Collagen fibrils are characterized by a 67-nm axial
periodicity; they also define the shape of the tissues in
which they occur. Fibrils are arranged in complex threedimensional
arrays such as orthogonal lattices (e.g., in
the cornea), parallel bundles (e.g., in ligament and tendon),
and concentric weaves (e.g., in bone). They are
characterized by three interwoven chains (a-chains) that
can be homotrimeric or heterotrimeric, depending on the
collagen type, each chain having a polyproline II-like
conformation [1, 2]. The tight triple helix configuration
is allowed by the repetitive Gly-X-Y triplet, where X can
be any other amino acid, but is usually a proline, and Y
is often a hydroxyproline [1]. Glycine is an absolute
requirement in every third position because it is the
smallest amino acid that can occupy the limited space in
the center of the triple helix