Depending on the pH, complexation involved a bidentate chelate, monodentate and bidentate bridging mode. Zn(II) binding was insufficiently modeled by the classic Stern&#8722;Volmer equation and well described by the double logarithmic equation (<i>R</i> &gt; 0.94) as well as by a modified Stern&#8722;Volmer formula assuming the existence of available and unavailable fluorophore populations (<i>R</i> &gt; 0.98). The fluorescence ratio of different fluorophores was proposed as an indicator of the binding affinity of various structures. A positive relationship was found between the fraction of accessible fluorophores and Zn(II) binding at pH 7 determined based on proton release (<i>R</i> = 0.91&#8722;0.97). The obtained results can find application in controlling the mobility and bioavailability of Zn in different conditions.