CEST properties of PemFE, Pem, and C12FE, each in a 10 mM 1× DPBS solution at 37 °C. (a) MTRasym at 5.2 ppm of all designed molecules at pH 7.4, measured at different saturation field strength. (b) PemFE MTRasym at 5.2 ppm measured with respect to presaturation time (Tsat) at pH 6, 7, and 8. (c) MTRasym at 5.2 ppm as a function of pH using B1 = 3.6 μT. A quantifying exchange using the saturation time (QUEST) method was used to fit data and obtain kex values. (d) pH dependence of proton exchange rate (kex) as a function of pH using B1 = 3.6 μT.