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Highly Sensitive Hill-Type Small-Molecule pH Probe That Recognizes the Reversed pH Gradient of Cancer Cells

Luo, Xiao, Yang, Haotian, Wang, Haolu, Ye, Zhiwei, Zhou, Zhongneng, Gu, Luyan, Chen, Jinquan, Xiao, Yi, Liang, Xiaowen, Qian, Xuhong, Yang, Youjun
Analytical chemistry 2018 v.90 no.9 pp. 5803-5809
biopsy, cell membranes, guidelines, histopathology, membrane permeability, neoplasm cells, neoplasms, pH
A hallmark of cancer cells is a reversed transmembrane pH gradient, which could be exploited for robust and convenient intraoperative histopathological analysis. However, pathologically relevant pH changes are not significant enough for sensitive detection by conventional Henderson–Hasselbalch-type pH probes, exhibiting an acid–base transition width of 2 pH units. This challenge could potentially be addressed by a pH probe with a reduced acid–base transition width (i.e., Hill-type probe), appropriate pKₐ, and membrane permeability. Yet, a guideline to allow rational design of such small-molecule Hill-type pH probes is still lacking. We have devised a novel molecular mechanism, enabled sequential protonation with high positive homotropic cooperativity, and synthesized small-molecule pH probes (PHX1–3) with acid–base transition ranges of ca. 1 pH unit. Notably, PHX2 has a pKₐ of 6.9, matching the extracellular pH of cancer cells. Also, PHX2 is readily permeable to cell membrane and allowed direct mapping of both intra- and extracellular pH, hence the transmembrane pH gradient. PHX2 was successfully used for rapid and high-contrast distinction of fresh unprocessed biopsies of cancer cells from normal cells and therefore has broad potentials for intraoperative analysis of cancer surgery.