Strategic incorporation of a meta-dimethylamino (-NMe2 ) group on the conformationally locked green fluorescent protein (GFP) model chromophore (m-NMe2 -LpHBDI) has drastically altered molecular electronic properties, counterintuitively enhancing fluorescence of only the neutral and cationic chromophores in aqueous solution. A ~200-fold decrease in fluorescence quantum yield of m-NMe2 -LpHBDI in alcohols (e.g., MeOH, EtOH and 2-PrOH) supports this GFP-derived compound as a fluorescence turn-on water sensor, with large fluorescence intensity differences between H2 O and ROH emissions in various H2 O/ROH binary mixtures. A combination of steady-state electronic spectroscopy, femtosecond transient absorption, ground-state femtosecond stimulated Raman spectroscopy (FSRS) and quantum calculations elucidates an intermolecular hydrogen-bonding chain between a solvent -OH group and the chromophore phenolic ring -NMe2 and -OH functional groups, wherein fluorescence differences arise from an extended hydrogen-bonding network beyond the first solvation shell, as opposed to fluorescence quenching via a dark twisted intramolecular charge-transfer state. The absence of a meta-NMe2 group twisting coordinate upon electronic excitation was corroborated by experiments on control samples without the meta-NMe2 group or with both meta-NMe2 and para-OH groups locked in a six-membered ring. These deep mechanistic insights stemming from GFP chromophore scaffold will enable rational design of organic, compact and environmentally friendly water sensors.
© 2021 American Society for Photobiology.