Thermal (In)Stability of Type I Collagen Fibrils

We measured the Young’s modulus at temperatures ranging from 20 to $100°\mathrm{C}$ for a collagen fibril that is taken from a rat’s tendon. The hydration change under heating and the damping decrement were measured as well. At physiological temperatures 25 to $45°\mathrm{C}$, the Young’s modulus decreases, which can be interpreted as an instability of the collagen. For temperatures between 45 and $80°\mathrm{C}$, the Young’s modulus first stabilizes and then increases when the temperature is increased. The hydrated water content and the damping decrement have strong maximums in the interval 70 to $80°\mathrm{C}$ indicating complex intermolecular structural changes in the fibril. All these effects disappear after heat-denaturation of the sample at $120°\mathrm{C}$. Our main achievement is a five-stage mechanism by which the instability of a single collagen at physiological temperatures is compensated by the interaction between collagen molecules.

Figure 1

(a) Hierarchic tendon structure: collagen triplexes ($M$), fibrils ($F$), fascicle, and fiber. (b) Fiber is composed of fascicles. (c) The fascicle is a composite of collagen fibrils in a proteoglycan-rich matrix (pg). (d) The fibril is composed of triplehelical collagen molecules ($M$). There is a succession of gap ($G$) and overlap ($O$) zones. The triplexes are stabilized in the fibril by intermolecular crosslinks, direct hydrogen bonds, and water-mediated hydrogen bonds. ${\epsilon }_T$, ${\epsilon }_F$, and ${\epsilon }_M$ denote strains on the tendon, fibril, and separate molecule [20].

Figure 2

The Young’s modulus $E$ versus temperature for the native and heat-denatured collagen fibrils. The heating speed is $1°\mathrm{C}/\min $. Arrows and indices 1–5 indicate on temperature intervals discussed in the text.

Figure 3

The logarithmic damping decrement $\vartheta$ versus temperature for the native and heat-denatured collagen fibrils. The heating speed is $1°\mathrm{C}/\min $. The arrow and indices 3 and 4 correspond to those in Fig. 2.

Figure 4

The hydrated water content (HWC) versus temperature for native and heat-denatured collagen fibrils. The initial temperature and water content in the chamber were $25°\mathrm{C}$ and 93%, respectively. At this temperature and humidity, the water content of the native and denatured microfibril is $h=0.3{\mathrm{gH}}_2\mathrm{O}/\mathrm{g}$ dry collagen and $h=0.22{\mathrm{gH}}_2\mathrm{O}/\mathrm{g}$ dry collagen, respectively. The change of the water content is given in percents relative to that water content at $25°\mathrm{C}$.