Efficient graphene oxide coating improves cryo-EM sample preparation and data collection from tilted grids

Recent technical developments have made single particle cryo-EM a major structural biology technique, especially in the characterization of challenging samples that resist crystallization, can only be obtained in small amounts, or suffer from compositional or conformational heterogeneity. However, a number of hurdles that often challenge sample preparation still need to be overcome in order to increase the applicability and throughput of cryo-EM. These technical hurdles include obtaining enough particles per image, with close to random orientation, and without damage from interaction with the air-water interface. While coating EM grids with graphene oxide is a promising procedure for the improvement of sample preparation, it suffers from some technical problems that limit its applicability. We have modified the established drop cast method for adhering graphene oxide onto holey patterned grids to increase graphene coverage. Our method relies on the use of a polycationic polymer to coat the surface of the grid prior to graphene oxide application, thereby preventing the repulsion of the negatively charged graphene oxide sheets from the negatively charged grid surface. With this improved preparation method, we show that graphene oxide supports can increase the number of particles in the field of view by an order of magnitude with respect to open holes, while keeping them away from the damaging air-water interface. We also show how graphene oxide coated gold foil grids can be used to collect tilted cryo-EM data in order to overcome preferred orientation issues, without experiencing the large amount of drift observed with conventional amorphous carbon supports, thus allowing data collection that can lead to high-resolution reconstructions.