Comparison of the Ca²⁺complexing properties of isosaccharinate and gluconate – is gluconate a reliable structural and functional model of isosaccharinate?

Abstract

The calcium complexation and acid-base properties of α-d-isosaccharinate (Isa) in neutral and in (hyper)alkaline solutions have been investigated via potentiometric titrations, multinuclear NMR, ESI-MS and quantum chemical calculations. Isa is the primary alkaline degradation product of cellulose, and may be present in radioactive waste repositories and therefore, it could contribute to the mobilization of radioactive nuclei. Because of its limited availability, d-gluconate (Gluc) is commonly used as a structural and functional model of Isa. Therefore, the thermodynamic and structural data obtained for Isa were compared with those of Gluc. The formation constants of the CaIsa and CaGluc complexes present in neutral solutions are practically identical, but the binding sites are in different positions and the CaIsa solution species cannot be detected. The stepwise formation constant of the CaIsaH complex (forming in alkaline medium) is somewhat larger than that of CaGlucH, which is in line with the observation that IsaH is a stronger base than GlucH. The most striking difference is that, unlike Gluc, Isa does not form polynuclear complexes with Ca. The structural reason for this is that the alcoholate groups on C2 and C3 adjacent to the carboxylate moiety on Gluc are able to simultaneously bind Ca, making the formation of polynuclear Ca-complexes possible. On Isa, only the alcoholate on C2 is involved, while the other one on C6 is not (supposedly for steric reasons). In conclusion, during the interactions of Gluc and Isa with Ca, differences rather than similarities prevail.

Complex formation between UO22+and α-isosaccharinic acid: insights on a molecular level

“…10 They concluded that both Ca 2+ and Eu 3+ form complexes with ISA via the carboxylic as well as the αand γ-hydroxy groups. Similar results were obtained for Ca 2+ by Dudás et al 26 Apart from the identification of the carboxylic, and αand γ-hydroxy groups as coordinating functionalities, their NMR results showed that the β-hydroxy group is not involved. Tasi et al…”

“…24 The pK a value of the carboxylic group is 3.27 while the deprotonation of alcohol functionalities is expected to occur above pH 12. 23,25,26 However, the acidity of alcohol functionalities is expected to increase upon their interaction with Lewis acidic metal centers. 23,27 The previously mentioned impact of ISA on the sorption and solubility was gen-erally traced back to the formation of strong complexes in which the hydroxyl groups of the ligand in addition to the carboxylic group play a crucial role.…”

“…28 NMR-measurements from Dudás et al revealed that ISA acts as a multidentate ligand for Ca(II) under alkaline conditions. 26 They suggested the coordination via the carboxylic group as well as the tertiary alcohol (C 2 -OH) and the secondary alcohol (C 4 -OH), of which one is deprotonated. Furthermore, the authors concluded that the primary alcohol in the β-position (C 6 -OH) to the carboxylic group is not involved in complex formation.…”

Uptake of selected organic ligands by hardened cement paste: Studies on proxy ligands for the degradation of polyacrylonitrile and general considerations on the role of different functionalities in the uptake process

“…Although they form weak binary complexes with Ca of the type CaL + in acidic to weakly alkaline conditions, several studies available in the literature provide evidence on the formation of stable complexes of ISA/GLU with Ca in hyperalkaline pH conditions, which involve the deprotonation (besides the carboxylic group) of at least one of the alcohol groups in those ligands [e.g. CaLH -1 (aq) or Ca 3 L 2 H -4 (aq)] (Hummel et al, 2005;Pallagi et al, 2014;Dudas et al, 2017;Kutus et al, 2020). A similar effect was suggested by Jo and co-workers for C-S-H-ISA surface complexes based on molecular dynamics calculations (Jo et al, 2022).…”

Effect of organic compounds on the retention of radionuclides in clay rocks: Mechanisms and specificities of Eu(III), Th(IV), and U(VI)

“…The ISA isomers are major degradation products of cellulose (Glaus et al, 1999;Van Loon et al 1999, Rout et al, 2015, Shaw 2013. Most of the complexation data on α-ISA and analogues, such as gluconate (Dudás et al, 2017), have been measured at high pH, e.g. pH = 13.3 in Tits et al (2005).…”