Helical structures from neutral biopolymers - Physical Gels from Biological and Synthetic Polymers - page 183

1977 ). The main source of raw material for commercial samples of gelatin is bovine or
porcine, especially bones or hides from beef, or pig skins. More recently, somewhat
different gelatins extracted from

fish skins have become available. We first examine the

features common to all gelatins, and then clarify the differences.

7.2.1 Molecular composition and native conformation

Collagen is proteinaceous (Dickerson and Geis, 1969 ) and is an assembly of tropocolla-
gen macromolecules containing a number of amino acids (Cantor and Schimmel, 1980 ).
The proportion of each amino acid and its exact position in the chain are predetermined.
These amino acids, also called residues, correspond to the monomer units in synthetic
polymers; the collagen single chain, for example, is made of about 1000 residues and
contains about 20 amino acids. Glycine (Gly) is the most abundant by weight, but the
speci c and unique feature of the sequence of amino acids in tropocollagen is that every

third residue is Gly, and in mammalian sourced material the imino acids proline (Pro)
and hydroxyproline (Hyp) constitute another quarter of the composition by weight. The
rings of these imino acids give enhanced rigidity to the tropocollagen chain and play
an important role in both the mechanical and the thermal stability of the collagen
rods. The most frequent sequences found on tropocollagen are therefore

- -

(GlyProX)

- -

or (GlyProHyp) , where X are other amino acids.

7.2.2 Structure of native tropocollagen fibrils

The secondary and tertiary structures of native collagen were established in 1955, inde-
pendently by Rich and Crick ( 1955 ) and by Ramachandran and Kartha ( 1955 ). The three-
stranded, triple-helical model for tropocollagen was rst suggested by Ramachandran and

Kartha, who showed that their X-ray diffraction results could be explained by a helix
comprising left-handed helical chains (secondary structure) related by a right-handed rope
twist (tertiary structure) so that, overall, collagen has a coiled-coil, rope-like conformation.
The Gly residues of the

repeat sequence are placed in the core of the structure
because the triple helix structure, in which the three chains are arranged about a common
central axis, leaves no space in the inner core for larger lateral groups.
The imino acid hydroxyproline (Hyp) was recognized as being associated uniquely
with the collagen triple-helical structure and having a role in stabilizing the structure.
Moreover, the three chains are linked through inter-chain hydrogen bonds perpendicular
to the common axis, although the nature and the number of these bonds is still a matter of
debate. For each

- Gly - X - Y -

triplet, one hydrogen bond forms between the amide
hydrogen atom of glycine in one chain and the carbonyl oxygen atom of residue X in
an adjacent chain. Hydrogen bonds involving the hydroxyl group of hydroxyproline may
also stabilize the collagen triple helix. Ramachandran

- Gly - X - Y -

'

s 1968 model (Ramachandran and
Chandrasekharan, 1968 ) proposes a pattern of hydrogen bonding also involving water
molecules in interstitial positions.
The left-handed helices have 10 residues per 3 turns and the pitch is approximately
0.9 nm. The right-handed superhelix has a pitch 10 times larger, around 9 nm. The