End chain and side chain interactions in the ordered aggregation of modified collagen macromolecules
Sonic irradiation of acid-soluble collagen (tropocollagen) markedly affects the aggregation properties of these macromolecules. Relatively brief irradiation prevents the formation of native-type (~ 700 Å axial period) fibrils. However, ordered aggregates of the segment long-spacing (SLS) type, in which the macromolecules are all „pointing“ in the same direction and are in register with respect to their ends, are still formed on addition of ATP; these have lengths and band patterns indistinguishable from those obtained from unirradiated preparations. Dimeric and polymeric SLS-type structures are commonly found. Detailed analysis of the band patterns of these forms led to the suggestion that one or more peptide chains may extend beyond the triple helix body of the tropocollagen macromolecule at each end and that polymerization, either of the normal type or of the abnormal types observed in irradiated collagen, involves the orderly interaction of these end chains — possibly by a coiling of the chains about each other.