Model compounds of PYP chromophore were synthesized and characterized to investigate the role of the Cys69 residue in the active center, which has the intramolecular NH⋯OC hydrogen bond to the conjugated carbonyl oxygen and thioester linkage of the chromophore. The results of UV-vis and 13C NMR spectroscopy of the model compounds indicated that they delocalized the negative charge of the chromophore and increased the contribution of the quinoid-like resonance structure in the phenolate anion state. Thus, the Cys69 residue plays an important role in the regulation of the color and the stabilization of the chromophore anion in the active center.
