Cyan Fluorescent Proteins (CFPs) are widely used in FRET-based live cell imaging experiments, in which they non-radiatively transfer their excitation energy to a Yellow Fluorescent Protein depending on their vicinity, thus providing a technique to probe protein-protein interactions. The prototypal CFP, Enhanced Cyan Fluorescent Protein (ECFP), suffers from non-optimal fluorescence properties that were not drastically improved in two later-developed CFPs, Cerulean and SCFP3A. Two additional rounds in optimization by structure-based mutagenesis yielded mTurquoise and mTurquoise2, the latter being the monomeric fluorescent protein with the highest fluorescence efficiency (fluorescence quantum yield). A structural analysis by X-ray crystallography revealed a flexibility of one of the beta-strand of the protein, whose interactions with the chromophore could affect its fluorescence properties. In the latest CFPs, this flexibility appeared to have been suppressed. By using neutron scattering, we would like to bring a definitive experimental proof of the mechanism of fluorescence-controlled of the chromophore by its protein matrix.