The close-packed triple helix as a possible new structural motif for collagen

TL;DR

This paper proposes a new close-packed triple helix structure for collagen, characterized by a specific pitch angle, which could explain various biological phenomena and resolve existing structural ambiguities.

Contribution

It introduces a geometrically optimal close-packed triple helix model for collagen, challenging conventional structures and suggesting new functional implications.

Findings

Close-packed triple helix has a pitch angle of 43.3°

Proposed structure features a central channel with negative charges

Supports potential for ion transport and biomineralization

Abstract

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.