pH Dependence of T₁ for ¹³C‐Labelled Small Molecules Commonly Used for Hyperpolarized Magnetic Resonance Imaging

Hyperpolarization is a method to enhance the nuclear magnetic resonance signal by up to five orders of magnitude. However, the hyperpolarized (HP) state is transient and decays with the spin‐lattice relaxation time ( T 1 ), which is on the order of a few tens of seconds. Here, we analyzed the pH‐dependence of T 1 for commonly used HP 13 C‐labelled small molecules such as acetate, alanine, fumarate, lactate, pyruvate, urea and zymonic acid. For instance, the T 1 of HP pyruvate is about 2.5 fold smaller at acidic pH (25 s, pH 1.7, B 0 =1 T) compared to pH close to physiological conditions (66 s, pH 7.3, B 0 =1 T). Our data shows that increasing hydronium ion concentrations shorten the T 1 of protonated carboxylic acids of most of the analyzed molecules except lactate. Furthermore it suggests that intermolecular hydrogen bonding at low pH can contribute to this T 1 shortening. In addition, enhanced proton exchange and chemical reactions at the p K a appear to be detrimental for the HP‐state.