Such differences are likely due to pH changes or differences in primary structure of the luciferase. Modification of the firefly luciferin substrate has led to "red-shifted" emissions (up to emission wavelength of 675 nm).
Differences like these are likely due to alterations in pH levels or variations in the initial sequence of luciferase. Changes to the firefly luciferin substrate have produced "red-shifted" emissions, reaching up to a wavelength of 675 nm.