Molecular engineering of the photo switching in the ortho chromophores of the nanostructured green fluorescence protein

Abstract

Green fluorescent protein (GFP) has attracted wide attention as an efficient fluorescent probe for photophysical properties studies in the biological and biochemical sciences. In this work, the effect of different substituents (OMe, OEt, NO2, Br, CN, and CF3) in the para position of the phenyl ring in 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), an analogue of the core chromophore of the GFP, on their photophysical properties were explored in both gas and solution phases by using TD-DFT method at PBE0/6-311++G(d,p) and M06-2X/6-311++G(d,p) levels of theory. The potential energy surfaces (PESs) were evaluated along the reaction coordinate (RC = dOH) at the ground S0 and excited S1 states of the three o-HBDI derivatives composed of electron donating and accepting substitutes in both gas and solution phases. The structural, electronic and photophysical properties, the atomic charges and electron density at critical points were examined. The results show the photophysical properties of o-HBDI are dependent on substitution patterns and solvents. In contrast to S0 state, the excited-state intramolecular proton transfer (ESIPT) process at S1 state in these compounds is expected to be approximately less barrier height. The ESIPT process in all three solvents is predicted to be an exergonic reaction. Depending on the strength of the electronic accepting or donating of the substituents, a wide tunable range of the fluorescence emissions from 460nm (visible blue light) to 642nm (visible red light) was predicted in the solvent media. By comparing wavelengths, the maximum value of the stoke shift is observed for CN-o-HBDI and TF-o-HBDI. Chromophores composed of the electron donating substituents (OMe and OEt) represent greatest red-shift in non-polar solvent cyclohexane. Our computed results are in good agreement with the experimental observations and have the potential applications in manipulating and tuning photo-physical properties of the technologically and biologically important fluorescent protein.