International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by E. Arnold, D. M. Himmel and M. G. Rossmann

International Tables for Crystallography (2012). Vol. F, ch. 10.1, p. 244   | 1 | 2 |

Section 10.1.2.5. Additional benefits from sub-77 K cooling with helium

H. Hopea* and S. Parkinb

aDepartment of Chemistry, University of California, Davis, One Shields Ave, Davis, CA 95616–5295, USA, and bDepartment of Chemistry, University of Kentucky, Lexington, Kentucky, USA
Correspondence e-mail:  hhope@ucdavis.edu

10.1.2.5. Additional benefits from sub-77 K cooling with helium

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As is the case with nitrogen cooling, it is unlikely that thermo­dynamic equilibrium will be reached by cooling to liquid-helium temperatures. The change that will certainly take place on cooling to liquid-helium temperature is that true thermal motion will be greatly reduced. One result of this is that individual atom peaks will become much sharper. For example, electron-density maxima for well ordered atoms will increase by a factor of about three on cooling from 90 to 10 K. Potentially, this can allow a more detailed interpretation of a structure with a resolution limit better than about 1.5 Å, and also for the better ordered regions of a structure with poorer overall resolution. In general, however, it is not realistic to expect a significant resolution improvement in low-resolution structures based on the effects of temperature alone. Improvements related to diminished radiation damage, on the other hand, can be significant. Two studies illustrate the effects discussed here.

The effects of helium cooling on a high-resolution structure are well illustrated in a study by Petrova et al. (2006)[link]. They studied a complex of human aldose reductase at 15, 60 and 100 K. The complex has yielded data to 0.66 Å resolution, and thus represents a generally highly ordered structure. The emphasis of the study was on the behaviour of the atomic displacement parameters (ADPs). It was found that the major ADP component for well ordered atoms is temperature driven, as it would be in normal small-molecule structures. A large proportion of the atoms at 15 K have B values of 2 Å2 or less (about 0.025 Å2 or less in terms of U values). At 100 K, the corresponding cutoff is about 5 Å2. Cooling to 15 K allows large portions of the structure to be determined with a precision that would be considered excellent for small molecules. However, the average isotropic B value for the `best' Cα atoms at 15 K is still 3.9 Å2 (a U value of about 0.05 Å2). This indicates that the positional parameters for many of these atoms in reality are composites of closely spaced positions. The best average protein ADPs at 15 K are about the magnitude of small-molecule ADPs at room temperature. This sets unfortunate limits to the attainable accuracy of structural and electron-density parameters.

Hexagonal hen egg-white lysozyme has a relatively well ordered structure, but there are significant regions with multiple conformations. Brinkmann et al. (2006)[link] measured diffraction data at 10 K to a resolution limit of 1.46 Å. The results indicate that major areas of disorder are present, illustrating that structural disorder persists at the lowest temperatures.

Although helium is more expensive than nitrogen as a coolant, the added cost for a helium-temperature data set is usually trivial. Equipment design and operating methods have developed to a stage where there is no significant operational difference between nitrogen and helium cooling when manual crystal handling is used.

References

Brinkmann, C., Weiss, M. S. & Weckert, E. (2006). The structure of the hexagonal crystal form of hen egg-white lysozyme. Acta Cryst. D62, 349–355.
Petrova, T., Ginell, S., Mitschler, A., Hazemann, I., Schneider, T., Cousido, A., Lunin, V. Y., Joachimiak, A. & Podjarny, A. (2006). Ultrahigh-resolution study of protein atomic displacement parameters at cryotemperatures obtained with a helium cryostat. Acta Cryst. D62, 1535–1544.








































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