Тhiacalix[4]arene phosphonate C-800 as a novel fluorescent probe for zinc in living cells

Zn ions are significant for maintaining the proper human organism functioning, thus monitoring­ the zinc content in living cells and the development of sensitive tracking systems and sensors for Zn is particularly important. The purpose of the work was to study the properties of synthetic thiacalix[4]arene C-800 (5,11,17,23-tetrakis[(hydroxy-ethoxyphosphonyl)methyl])-25,26,27,28-tetrahydroxythiacalix[4]arene) as a fluo­rescent sensor for zinc ions in living cells. Our studies demonstrated that thiacalix[4]arene C-800 containing­ four hydroxy-ethoxyphosphonylmethyl groups on the upper rim exhibited fluorescent properties at 340 nm excitation wavelength. Fluorescence intensity of thiacalix[4]arene C-800 was increased significantly in the presence of Zn cations, while cations of other metals, such as Mg2+, Ca2+, Cd2+, and Pb2+ did not affect it. Computer modeling demonstrated that two Zn cations interact with the oxygen atoms of four hydroxy-ethoxyphosphonylmethyl groups. It was shown that thiacalix[4]arene C-800 quickly penetrated rat myometrial cells that led to an increased intracellular fluorescence level. The addition of Zn2+ to cells, stained with thiacalix[4]arene C-800, was followed an even greater increase of intracellular fluorescent signal intensity. No effect of thiacalix[4]arene C-800 on reactive oxygen species production in myometrial cells was detected as well as on cells viability in the range of its 50-250 μM concentrations. Thus, thiacalix[4]arene C-800 can potentially be used as a selective fluorescent probe for the detection of Zn2+ in living cells.