Biology:Henderson limit

Short description: Dose limit in X-ray crystallography studies

The Henderson limit is the X-ray dose (energy per unit mass) a cryo-cooled crystal can absorb before the diffraction pattern decays to half of its original intensity. Its value is defined as 2 × 10⁷ Gy (J/kg).

Decay of diffraction patterns with increasing X-ray dose

Although the process is still not fully understood, diffraction patterns of crystals typically decay with X-ray exposure due to a number of processes which non-uniformly and irreversibly modify molecules that compose the crystal. These modifications induce disorder and thus decrease the intensity of Bragg diffraction. The processes behind these modifications include primary damage via the photo electric effect, covalent modification by free radicals, oxidation (methionine residues), reduction (disulfide bonds) and decarboxylation (glutamate, aspartate residues).

Practical significance

Although generalizable, the limit is defined in the context of biomolecular X-ray crystallography, where a typical experiment consists of exposing a single frozen crystal of a macromolecule (generally protein, DNA or RNA) to an intense X-ray beam. The beams that are diffracted are then analyzed towards obtaining an atomically resolved model of the crystal. Such decay presents itself as a problem for crystallographers who require that the diffraction intensities decay as little as possible, to maximize the signal to noise ratio in order to determine accurate atomic models that describe the crystal.

References

• Henderson, Richard (23 July 1990). "Cryo-protection of protein crystals against radiation damage in electron and X-ray diffraction". Proceedings of the Royal Society of London. Series B: Biological Sciences 241 (1300): 6–8. doi:10.1098/rspb.1990.0057. Bibcode: 1990RSPSB.241....6H. 
• Claudio Nicolini (2009). Nanobiotechnology and Nanobiosciences. Pan Stanford Publishing. p. 128. ISBN 978-981-4241-38-0. https://books.google.com/books?id=YcEoy1X96KgC&pg=PA128. 
• Murray, James W.; Rudiño-Piñera, Enrique; Owen, Robin Leslie; Grininger, Martin; Ravelli, Raimond B. G.; Garman, Elspeth F. (14 April 2005). "Parameters affecting the X-ray dose absorbed by macromolecular crystals". Journal of Synchrotron Radiation 12 (3): 268–275. doi:10.1107/S0909049505003262. PMID 15840910.