1. 1.|Tropocollagen has been treated with pepsin, carboxypeptidase and leucine aminopeptidase in order to determine whether alterations in the molecule would give rise to modified segment-long-spacing and fibrillar precipitates. 2. 2.|Examination of segment-long-spacing from pepsin-digested tropocollagen clearly revealed that shortening at both ends of the molecule had occurred. Treatment with leucine aminopeptidase resulted in shortening at the A (N-terminal) end of the molecule. Carboxypeptidase digestion revealed a shortening of the B (C-terminal) end of the molecule. 3. 3.|Fibrils from pepsin-digested tropocollagen showed changes in morphology dependent on the length of exposure to pepsin. Short digestion (1-5 h) gave rise to fibrils with a symmetric band pattern. At 20 h tactoidal structures with both polarised and symmetric band patterns were formed. Prolonged digestion (5 days) gave rise to symmetric tactoids predominantly. Digestion of tropocollagen with leucine aminopeptidase gave rise to symmetric fibrils. Digestion. with carboxypeptidase gave rise to polarised tactoids. 4. 4.|A hypothesis suggesting that the A end of the collagen molecule is required for orientation of molecules within a fibril and that the B end is required for a fibril diameter-limiting mechanism is proposed on the basis of these results.
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