Water exchange kinetics on lanthanide(III) ions: a variable temperature and pressure ¹⁷O NMR study

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca²⁺-selective flux, they possess an elaborate mechanism for selection of Ca²⁺ over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca²⁺ transfer remains unanswered. By virtue of their similarity to Ca²⁺, polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca²⁺-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Ca_vα₁-subunits (Ca_v2.3, Ca_v3.2) by Zn²⁺ and Cu²⁺ may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu²⁺ and Zn²⁺ or exposure to non-physiological toxic metal ions.

Hydration studies of lanthanides and actinides are reviewed. In their trivalent state, the two series of elements have similar hydration and hydrolytic behavior. The solid hydrates adopt the tricapped trigonal prism (CN=9) structures whereas in solution, diffraction and solution measurements support a change in hydration number from 9 to 8 with decreasing ionic radii. Thermodynamics of hydration, rates of solvolysis and preferential solvation in mixed solvents are discussed. Changes in hydration and hydrolytic characterization with oxidation state of the central ion are also reviewed.

A ¹H NMR study of the kinetics of exchange of 1,1,3,3-tetramethylurea (tmu) on six-coordinated [Ln(tmu)₆]³⁺ in CD₃CN is reported for Ln = Tb, Dy, Ho, Er and Yb. When Ln = Er and Yb, the rate of ligand exchange = k6 [Ln(tmu)₆³⁺], and k (298.2 K) = 214 ± 2 and 65.5 ± 0.9 s⁻¹, ΔH≠ = 35.5 ± 0.5 and 38.3 ± 1.0 kJ mol⁻¹, and ΔS≠ = −81.3 ± 1.7 and −81.8 ± 3.1 J K⁻¹ mol⁻¹, respectively. Although overall ligand exchange rate laws have not been determined for Ln = Tb, Dy and Ho, observed first order exchange rate constants, k = 1380, 1290 and 510 s⁻1 (298.2 K) have been determined. From these data, and that previously published, it is clear that a dominant factor determining the rate of ligand exchange on [Ln(tmu)₆]³⁺ is the ionic radius of Ln³⁺. The lability of [Ln(tmu)₆]³⁺ towards ligand exchange is several orders of magnitude less than is the case for eight-coordinated [Ln(dmf)₈]³⁺ and nine-coordinated [Ln(H₂O)₉]³⁺, and this is attributed to an increase in Ln³⁺-ligand dipole attraction as the coordination number decreases.

Literature data on the nature and thermodynamics of inorganic complex species of the REE and Y have been critically reviewed. Theoretical considerations suggest that trivalent REE and Y should exhibit strong, predominantly electrostatic complexing with “hard” ligands such as fluoride, sulfate, phosphate, carbonate and hydroxide and this is borne out by the available experimental data. Complexing of these metals with chloride and nitrate is moderately weak and with ammonia and bisulfide is extremely weak to nonexistent. A considerable amount of concordant thermodynamic data are available at 25°C for the sulfate complexes LnSO⁺₄ and Ln(SO₄)⁻₂, the fluoride complexes LnF²⁺, LnF⁺₂ and LnF⁰₃, the nitrate complex LnNO²⁺₃ and the chloride complex LnCl²⁺, where Ln signifies the REE or Y. There are much fewer reliable data available for the hydroxide, carbonate and phosphate complexes and in fact, the exact stoichiometries of REE and Y complexes with these ligands are still somewhat uncertain. However, it appears that the LnOH²⁺, LnCO⁺₃, Ln(CO₃)⁻₂, LnHCO²⁺₃ and LnH₂PO²⁺₄ complexes have been identified.

Calculation of the speciation of Eu (βEu = 10⁻⁷ m) in a typical groundwater with βSO²⁻₄ = 10⁻⁴ m, βC⁻ = 2·10⁻⁴ m, βF⁻ = 10⁻⁶ m, βCO²⁻₃ = 10⁻⁴ m, βNO⁻₃ = 10⁻⁴ m and βPO³⁻₄ = 10⁻⁶ m shows that the simple ion and the sulfate complexes are most important at acidic pH and that the carbonate complexes become predominant at near-neutral to basic pH. Even in relatively saline waters, chloride complexes do not account for a significant fraction of REE in solution and are completely negligible in most surface waters and groundwaters. Nitrate complexes are also negligible even in environments where the concentration of this ligand is artificially high due to pollution. Phosphate and fluoride complexes can attain importance where concentrations of these ligands are somewhat anomalous (i.e. 2–4 orders of magnitude higher than in the model groundwater) due to either natural or anthropogenic factors.

Data on the nature and stabilities of REE and Y complexes at low temperature are of critical importance to those concerned with safe nuclear waste disposal, geochemical exploration for REE and Y deposits, and the use of REE and Y as tracers in seawater and fresh water. The information most needed at the present time includes: (1) reliable hydrolysis constants; (2) additional data on phosphate, carbonate and the higher fluoride complexes; (3) data on mixed ligand complexes; and (4) more complete data on the complexation behavior of the +4 and +2 oxidation states of the REE.