The present invention is directed to labeled compounds of the formulae; wherein Q is selected from the group consisting of —S(═O)—, and —S(═O)2—, Z is selected from the group consisting of 1-naphthyl, substituted 1-naphthyl, 2-naphthyl, substituted 2-naphthyl, and phenyl groups with the structure ; wherein R1, R2, R3, R4 and R5 are each independently selected from the group consisting of hydrogen, a C1-C4 lower alkyl, a halogen, and an amino group selected from the group consisting of NH2, NHR and NRR′ where R and R′ are each independently selected from the group consisting of a C1-C4 lower alkyl, an aryl, and an alkoxy group, and X is selected from the group consisting of hydrogen, a C1-C4 lower alkyl group, and a fully-deuterated C1-C4 lower alkyl group.
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