An improved protocol for rapid freezing of protein samples for long-term storage.

Freezing of purified protein drops directly in liquid nitrogen is a convenient technique for the long-term storage of protein samples. Although this enhances reproducibility in follow-up crystallization experiments, some protein samples are not amenable to this technique. It has been discovered that plunging PCR tubes containing protein samples into liquid nitrogen results in more rapid freezing of the samples and can safely preserve some proteins that are damaged by drop-freezing. The PCR-tube method can also be adapted to a PCR-plate freezing method with applications for high-throughput and structural genomics projects.