Radiation decay of thaumatin crystals at three X‐ray energies

Radiation damage is an unavoidable obstacle in X‐ray crystallographic data collection for macromolecular structure determination, so it is important to know how much radiation a sample can endure before being degraded beyond an acceptable limit. In the literature, the threshold at which the average intensity of all recorded reflections decreases to a certain fraction of the initial value is called the `dose limit'. The first estimated D 50 dose‐limit value, at which the average diffracted intensity was reduced to 50%, was 20 MGy and was derived from observing sample decay in electron‐diffraction experiments. A later X‐ray study carried out at 100 K on ferritin protein crystals arrived at a D 50 of 43 MGy, and recommended an intensity reduction of protein reflections to 70%, D 70 , corresponding to an absorbed dose of 30 MGy, as a more appropriate limit for macromolecular crystallography. In the macromolecular crystallography community, the rate of intensity decay with dose was then assumed to be similar for all protein crystals. A series of diffraction images of cryocooled (100 K) thaumatin crystals at identical small, 2° rotation intervals were recorded at X‐ray energies of 6.33 , 12.66 and 19.00 keV. Five crystals were used for each wavelength. The decay in the average diffraction intensity to 70% of the initial value, for data extending to 2.45 Å resolution, was determined to be about 7.5 MGy at 6.33 keV and about 11 MGy at the two higher energies.