The exchange rates of protein guanidinium protons in the mouse brain

We develop a novel two-step multi-B1 Bloch-McConnell fitting approach for calculating the exchange rate of CEST protons in vivo, and apply it to guanidinium protons, the exchange rate of which is 70.1 ± 5.5 s-1 with a concentration of 40.4 ± 5.2 mM in mouse brain at an optimized B1 of 0.8 µT. Guanidinoacetate N-methyltransferase deficiency (GAMT-/-) mice that have low creatine and phosphocreatine concentrations in brain are studied for protein guanidinium, i.e., arginineCEST (ArgCEST). The low exchange rate of ArgCEST suggests that the inverse pH dependence in GuanCEST with low B1 is dominated by CrCEST compared to ArgCEST.