A Subunit Model for the Tropocollagen Macromolecule.

In this article is presented a novel subunit model for the tropocollagen (TC) macromolecule, the monomeric unit of soluble collagen.1 The TC macromolecule is a rigid rod of definite length composed of three helical polypeptide strands that are equal (or very nearly equal) in length.2 4 The strands are of two types chemically, designated a1 and a2.4 Each macromolecule contains two al strands and one a2 strand.4 Our thesis is that each a1 strand is a repeating sequence of a subunit a,, and that the a2 strand is a repeating sequence of a shorter subunit 02. It is proposed specifically, after close examination of experimental data, that the subunits 01 and 02 have lengths in the ratio 7:5. This proposal leads to the "selflimiting" model for the TC macromolecule illustrated in Figure 1. In this model the strands terminate in register when each al strand comprises five a1 subunits and the a2 strand comprises seven 02 subunits. The over-all molecular length L is the "beat period" of the two subunit lengths,