D-Luciferin 6′-methyl ether (6′-Methoxyluciferin; compound 19a) is a potent luciferase from the North American firefly Photinus pyralis ( PpyLuc ) inhibitor with an IC 50 of 0.1 µM. D-Luciferin 6′-methyl ether, a D-luciferin analog, shows non-specific interactions at ATP- and luciferin-binding sites of the PpyLuc active site.