Phonons and the elastic moduli of collagen and muscle

BIOLOGICAL fibres make a suitable subject for the development of the science of biomaterials. The amino-acid sequence of the collagen molecule is known1,2, and has been used to account for the self-assembly of collagen into native fibrils1 and polymorphic forms3, the pattern of bands due to positive staining in electron micrographs4 and the low-angle meridional X-ray diffraction pattern5. The collagen structure to amino acid resolution in axial projection is thus completely understood. Here we relate this structural picture to the mechanical properties (and since we are dealing with a biological fibre, these are also the biological properties) of the fibres. This can be done by investigating the vibrational and other motions of the molecular assembly in collagen. The force constants of hydrogen bonds in some polypeptides have been estimated on the basis of models6 but no experimental measure of their magnitude has been reported. We describe measurements of the inelastic light scattering (Brillouin) spectra of collagen from rat rail tendon from which we obtain elastic moduli of the tropocollagen molecule and an estimate of hydrogen bond force constants. We have also studied effects of hydration on the spectra to obtain information about the association of water with collagen in the native state. For comparison we have made preliminary measurements on molluscan ‘catch’ muscle in which the polypeptide conformation is different from collagen.