Why is Fura-2 unable to cross cell membranes?

Understanding the Limitation of Fura-2 in Crossing Cell Membranes

Fura-2 is a widely used fluorescent calcium indicator, particularly in the field of cell biology for measuring intracellular calcium concentrations ([Ca2+]i). Despite its extensive application, Fura-2 is inherently unable to cross cell membranes due to its chemical structure and properties.

Key Reasons

  • Charge: Fura-2 is a negatively charged molecule at physiological pH. The cell membrane is composed of a lipid bilayer that is hydrophobic and repels charged and polar molecules, preventing their passive diffusion across the membrane.
  • Size: Fura-2 is relatively large, further hindering its ability to passively diffuse through the lipid bilayer of cell membranes.
  • Hydrophilicity: Being hydrophilic, Fura-2 does not interact favorably with the hydrophobic core of the cell membrane, limiting its ability to cross into cells without assistance.

To circumvent this limitation, a derivative of Fura-2, known as Fura-2 AM (acetoxymethyl ester), is used. Fura-2 AM is a membrane-permeable esterified form of Fura-2. Once inside the cell, cellular esterases cleave the ester bonds, converting Fura-2 AM back into the charged, membrane-impermeable form of Fura-2, thus trapping it inside the cell. This process allows for the effective measurement of intracellular calcium levels using Fura-2.

Back to blog