In mEosFP, the main role of Met159 is to stabilize the neutral state of the chromophore and the M159A mutation cancels this stabilization. On the other hand, the anionic form may take advantage of the mutation by reorganizing and optimizing the hydrogen bond network surrounding the hydroxylate group of the chromophore. As a consequence, the M159A mutation disfavors photoconversion by lowering the fraction of neutral forms in the sample. An opposite effect has been observed in the structural comparison between EosFP and Dendra2, where an electrostatic interaction in EosFP is disrupted in Dendra2. This disruption decreases the stability of the anionic chromophore, resulting in a larger fraction of the neutral form and thus promoting photoconversion (Adam et al., 2009).