Langmuir monolayer characterization of metal chelating lipids for protein targeting to membranes

Targeting and organization of proteins on lipid membranes led to applications in both biological and materials sciences. Coordination of membrane-bound metal ions by surface histidine residues provides a general method for targeting of proteins to membrane surfaces. Here we report the Langmuir monolayer properties of a new class of metal-chelating lipids. The lipids utilize the metal chelator iminodiacetate (IDA) as the hydrophilic headgroup, allowing display of divalent transition metal ions on the aqueous side of the membrane. Changes in surface pressure-molecular area isotherms were used to observe metal binding, and an association constant for Cu²⁺ binding to the IDA lipids of 10^7-8 M^-1 was estimated. The ability to control binding site density is important for many applications. The IDA lipid was found to be miscible with both distearoylphosphocholine (DSPC) and 1-stearoyl-2-oleoyl-phosphocholine (SOPC) at most compositions and surface pressures.