Fate of the fluorescent state of p-amido analogue of green fluorescence protein chromophore

When the wild-type green fluorescent protein (wtGFP) chromophore, p-hydroxybenzylidene-imidazolinone (p-HBDI), was modified into various analogues, its fluorescent state and relaxation pathways were also changed dramatically. In contrast to p-HBDI and its various analogues, such as the o-hydroxy analogue (o-HBDI), the o-sulfonamide analogue (o-TsABDI), the p-sulfonamide analogue (p-TsABDI) and the p-amino analogue (p-ABDI), the p-amido analogue (p-AABDI) of wtGFP chromophore displays single fluorescence and its electronic absorption and fluorescence emission do not involve a charge transfer. Its ground-state N–H acid strength (pK_a) is 19.6 in DMSO, which is not very acidic, so, unlike p-TsABDI, p-AABDI does not undergo an excited-state proton transfer. Its S₁ excited state decays mainly along τ torsion through the S₁/S₀ conical intersection with a barrier of 2.8 kcal/mol. This was confirmed by the cis-trans photoisomerization experiment, in which 47% of cis-p-AABDI was converted to its trans-form right after 20 min irradiation with 350 nm UV light.