Spectroscopic Study of the Hydration Equilibria and Water Exchange Dynamics of Lanthanide(III) Complexes of 1,7-Bis(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO2A)

Abstract

The hydration state of a series of [Ln(DO2A)(H2O)n]+ complexes in aqueous solution at pH = 6.4–7.0 was studied by measuring the lanthanide‐induced 17O shifts (LIS) of water [Ln includes elements from Ce to Yb; DO2A = 1,7‐bis(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane]. Their contact contribution, obtained from Reilley plots, indicated a decrease in the inner‐sphere water coordination number of the [Ln(DO2A)(H2O)n]+ complexes from n = 3 (Ce–Eu), to n = 2 (Tb–Yb). A temperature‐dependent UV/Vis absorption study of the 578–582 nm 7F0 → 5D0 transition band of [Eu(DO2A)(H2O)n]+ in aqueous solution showed that this complex is present in an equilibrium between eight‐ and nine‐coordinate species with n = 2 and n = 3, respectively. The hydration equilibrium parameters (2 ↔ 3), K2–3298= 4.0 ± 0.2, ΔH2–30 = –12.1 ± 1 kJ mol–1 and ΔS 2–30 = –28.9 ± 3 J mol–1 K–1,correspond to an average hydration number of 2.65–2.85 in the temperature range 273–363 K. A variable temperature, multiple field 17O NMR study combined with direct EPR measurements of the transverse electronic relaxation rates has been used to obtain the parameters characterizing water exchange, rotation and electronic relaxation, all influencing the proton relaxivity of [Gd(DO2A)(H2O)2–3]+. The small increase in the water exchange rate of [Gd(DO2A)(H2O)2–3]+ (k ex298 = (10 ± 5) × 106s–1) relative to that of[Gd(DOTA)(H2O)]– (4.8 × 106 s–1) is a consequence of an unfavorable interplay of charge and hydration equilibria. The value of τ R298 = 40 ± 1 ps is short, and the electronic relaxation rate (1/T2e ≈︁ 1.2 × 1010 s–1) is fast relative to [Gd(DOTA)(H2O)]– (1.3–2.4 × 109 s–1 for B = 0.34 T). These parameters negate to some extent the expected increase in proton relaxivity of the [Gd(DO2A)(H2O)2–3]+ complex.

Interpretation of europium(III) spectra

“…At higher temperatures, the complex with the smaller number of coordinated water molecules is favored, so that the band with decreasing intensity as a function of temperature can be assigned to this complex. showed the existence of a temperature dependent equilibrium between eight-coordinate (n = 2) and nine-coordinate (n = 3) species [510]. At lower temperatures, the nine-coordinate species dominates the mixture, whereas the contribution of the eight-coordinate species increases with increasing temperatures.…”

Enhancement of Anion Binding in Lanthanide Optical Sensors

“…This phenomenon, known as the “gadolinium break,” 51 has emerged from various studies of stability constants across the lanthanide series, such as those with acetate and anthranilate, 52 as well as in investigations of hydration enthalpy, 53 vaporization energy, 54 viscosity, 55 molar entropy 56 and enzyme inhibition properties (Figure 4). 57 Consistent with past potentiometric work, 58 binding affinities of DPA 2− to Ln 3+ aqua ions were relatively uniform and did not exhibit any gadolinium break.…”

Molecular Dynamics Simulations of the Internal Mobility of Gd3+-Based MRI Contrast Agents: Consequences for Water Proton Relaxivity

“…Beside experimental studies (potentiometric titrations [17], UV-visible [18], luminescence [19], NMR [20] [21], and EPR [15][16] spectroscopies), computational chemistry has proven to be a uniquely useful tool to gain detailed information about such complexes, for example their structure [22][23] and solvation [24]. The recent development of a force field able to reproduce the intramolecular behaviour of a Gd 3+ complex in aqueous solution by molecular dynamics (MD) simulation [25][26] has opened a new way to investigate the domain of MRI contrast agents.…”