Predicting ¹H NMR relaxation in Gd³⁺-aqua using molecular dynamics simulations

Atomistic molecular dynamics simulations are used to predict ¹H NMR T₁ relaxation of water from paramagnetic Gd³⁺ ions in solution at 25 °C. Simulations of the T₁ relaxivity dispersion function r₁ computed from the Gd³⁺-¹H dipole-dipole autocorrelation function agree within ≃8% of measurements in the range f₀ ≃ 5 ↔ 500 MHz, without any adjustable parameters in the interpretation of the simulations, and without any relaxation models. The simulation results are discussed in the context of the Solomon-Bloembergen-Morgan inner-sphere relaxation model, and the Hwang-Freed outer-sphere relaxation model. Below f₀ ≲ 5 MHz, the simulation overestimates r₁ compared to measurements, which is used to estimate the zero-field electron-spin relaxation time. The simulations show potential for predicting r₁ at high frequencies in chelated Gd³⁺ contrast-agents used for clinical MRI.