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Collagen is the most abundant structural protein in vertebrates. The specific shape of its stress-strain curve is crucial for the function of a number of organs. Although the macroscopic mechanical behavior of collagen is well known, there is still no explanation of the elastic process at the supramolecular level. We have performed in situ synchrotron x-ray scattering experiments, which show that the amount of lateral molecular order increases upon stretching of collagen fibers. In strain cycling experiments the relation between strain and diffuse equatorial scattering was found to be linear in the "heel" region of the stress-strain curve. A new molecular model for collagen elasticity is proposed, which, based on the existence of thermally activated molecular kinks, reproduces this linearity and gives a simple explanation for the form of the stress-strain curve of collagen.

Osteogenesis imperfecta (OI) is a disease attributable to any of a large number of possible mutations of type I collagen. The disease is clinically characterized in part by highly brittle bone, the cause of this feature being unknown. Recently a mouse model of OI, designated as osteogenesis imperfecta murine (oim), and having a well defined genetic mutation, has been studied and found to contain mineral crystals different in their alignment with respect to collagen and in their size. These observations are consistent with those reported in human OI and the unusual crystal alignment and size undoubtedly contribute to the reduced mechanical properties of OI bone. While the mineral has been investigated, no information is available on the tensile properties of oim collagen. In this study, the mechanical properties of tendon collagen under tension have been examined for homozygous (oim/oim), heterozygous ( ϩ /oim), and control ( ϩ / ϩ ) mice under native wet conditions. The ultimate stress and strain found for oim/oim collagen were only about half the values for control mice. Assuming that prestrained collagen molecules carry most of the tensile load in normal bone while the mineral confers rigidity and compression stability, the reported results suggest that the brittleness of OI bone in the mouse model may be related to a dramatic reduction of the ultimate tensile strain of the collagen. ( J. Clin. Invest. 1997. 100:40-45.) Key words: collagen • osteogenesis imperfecta • mechanical properties • stress/strain • x-ray scattering

Monolithic silica aerogels modified by functional organic groups were prepared by solgel processing of Si(OMe) 4 /A(CH 2 ) n Si(OR) 3 mixtures under identical experimental conditions, followed by drying of the wet gels with supercritical CO 2 . The employed functional groups and PPh 2 (n ) 2). These groups were retained in the aerogels except the isocyanate groups, which reacted with methanol to the corresponding carbamate. The properties of the obtained aerogels are rather independent of the kind of functional group, but strongly depend on the Si(OMe) 4 /A(CH 2 ) n Si(OR) 3 ratio, which was varied between 9:1 and 6:4. The density of the aerogels was 0.2-0.3 g cm -3 ; some aerogels with lower densities were also prepared for comparison. Gelling of the precursor mixtures is drastically slowed with an increasing portion of A(CH 2 ) n Si(OR) 3 , and the water consumption is retarded. During supercritical drying, shrinkage of ∼10% was observed for the aerogels prepared from the 9:1 precursor mixtures. Increasing the portion of A(CH 2 ) n Si(OR) 3 or decreasing the aerogel density lead to a larger shrinkage and an incomplete incorporation of the functional organic groups. The chemical composition of the resulting aerogels was investigated by infrared (IR) and Raman spectroscopy, elemental analysis, and titration of the functional groups, and their structure by nitrogen sorption and small-angle X-ray scattering (SAXS). The Brunauer-Emmett-Teller (BET) surface areas generally decreased with an increasing portion of A(CH 2 ) n Si(OR) 3 , whereas the C parameter showed a saturation behavior. An analysis of the pore volumes indicated that with an increasing portion of A(CH 2 ) n Si(OR) 3 or a decreasing bulk density, the gel skeleton is increasingly compressed during the N 2 sorption experiments. SAXS measurements showed larger particles upon increasing the A(CH 2 ) n -Si(OR) 3 /Si(OMe) 4 ratio, which correlates with the observed decrease of the specific surface areas. The results were interpreted that an increasing portion of A(CH 2 ) n Si(OR) 3 has the same kinetic effects on the hydrolysis and condensation reactions and the same structural consequences for the network formation as decreasing the bulk density of an aerogel obtained from the one-component Si(OMe) 4 system. The fractal dimension increased with an increasing portion of A(CH 2 ) n Si(OR) 3 ; it was significantly larger for A ) NCO and NHC(O)OMe than for A ) SH or OC(O)C(Me)dCH 2 .

Purely f.c.c. C70 was irradiated with high-energy heavy ions at low temperature. This led to the appearance of additional Bragg peaks attributed to an irradiation-induced f.c.c.-to-rhombohedral phase transition. On heating to about 570 K the retransition to the pure f.c.c. phase occurred. Since no irradiation-induced phase transition was observed in C60, the rhombohedral phase appears to be connected with the “rugby-ball shape” of C70 which favours the alignment of the molecules. We propose that irradiation provides the agitation enabling the C70 molecules to overcome the small energy barriers which otherwise prevent the alignment of the rugby balls over larger regions.

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