A speciation study of the aqueous H+/H2VO4−/H2O2/L-α-alanyl-L-serine system

Abstract

A detailed study of the quaternary aqueous H⁺/H₂VO₄⁻/H₂O₂/L-α-alanyl-L-serine (Alaser) system has been performed at 25 °C in 0.150 M Na(Cl) medium, representing the ionic strength of human blood, using quantitative ⁵¹V NMR and potentiometric data (glass electrode). Data were evaluated with the computer program LAKE, which is able to treat combined EMF and NMR data. The pK_a-values for Alaser were determined as 8.04 ± 0.01 and 3.07 ± 0.01. The errors given are 3σ. In the ternary H⁺/H₂VO₄⁻/Alaser system, two complexes, (H⁺)_p(H₂VO₄⁻)_q(Alaser)_r, having (p, q, r) values (0, 1, 1) and (−1, 1, 1) with log β_0,1,1 = 2.42 ± 0.01 and log β_−1,1,1 = −5.80 ± 0.05 explain all data in the pH region 2.5–9.5. Equilibrium conditions are illustrated in distribution diagrams and structure proposals are given based on ¹H and ¹³C NMR investigations. In the quaternary H⁺/H₂VO₄⁻/H₂O₂/Alaser system, six complexes could be found in addition to all binary and ternary complexes over the pH region 2.6–11.1, four with a V/X/Alaser ratio 1 ∶ 1 ∶ 1 and two with a ratio 1 ∶ 2 ∶ 1 (X = peroxo ligand). The formation of the monoperoxo vanadium species is very slow, requiring up to 10 days for complete equilibrium. Significant decomposition of peroxide occurs only in acidic solutions. Chemical shifts, compositions and formation constants for the six quaternary complexes are given, and equilibrium conditions are illustrated in distribution diagrams. The H⁺/H₂VO₄⁻/H₂O₂/Alaser/Alahis system, where Alahis denotes L-α-alanyl-L-histidine, was briefly investigated and no mixed ligand species were detected.