WasCFP, a pH-dependent green fluorescent protein with a tryptophan-based chromophore (Thr65-Trp66-Gly67) in anionic state, was designed from a cyan precursor mCerulean. In this study, the three-dimensional structure of WasCFP has been determined by an X-ray method at pH 5.5, pH 8.0 and pH 10.0, with a resolution of 1.14, 1.25 and 1.5 Å, respectively. We show that changes in the acidity of the media are accompanied by a synchronous change of the side chain conformations of the residues in the near-chromophore environment. Subsequent changes in the local H-bond network interacting with the chromophore lead to considerable alterations in the protein spectral properties as a consequence of reversible processes of ionization-protonation of the Trp chromophore. These experimental results have been supported by quantum chemistry calculations.