Figure 2.

UPLC-MS chromatograms (MRM mode; acetic acid program) of 4-Cl-IPyA standard (top channel), an in vitro assay mixture using PsTAR1 as the enzyme and 4-Cl-Trp as the substrate (middle channel), and a mixture of the two samples (bottom channel). Mixing the two samples showed coelution of the product with the standard. Given the relative signal strengths of the keto and enol tautomers in the mixture, compared with the individual samples, the mixture represents approximately a 1:1 ratio of the two samples. Identical mass transitions and coelution of the product with the standard confirm its identity as keto 4-Cl-IPyA.