Relaxivity Study and X-ray Structure of Gd(III)-diethylenetriamine-N,N′,N″-triacetic-N,N″- bis(benzylamide). A Potential MRI Contrast Agent

The new bis(amide) derivatives of DTPA (diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid), diethylenetriamine-N,N′,N″-triacetic-N,N″-bis(benzylamide) (DTPA-BBA) have been synthesized. The crystal structure of gadolinium(III) complex of DTPA-BBA ([Gd(DTPA-BBA)]) has been determined by X-ray crystallography: C₂₈H₅₂GdN₅O₁₇, M_w = 889 g mol^-1, space group P 1̄ (#2) (triclinic), a = 12.645(4), b = 14.125(8), c = 12.623(4) Å, α = 111.60(3), β = 114.79(3), γ = 88.39(4)°, V = 1881(1) ų, Z = 2, D_x = 1.569 g/cm³, λ(Mo Kα) = 0.71069 Å, μ = 18.44 cm^-1, final R = 0.047, R_w = 0.046 for 3755 independent observed reflections at 23°C. The coordination sphere of Gd(III) comprises three amine nitrogens, two amide oxygens, three carboxylic acid oxygens, and one water. The relaxivitiy of Gd(III) complex was determined to be R1 = 4.08(4) and R2 = 6.06(5) dm³ mmol^-1 s^-1 at pH = 7.0, 20 MHz, and 37(1)°C. Additionally, the R1 relaxivity for Gd(III) chelate was found to be invariant with respect to pH changes over the range of 2-10. This indicates that a constant inner-sphere hydration number is associated with the [Gd(DTPA-BBA)] complex. Hence the high stability of the complex is demonstrated.