A YFP variant, EYFP(V68L/Q69K) was subjected to circular permutation. The original N and C termini were linked through a pentapeptide linker GGSGG, rendering Y145 and N144 new N and C termini, respectively. The blue-shift in the absorbance suggests that the chromophore was protonated in cpEYFP(V68L/Q69K). The excitation spectrum has two peaks at 417 and 506 nm which is reminiscent of the bimodal excitation spectrum of WT-GFP. The protonated species of cpEYFP(V68L/Q69K) absorbs around 420 nm fluoresced, whereas protonated species of most YFPs do not.