# Unraveling and Sliding of Polypeptide Strands Underlies the Exceptional Toughness of the Triple-Helix Collagen Molecule

## TL;DR

Collagen's toughness comes from its triple-helix structure unraveling and sliding under stress, allowing it to stretch significantly without breaking.

## Contribution

The study reveals a controlled slippage mechanism in collagen's triple-helix structure as the source of its exceptional toughness.

## Key findings

- Individual tropocollagen molecules can stretch up to three times their native length without breaking.
- Slippage of α-chains in the triple-helix occurs irreversibly before being stabilized by chain-end interactions.
- Electrostatic interactions with mica prevent aggregation and support the stretching mechanism.

## Abstract

Fibril-forming tropocollagens (TCs) play an essential
role in tissue
biomechanics. They are ubiquitous in mammals and other animal tissues,
where they provide passive mechanical functions. While molecular dynamics
simulations have targeted the mechanics of individual TCs, experimental
data on their tensile mechanical properties remain scarce. As a consequence,
the link between the unique triple-helix structure of the collagen
molecule and macro-mechanical properties of collagenous tissues is
not well understood. To close this gap, we have investigated isolated
TCs grafted on the tip of atomic force microscopy (AFM) probes as
well as adsorbed TC films using a surface force apparatus (SFA). AFM
force spectroscopy showed that an individual TC can be stretched without
failing to a contour length of up to 900 nmnearly three times
its native lengthover thousands of stretching cycles. The
molecule was retracted from a strongly adhering mica surface by pulling
on one of the α-chains, forcing the triple-helix to unravel.
During this process, the α-chains slipped progressively, irreversibly,
and almost entirely past each other before being caught by strong
physical interactions between overlapping chain ends. SFA measurements
showed that strong electrostatic interactions bind TC to mica and
prevent TC aggregation, supporting the AFM results. These findings
indicate that a controlled slippage mechanism underpins the exceptional
toughness of TCs, collagen fibrils, and collagen-rich tissues such
as tendons and skin.