An ESIPT Fluorescent Probe Sensitive To Protein α-helix Structures

A large majority of membrane proteins have one or more transmemberane regions consisted of a-helices. Membrane protein levels are different from one type of cells to another, and the expression of membrane proteins is also altered 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 prostate normal cells. Based on these, we designed a fluorescence probe sensitive to protein a-helical structures to discriminate individual prostate cancer cells from normal ones. A benzazole derivative (1in this paper) was observed to emit strong fluorescence resulted from an excited-state intramolecular proton transfer (ESIPT) in protein a-helical environments. The intensity of ESIPT fluorescence of1was observed to be positively correlated with the a-helix contents of proteins. The molecular docking simulation suggested that it has low energy for the binding of1to proteins when the binding sites are localized within the a-helical regions of protein via H-bonds. Furthermore,1was found to be localized in cell membranes through binding to transmembrane a-helical regions of membrane proteins and to be capable of probing a difference in the membrane protein a-helix contents between prostate normal and cancer cells by its changes in the ESIPT emission intensity. These results indicated that1can distinguish individual prostate cancer cells from normal ones because the changes in the ESIPT fluorescence intensity of1can reflect the regulation in expression of the membrane proteins including zinc transporters. The discriminating strategy of individual prostate cancer cells might contribute to the early diagnosis approaches of prostate cancer.