These units are specifically designed for the accurate representation of large protein structures. The use of the white peptide bond unit and alpha carbon unit for the main chain atoms enables this backbone to be easily followed and identified on the completed model. The side chains are an unobtrusive grey whilst different colours for the functional groups ensures that these are easily apparent. Thus 'new' features are continually coming to notice, such as the complete ring of charged amino groups around cytochrome c: the high concentration of hydroxyl groups towards one corner of the chymotrypsin model, in addition of course to the well known clustering of hydrophobic residues (grey and purple) in the centre of the molecule.

The units are used in sequence studies and in the investigation of possible structures as well as the representation of predetermined structures. In the latter case model building requires the atomic coordinates of the molecule. It is necessary to know the scale and orientation of the coordinate axes i.e. what angles they make with each other. Results from early crystallographic studies may appear in fractional distances of the edge of the unit cell. Ultimately, coordinates are usually expressed in Angstroms on axes at right angles to each other i.e. orthogonal :cartesian. These coordinates may be for the complete molecule, for the backbone only, or just the alpha carbons of the backbone. Models can be built in all three cases, but obviously the accuracy will increase with the amount of information available, and this increases with the degree of refinement undertaken by the group concerned. Knowledge of the backbone coordinates enables the dihedral angles to be calculated. These facilitate model building since the backbone conformation can be dialled up using a protractor as mentioned above.

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