An ESIPT fluorescent probe sensitive to protein α-helix structures

Abstract

A large majority of membrane proteins have one or more transmembrane regions consisting of α-helices. Membrane protein levels differ from one type of cell to another, and the expression of membrane proteins also changes from normal to diseased cells. For example, prostate cancer cells have been reported to have downregulated expression of membrane proteins, including zinc transporters, compared with normal prostate cells. These reports inspired us to design a fluorescence probe sensitive to protein α-helical structures to discriminate individual prostate cancer cells from normal ones. A benzazole derivative (1 in this study) was observed to emit strong fluorescence resulting from an excited-state intramolecular proton transfer (ESIPT) in protein α-helical environments. The intensity of ESIPT fluorescence of 1 was observed to be positively correlated with the α-helix content of proteins. The molecular docking simulation suggested that it had low energy for the binding of 1 to proteins when the binding sites were localized within the α-helical regions of protein via H-bonds. Furthermore, 1 was found to be localized in cell membranes through binding to transmembrane α-helical regions of membrane proteins, and was capable of probing differences in the α-helix contents of membrane proteins between normal and cancerous prostate cells through changes in the ESIPT emission intensity. These results indicated that 1 could distinguish individual prostate cancer cells from normal ones, as the changes in the ESIPT fluorescence intensity of 1 could reflect the regulation in expression of the membrane proteins including zinc transporters. This recognition strategy of individual prostate cancer cells might contribute to early diagnosis techniques for prostate cancer.