A Metal-Chelating Lipid for 2D Protein Crystallization via Coordination of Surface Histidines

Two-dimensional protein crystallization on lipid monolayers is becoming a powerful technique for structure determination as well as materials applications. However, progress has been hindered by the requirement of a unique affinity lipid for each new protein of interest. Metal ion coordination by surface-accessible histidine side chains provides a convenient and general method for targeting of proteins to surfaces. Here we present the synthesis and characterization of a metal-chelating lipid which has been designed to target proteins to Langmuir monolayers and promote their two-dimensional crystallization based on histidine coordination. The lipid utilizes the metal chelator iminodiacetate (IDA) as the hydrophilic headgroup and contains unsaturated, oleyl tails to provide the fluidity necessary for two-dimensional protein crystallization. The lipid is shown to bind copper from the subphase strongly when incorporated in Langmuir monolayers. In addition, it is possible to form copper-containing monolayers by spreading the premetalated lipid on the subphase in the absence of copper. Fluorescence microscopy reveals the binding and crystallization of the protein streptavidin, promoted by the simultaneous coordination of two surface-accessible histidine side chains to the IDA−Cu lipid.