Biomedical Engineering Reference
In-Depth Information
O
2
O
3
O
1
N
1
C
′
2
C
3
C
′
1
N
2
C
2
C
3
C
1
N
3
Figure 1.32
The three-chain configuration proposed for
collagen and gelatin by Pauling and Corey in 1951 [204].
uncoil, forming an extended state (which he called the
-helix has
now been recognized as a feature of the majority of protein structures. Astbury's
models were correct in essence and correspond to modern elements of secondary
structure, the
β
-form
) [200]. The
α
-strand (Astbury's nomenclature was used), which
were developed later by Pauling and Corey. These authors have attempted to
account for the positions and intensities of the X-ray diffraction maxima, and
pointed out that the equatorial reflections correspond to a hexagonal packing of
circular cylinders [204]. They proposed a structure (now withdrawn) having three
chains helically intertwined, see Figure 1.32.
The X-ray pattern of an unstretched collagen fiber is rather diffuse and poor in
detail. Its principal features are a strong meridional axe at about 2.9
˚
Aandweaker
arcs at 4.0 and 9.5
˚
A. There are also strong equatorial reflections with spacings
corresponding to approximately 6 and 12
˚
A (which depend on the humidity), and a
diffuse distribution of intensity around 4.5
˚
A, mainly near the equator. This pattern
is improved in orientation and detail if the fiber is kept stretched during the X-ray
exposure [205].
A structural model was built based on various stereochemical properties of the
polypeptide chains, and the intensity distribution expected of it was calculated
using the helix diffraction theory proposed in 1952 by Cochran
et al
. [206]. Next, the
calculated intensity distribution was compared with the observed one [207-211].
The effect of introducing the various side chain atoms on the calculated intensity
distribution has been studied in detail by the Madras group of Ramachandran
[207].
An interpretation of the X-ray pattern led Rich and Crick in 1955 to the conclusion
that the collagen helix has a unit height of approximately 3
˚
A, the number of units
per turn being close to 10/3, which corresponds to a unit twist of 108
◦
[208, 209].
However, the determination of the molecular structure of collagen from X-ray
diffraction data has proven extremely difficult, despite the progresses of fiber
α
-helix and the
β
