We introduce a method for the detection of weak interactions of small molecules such as metabolites or medicaments that contain deuterated methyl groups with proteins in solution. The technique relies on long-lived imbalances of spin state populations, which are generated by dissolution dynamic nuclear polarization (D-DNP) and feature lifetimes that depend on the frequency of internal rotation of deuterated methyl groups. We demonstrate the technique for interactions between deuterated dimethyl sulfoxide (DMSO- d6) and bovine serum albumin (BSA) or trypsin, where the methyl group rotation is slowed down upon protein binding, which causes a marked reduction in the lifetime of the population imbalances.