Scanning Microscopy


By analysing the positions of individual unit cells in the image of a distorted macromolecular crystal, it is possible to achieve considerably more than is achieved by the correlation averaging or unbending now widely practised. It is possible partly to compensate for individual molecular distortion; and it is possible to identify molecules in equivalent environments (which can be expected to be in equivalent states of strain), selective averaging of which yields images that show how strain is accommodated at the sub-molecular level. The possible presence of surface forces applied to the crystal by its support film complicates the analysis and adds two additional parameters, not previously identified, to those necessary to characterise the environment of each molecule fully; these surface stress parameters can be estimated on the basis of a simple (isotropic) model of the elastic behaviour of a 2-D crystal. The appropriate mathematical description of strain and elasticity in 2-D crystals has been assembled concisely, and a set of new procedures developed allowing their practical exploitation within the Semper image processing system.

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