Alkali Metal Ion Complexes with Phosphates, Nucleotides, Amino Acids, and Related Ligands of Biological Relevance. Their Properties in Solution

Abstract

Alkali metal ions play very important roles in all biological systems, some of them are essential for life. Their concentration depends on several physiological factors and is very variable. For example, sodium concentrations in human fluids vary from quite low (e.g., 8.2 mmol dm(-3) in mature maternal milk) to high values (0.14 mol dm(-3) in blood plasma). While many data on the concentration of Na(+) and K(+) in various fluids are available, the information on other alkali metal cations is scarce. Since many vital functions depend on the network of interactions occurring in various biofluids, this chapter reviews their complex formation with phosphates, nucleotides, amino acids, and related ligands of biological relevance. Literature data on this topic are quite rare if compared to other cations. Generally, the stability of alkali metal ion complexes of organic and inorganic ligands is rather low (usually log K < 2) and depends on the charge of the ligand, owing to the ionic nature of the interactions. At the same time, the size of the cation is an important factor that influences the stability: very often, but not always (e.g., for sulfate), it follows the trend Li(+) > Na(+) > K(+) > Rb(+) > Cs(+). For example, for citrate it is: log K ML = 0.88, 0.80, 0.48, 0.38, and 0.13 at 25 °C and infinite dilution. Some considerations are made on the main aspects related to the difficulties in the determination of weak complexes. The importance of the alkali metal ion complexes was also studied in the light of modelling natural fluids and in the use of these cations as probes for different processes. Some empirical relationships are proposed for the dependence of the stability constants of Na(+) complexes on the ligand charge, as well as for correlations among log K values of NaL, KL or LiL species (L = generic ligand).

Modeling solubility and acid–base properties of some amino acids in aqueous NaCl and (CH3)4NCl aqueous solutions at different ionic strengths and temperatures

“…As known solvation-desolvation processes influence protonation thermodynamic parameters of amino acids in aqueous solutions of tetralkylammonium salts. Moreover, as described in our previous papers, unprotonated amino group can interact with (CH 3 ) 4 N + and carboxylate anion with Na + forming weak complexes (Berto et al 2012 ; Bretti et al 2013 , 2014a , 2015 ; Crea et al 2016 ).…”

Thermodynamic Solution Properties of a Biodegradable Chelant (L-glutamic-N,N-diacetic Acid, L-GLDA) and Its Sequestering Ability toward Cd2+

“…The chemical speciation of a ligand in a multi-component solution must be drawn considering the “pure” protonation constants and the weak species relative to all the components of the solution calculated at the ionic strength of such solution and considering the concentration of all the interacting ions. Theoretical bases about this approach are available in the literature, including the requirements of using the molar concentration scale, both for ionic strength and equilibrium constants [ 48 , 49 ], the presence of a background electrolyte not—or very weakly—interacting with the ligand (generally tetraethylammonium iodide), and the ionic strength range not exceeding I = 1.0 mol dm −3 [ 50 , 51 , 52 , 53 ]. Some successful examples of applications of this model to other molecules are reported in the literature (e.g., [ 44 , 45 , 54 ]), where the readers may also find extensive discussion about the fitting ability and the analogy with the hybrid SIT model in terms of the chemical information carried.…”

Interaction of N-acetyl-l-cysteine with Na+, Ca2+, Mg2+ and Zn2+. Thermodynamic aspects, chemical speciation and sequestering ability in natural fluids

“…The second approach is a purely chemical model (“weak complex model”), in which the activity coefficients vary with ionic strength according to an equation that is independent of the nature of a species (generally for I < 1 mol dm −3 ), but only on its charge and the medium effect is interpreted in terms of formation of weak species between the molecule under study and the ions of the supporting electrolyte. From this basis, a background electrolyte, that it is assumed to be not (or very weakly) interacting with the molecule under study, is required; for this purpose, when dealing with O- and S- donor ligands, the choice falls on (C 2 H 5 ) 4 NI (aq) or, more rarely, on (CH 3 ) 4 NCl (aq) [ [49] , [50] , [51] , [52] , [53] , [54] , [55] , [56] , [57] , [58] , [59] ]. Moreover, since this approach is based on the direct comparison of data obtained in different media (the so-called Δp K method), the use of the molar concentration scale is more reliable [ 60 , 61 ].…”

“…A direct determination of the stability of these species is often hard since they rarely exceed the value of log K = 1.0. A detailed description of the basic principles of this approach [ 41 , 49 , 51 , 54 ] and some examples can be found, for example, in refs [ 50 , 52 , 55 – 59 ]. Briefly, for a simple monoprotic acid (HL), the lowering effect of the “apparent” protonation constant (log K i H app ) in an “interacting” medium ( e.g.…”

“…, Ca 2+ /NAC 2− ) should be known at S = 35 (~ 0.72 mol dm −3 ) and T = 298.15 K. However, according to this approach the ionic strength dependence for these species is often very similar and, for some species with unknown stability, predictive equations are also available ( e.g. , [ 54 ]). For example, in this work the stability of the K + /NAC 2− species is estimated using the corresponding values for the Na + /NAC 2− ones.…”