Ferenc Papp

Voltage‐clamp fluorometry (VCF) enables the study of voltage‐sensitive proteins through fluorescent labeling accompanied by ionic current measurements for voltage‐gated ion channels. The heterogeneity of the fluorescent signal represents a significant challenge in VCF. The VCF signal depends on where the cysteine mutation is incorporated, making it difficult to compare data among different mutations and different studies and standardize their interpretation. We have recently shown that the VCF signal originates from quenching amino acids in the vicinity of the attached fluorophores, together with the effect of the lipid microenvironment. Based on these, we performed experiments to test the hypothesis that the VCF signal could be altered by amphiphilic quenching molecules in the cell membrane. Here we show that a phenylalanine‐conjugated flavonoid (4‐oxo‐2‐phenyl‐4H‐chromene‐7‐yl)‐phenylalanine, (later Oxophench) has potent effects on the VCF signals of the Ciona intestinalis H V 1 (CiHv1) proton channel. Using spectrofluorimetry, we showed that Oxophench quenches TAMRA (5(6)‐carboxytetramethylrhodamine‐(methane thiosulfonate)) fluorescence. Moreover, Oxophench reduces the baseline fluorescence in oocytes and incorporates into the cell membrane while reducing the membrane fluidity of HEK293 cells. Our model calculations confirmed that Oxophench, a potent membrane‐bound quencher, modifies the VCF signal during conformational changes. These results support our previously published model of VCF signal generation and point out that a change in the VCF signal may not necessarily indicate an altered conformational transition of the investigated protein.