Enhanced nuclear-spin hyperpolarization of amino acids and proteins via reductive radical quenchers

•Vitamin C (VC) and 2-mercaptoethylamine (MEA) prevent photodamage in LC-photo-CIDNP.•Only low-micromolar concentrations of the VC and MEA additives are necessary.•VC or MEA enhance LC-photo-CIDNP sensitivity by ≥2-fold in long expts (256 scans)•VC acts mainly on the NMR molecule of interest (aromatic amino acids, proteins)•We propose that VC acts primarily by reducing transient Trp/Tyr radical populations.