Characterizing the dual-wavelength dye indo-1 for calcium-ion sensing under pressure

Abstract

Indo-1 is a dual-wavelength fluorophore widely used for calcium-ion sensing at ambient pressure. Because understanding the physico-chemical nature of high-pressure effects on probe dyes extends the range over which quantitative sensing is possible and informs the development of more robust probes, we report the high-pressure characterization of indo-1 for potential use in calcium-ion sensing under pressure. We find that indo-1 emission remains consistent with a two-state binding model when pressurized to 510 atm, accompanied by a decrease in calcium-binding affinity and an observation of piezochromic behavior. An estimate of the thermodynamic volume change of the indo-1 calcium dissociation reaction is consistent with values for other metal-ion chelators, and piezochromic shifts appear to be due to a pressure-induced change in solvent polarity versus a change in solvent viscosity. The two-state behavior and the linear response of parameters needed for dye calibration makes indo-1 amenable for use under pressure.