Fig. 1. In vitro characterization of the GFP³⁶⁺–dLBT protein. (A) ICP-MS measurement of the Gd(iii) content in Gd-DTPA (blue), Gd–GFP³⁶⁺–dLBT (green) and Gd–GFP³⁶⁺ (brown). Both GFP³⁶⁺–dLBT and GFP³⁶⁺ proteins contain a (His)₆ tag. (B) Fluorescence spectra of GFP³⁶⁺–dLBT with Gd³⁺ titration. The dashed line denotes the GFP³⁶⁺ control; the solid lines indicate the 10 μM GFP³⁶⁺–dLBT with the addition of Gd³⁺ ions. The molar ratios of [protein] : [Gd³⁺] are given. The spectra were recorded in HEPES buffer from 400–580 nm with an excitation wavelength of 395 nm. (C) T₁-weighted MR image produced with a spin echo sequence (TR 300 ms, TE 14 ms) of the GFP³⁶⁺–dLBT protein at different concentrations of gadolinium. The molar relaxivity rates, r₁ (1/T₁), were obtained by linear fitting of the experimental data. Error bars denote the standard deviation.