D-Luciferin 6′-methyl ether (6′-Methoxyluciferin; compound 19a) sodium serves as a potent inhibitor of the North American firefly Photinus pyralis (PpyLuc) luciferase, exhibiting an IC50 of 0.1 μM. As an analog of D-luciferin, this compound demonstrates non-specific interactions at the ATP- and luciferin-binding sites within the active site of PpyLuc [1].