"Ribonuclease II. Accuracy of measurement and shrinkage."

Date:
1954-1955
Reference:
PP/CRI/H/1/32
Catalogue details

Licence: In copyright

Credit: "Ribonuclease II. Accuracy of measurement and shrinkage.". Source: Wellcome Collection.

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    (1) errors due to technical limitations, e.g. variation of X-ray in tensity of the source, absorption errors, setting errors, errors in measuring intensities, and so forth. (2) changes due to lack of strict isomorphism, apparently due to s variation in the composition of the solvent and to the added molecules. (3) errors due to slight changes in the vapor pressure surrounding the crystal. This produces the new shrinkage phenomenon already men tioned. We can summarize our results by saying that the errors due to (3) can under certain circumstances be considerable, but can be eliminated; that errors due to (l) are usually very small; but those due to (2) appear difficult to avoid in the case of ribonuclease II, and will prob ably be the limiting factor in a structure determination by isomorphous replacement. The shrinkage phenomenon can be described briefly as follows: It has been shown for monoclinic (C^2) horse haemoglobin (Perutz, 1946, Huxley -anrd Kendrew, 1953) that the shrinkage of the crystals occurs in a number of discrete steps as the humidity of the surrounding air is lowered. This also seems to be the case for ribonuclease II. Typically, in shrinkage of this type, the cell dimensions which change do so by several Angstroms. We have discovered that, in ad dition to discontinuous shrinkage, we can produce very small and, ap parently continuous changes of the cell dimensions up to, say, 1/2 A, and that the intens i ti es_of_the_reflect ions_also_change. Moreover, if the crystals have been mounted rather 'dry'—that is, with only a small amount of excess mother liquor in the mounting capillary—these changes occur spontaneously. We believe that they are due to small