Information about crystal structures from other methods

3 Information about crystal structures from other methods 

Obtain a general expression for the number of lattice modes for a polymer with m chains passing through the unit cell and hence derive the numbers for m = 1 and 2. 

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In order to obtain the maximum amount of information about the crystal structure it is necessary to align the crystallites, which can be done by methods described in detail in chapter 10. It is sufficient to note here that suitable orientation is often produced by stretching a fibre of the polymer. In the simplest cases the chain axis of each crystallite, which is designated the c-axis, becomes aligned towards the fibre axis, but there is no preferred orientation of the other two axes around the c-axis. From such a sample a fibre pattern can be obtained, of the type shown in fig. 3.10. 

Crystals of several amino-acids and of a few simple peptides and other compounds related to proteins have been analyzed recently by three-dimensional methods. These and other accurate determinations of structure constitute the best experimental sources of information about the dimensions and configuration of the polypeptide chain. The present paper comprises a critical summary of this information— in particular, of the principal structural features of the amide group and the N—H---0 hydrogen bonds, as derived from X-ray diffraction analyses of crystals of amino-acids, peptides, and other organic compounds. 

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