Vitrification of thick samples for soft X-ray cryo-tomography by high pressure freezing

Soft X-ray cryo-microscopy (cryo-XT) offers an ideal complement to electron cryo-microscopy (cryo-EM). Cryo-XT is applicable to samples more than an order of magnitude thicker than cryo-EM, albeit at a more modest resolution of tens of nanometers. Furthermore, the natural contrast obtained in the "water-window" by differential absorption by organic matter vs water yields detailed images of organelles, membranes, protein complexes, and other cellular components. Cryo-XT is thus ideally suited for tomography of eukaryotic cells. The increase in sample thickness places more stringent demands on sample preparation, however. The standard method for cryo-EM, i.e., plunging to a cryogenic fluid such as liquid ethane, is no longer ideally suited to obtain vitrification of thick samples for cryo-XT. High pressure freezing is an alternative approach, most closely associated with freeze-substitution and embedding, or with electron cryo-microscopy of vitreous sections (CEMOVIS). We show here that high pressure freezing can be adapted to soft X-ray tomography of whole vitrified samples, yielding a highly reliable method that avoids crystallization artifacts and potentially offers improved imaging conditions in samples not amenable to plunge-freezing.