Chemistry Reference
In-Depth Information
Figure 9.
Electron micrographs of type I collagen in various states. (A) Rotary shadowed monomer.
Note that the individual chain is not entirely a rigid rod, but has some kinks and flexibility. In this EM
the N-terminus is on the left. The sharp kink is typical of the N-terminal region. In solution a collagen
monomer always has a shorter end-to-end length than its contour length. The difference in length
between free molecule and extended length in either native fibril or SLS aggregate shows the stabilizing
role of the intermolecular interactions in stiffening the triple helix. (B) Collagen molecules aggregated
in side-by-side, axially aligned segment-long-spacing form, SLS. The elongated molecules have a
length 4.4 times the D-periodicity noted in native fibrils. (C) A section of the band pattern of a native
collagen fibril, doubly positively stained with phosphotungstate and uranyl acetate. The length of the
repeat D-period, ~70 nm, is denoted by the double arrow. The major stained bands are denoted by
electron microscopists as the a-, b-, c-, d-, and e-bands, although the bands may be complexes of bands
as in the a-band. (D) The molecular packing in a native fibril in the Petruska-Hodge model, arranged to
correspond to the band pattern of the native fibril. Each molecule has the length shown in (A) or (B),
4.4D. The numbers, 1,2,3,4,5, are the relative axial positions of the domains within the D-period.
Fibril structure
Figure 9 shows electron micrographs of the collagen monomer (Fig. 9A), the
parallel-packed SLS form (Fig. 9B) and the structure of a typical fibril (Fig. 9C). The
SLS aggregate (Fig. 9B), has an asymmetric banding pattern, in a combination positive/
negative staining procedure, showing 52 or 53 clearly demarcated bands. This asymmetry
of the band pattern confirms the identical orientation of the molecules within the SLS
aggregate, with all of the molecules in the same N to C orientation. The stain pattern is
directly related to the sequence of charged amino acid side chains. The length of the SLS
corresponds to the contour length of the monomers, 290 nm, as demonstrated in Fig. 9A.
On the other hand, the fibril (Fig. 9C) has a periodicity of 67 nm. The problem of packing
rigid rods ~ 300 nm long to achieve a ~67 nm periodic structure (D-periodic) was solved
