Contractual obligation: Raman spectroscopy reveals collagen secret

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  • Published: Feb 1, 2015
  • Channels: Raman
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Collagen investigation

X-ray view of collagen: from the patterns of the two-dimensional X-ray diffraction, information about changes in the molecular and nanoscopic collagen structure can be gained when the protein dries. The structure of collagen is crucial for power generation. © Nature Communications 2015 / MPI of Colloids and Interfaces

Raman spectroscopy coupled to synchrotron X-ray diffraction has been used to investigate the conformation of molecular chains of collagen and how this purportedly inactive protein can under contraction.

Collagen is the main structural protein in animals and present in bone, tendons, ligaments and skin. It is also present in the cornea, cartilage, blood vessels, the gut, intervertebral discs and in dentin. In mammals, it represents between 25 and 35 percent of total protein mass. Broadly, it is considered to be an essential but "inactive" protein. Understanding the chemistry and the physical properties of collagen is important in physiology, in understand bone and joint and other medical problems and how they might be remedied, in tissue engineering and in the development of novel engineering materials that mimic collagen for robotics and other applications.

A force to be reckoned with

Now, researchers at the Max Planck Institute of Colloids and Interfaces in Potsdam-Golm, Germany, together with scientists from the Massachusetts Institute of Technology in Cambridge, USA, led by Admir Masic and Luca Bertinetti have shown that removing water from collagen fibres has a dramatic effect on its molecular and nanoscopic features. The fibres contract and generate tensile forces that are three hundred times greater than those that might be exerted by a human muscle. The discovery suggests that collagen may not be quite the inactive protein that science has previously thought and may have a much more active role in various physiological processes.

It is well known that collagen has a hierarchical structure comprising a complex arrangement of individual molecular components. The basic building block of which is the collagen molecule itself with its twisted structure reminiscent of rope wherein three strands are intertwined to form a triple helical motif. These motifs in turn combine to form thicker coils, collagen fibrils some 100 to 500 nanometres thick. Within each fibril, adjacent collagen molecules stack to form a staggered arrangement, which gives rise to alternating dense and less zones along its length. Many fibrils then combine to form collagen fibres.

The researchers have investigated the influence of water content on the properties of collagen and used various techniques to look at collagen at each of its hierarchical levels while, for the first time, maintaining control of the overall water content. The Golm team's data was compared with computer models developed by their colleagues at MIT.

Water, water

"Water is an integral component of collagen," explains Admir Masic. Water molecules bind tightly and follow the helical structure. In collagen's natural state, water represents almost two-thirds of its mass. Given the high water content of collagen, the team was not surprised that removing water had a dramatic effect. When they reduced the relative humidity of their collagen samples from 95 to 5 percent, the collagen almost dries out, the collagen molecules shortening by 1.3 percent and the corresponding fibrils by 2.5 percent. Although this is a relatively small change in length, the corresponding tensile force this represents is equivalent to around 100 megapascals, a much greater development of power than in contracting muscle fibres.

Usefully, Masic and Bertinetti's team have identified the mechanism underlying this contraction process using Raman spectroscopy. This technique allowed them to see the conformational changes that took place during the dehydration process. Intriguingly, the denser regions of the fibrils become longer, while the thinner regions shorten with a net contraction. "With this knowledge, researchers could develop materials that behave in a opposite ways when water is removed from them," explains Bertinetti.

Although such a degree of dehydration does not occur in living things, even under biological conditions, loss of water from collagen can generate high tensile forces. Collagen might therefore play an active role during bone synthesis, water is removed, causing the collagen matrix to contract and preventing new and fragile bone, from fracturing under stresses.

Related Links

Nature Commun, 2015, 6, online: "Osmotic pressure induced tensile forces in tendon collagen"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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