TYPE-I COLLAGEN PACKING, CONFORMATION OF THE TRICLINIC UNIT-CELL

The X-ray diffraction pattern of tendon collagen can contain a number of sharp Bragg peaks indicating three-dimensional crystallinity of the sample. Optimal diffraction images have been obtained with a high flu x synchrotron X-ray source and a carefully maintained sample environme nt and staining techniques. The Bragg peaks are always superimposed on a diffuse background This makes interpretation of data difficult and a number of conflicting models of collagen packing have been proposed. The removal of the diffuse scatter from the images allows the Bragg p eaks to be seen on a relatively flat background. This was conducted by modelling the background points as a series of two-dimensional polyno mial functions. The resultant set of observed Bragg reflections serves as an excellent basis to test the validity of two contradictory packi ng modes; (1) the triclinic model, Eraser et al., (2) the microfibril model, Kajava. From this it can easily be seen that the model proposed by Kajava is inappropriate, since there is limited agreement between predicted positions of reflections and the positions of observable ref lections on film. The packing of collagen molecules on a triclinic lat tice is favoured by this criterion.