Stabilization of a Fragile Two-Dimensional Protein Crystal at the Water−Air Interface:  The Square Lattice of Apoferritin

Two-dimensional (2-D) crystallization experiments at the water−air interface with recombinant apoferritin (horse liver) are described using a recently developed protein spreading technique, which is based on density and surface tension differences of the subphase and the injected protein solution. The preparation protocol was modified to investigate the influence of additives such as glutaraldehyde and CdSO<INF>4 </INF>on the crystallization process. The crystalline arrays were examined by electron crystallographic techniques. It turned out that addition of the cross-linker glutaraldehyde to the subphase buffer was crucial for maintaining the integrity of a fragile 2-D crystal of the square lattice type, which was not observed previously during transfer onto electron microscopy grids.