Localization of stress-induced molecular rearrangements in collagen

Molecular rearrangements in native fibres of rat-tail tendons (RTT) and human finger flexor tendons are registered with the help of short time diffraction spectra using synchrotron radiation. There is a tension-induced increase of the 67 nm long period as well as changes in the intensities of some meridional small angle reflections (fig. 1). Both effects are reversible when unloading the fibre, but are preserved when the load is held constant until the fibre tears. The increase in the long period is due partly to an inhomogeneous molecular process as indicated by the change in the intensities and partly to a stretching of the triple helices themselves. The inhomogeneous behaviour is due to an increase of the D-stagger from 234 to for example 236 amino acid residues, caused by sliding of the triple helices relative to each other, which results in a change of the length of the gap and overlap regions. This sliding of triple helices which are aligned according to the scheme of Hodge and Petruska is coupled with a stretching of the cross-linked telopeptides. This interpretation is supported by calculated axial electron density distributions under consideration of the relative intensities derived from models with varying length of gap and overlap regions, as well as by comparison with model calculations which include telopeptides.