The intermolecular volume of fully hydrated collagen fibrils from a number of mineralized and non-mineralized tissues of adult rats has been determined both by an exclusion technique and by a method which involves the monitoring of specific X-ray diffraction parameters. The intermolecular volume of either bone or dentinal fibrils is approximately twice that of either tail or achilles tendon, and the most frequent intermolecular distance in bone or dentine fibrils is approximately 3 Å larger than of the tendons.

A number of fibrillar structures are most compatible with the intermolecular volume of rat tail tendon. These include hexagonal molecular packing and orthogonal arrays of microfibrils comprising seven parallel molecular strands. The intermolecular volume of bone or dentinal collagen fibrils, on the other hand, appears to arise from structures having a disordered or pseudo-hexagonal molecular packing, in which the most frequent intermolecular distance is about 19 Å.

The space associated with collagen fibrils in adult bone is such that 70 to 80% of the mineral is located within the intermolecular space of the fibrils—approximately equal amounts of mineral being in spaces having lateral dimensions of 25 to 75 Å and 6 to 12 Å, respectively. Particles located in the latter kind of intermolecular space probably constitute, to a large extent, the non-crystalline mineral phase of adult bone.

The stereo-chemical constraints on the transport of mineral ions into and within collagen fibrils of bone and tendon support the postulate that bone collagen is an in vivo catalyst for mineral deposition and further suggests that its catalytic activity may be partially regulated through its molecular packing.