Glassy state of native collagen fibril

Our micromechanical experiments show that viscoelastic features of type-I collagen fibril at physiological temperatures display essential dependence on the frequency and speed of heating. For temperatures of 20–30 ◦C the internal friction has a sharp maximum for a frequency less than 2 kHz. Upon heating the internal friction displays a peak at a temperature Tsoft(v) that essentially depends on the speed of heating v: Tsoft ≈ 70 ◦C for v= 1 ◦C/min, and Tsoft ≈ 25 ◦C for v= 0.1 ◦C/min. At the same temperature Tsoft(v) Young’s modulus passes through a minimum. All these effects are specific for the native state of the fibril and disappear after heat-denaturation. Taken together with the known facts that the fibril is axially ordered as quasicrystal, but disordered laterally, we interpret our findings as indications of a glassy state, where Tsoft is the softening transition. Copyright c © EPLA, 2011 Connective tissues (tendon, chord, skin, bones, cornea and dentine) are complex, hierarchical structures, which have widely different mechanical and biochemical demands with respect to strength, elasticity and energy storage [1]. These demands are met via adaptation of the tissue’s hierarchical structure. Its major building block is the fibril made from triple-helical type-I collagen (macro)molecules, the most abundant protein in mammals [1]. The fibril combines axially ordered quasicrystalline structure with lateral disorder [2,3]. Both these aspects have been actively studied in the last decades [1–5], but many issues are still open. This subject is relevant for medicine, since a number of diseases (e.g., arthritis) relate to abnormalities in the fibril structure [1]. Here we shall study viscoelastic features of the native type-I collagen fibril. Our experimental results point out the existence of a glassy state at physiological temperatures. This state is displayed via frequency-dependent viscoelastic characteristics (Young’s modulus and the damping decrement) of the native fibril. Upon heating the fibril goes out of the glassy state, an effect known as the softening transition [6,7]. The temperature of this transition depends essentially on the speed of heating. (a)E-mail: [email protected] We confirmed that glassy features are not seen for heatdenaturated fibril. This will be the first example of a room temperature biopolymer glassiness, because the glass transition in globular proteins was experimentally observed at ≈ 200K [8–13]. Several methods contributed to the understanding of this transition: micro-mechanical experiments [8], NMR [9], Moessbauer spectroscopy [10], calorimetric studies [11], and X-ray scattering of synchrotron radiation [12]; see [13] for reviews. It is believed that the large-scale conformational motion of proteins freezes at ≈ 200K, analogously to freezing of cooperative motion in glass-forming liquids [13] and segmental motion in synthetic polymers [6,7]. The fibril’s structure. – In rat tail tendon the fibril consists of 10 triple-helical collagen molecules of diameter 1 nm and length 300 nm staggered together. We conventionally separate the collagen molecule into 5 segments [2]. The segments from 1 to 4 have equal length D= 67nm (this value of D is slightly tissue dependent), segment 5 is of length 0.5D. The fibril is ordered along its axis: nearby molecules overlap by distance (0.5+ k)D, where k= 0, . . . , 4 can assume any value between 0 and 4. This is a quasicrystalline order, because the long-range order is displayed without strict translational symmetry, and the four possible types of overlaps are distributed along the fibril. The most frequent overlap is 0.5D [14]. It