Fluorescence bands of a Trp110Ile mutant of the isolated heme domain of a heme-regulated phosphodiesterase (Ec DOS) from Escherichia coli and its complex with 8-anilino-1-naphthalenesulfonic acid (ANS) were very weak, compared to the wild-type protein, suggesting that the fluorescence of the remaining Trp53 residue is quenched by interactions with heme, and that the Trp110 residue is exposed to the solvent.