  The one-dimensional problem of selecting the triple helix with the highest
volume fraction is solved and hence the condition for a helix to be
close-packed is obtained. The close-packed triple helix is shown to have a
pitch angle of $v_{CP} =43.3 ^\circ$. Contrary to the conventional notion, we
suggest that close packing form the underlying principle behind the structure
of collagen, and the implications of this suggestion are considered. Further,
it is shown that the unique zero-twist structure with no strain-twist coupling
is practically identical to the close-packed triple helix. Some of the
difficulties for the current understanding of the structure of collagen are
reviewed: The ambiguity in assigning crystal structures for collagen-like
peptides, and the failure to satisfactorily calculate circular dichroism
spectra. Further, the proposed new geometrical structure for collagen is better
packed than both the 10/3 and the 7/2 structure. A feature of the suggested
collagen structure is the existence of a central channel with negatively
charged walls. We find support for this structural feature in some of the early
x-ray diffraction data of collagen. The central channel of the structure
suggests the possibility of a one-dimensional proton lattice. This geometry can
explain the observed magic angle effect seen in NMR studies of collagen. The
central channel also offers the possibility of ion transport and may cast new
light on various biological and physical phenomena, including
biomineralization.
