is still a matter of some debate; a fairly well-established model is in Figure
6.21. The average length of the branches is a few tens of monomers,
although some are smaller and some larger. The figure shows that most of
the branches take part in only one ‘‘cluster.’’ The longer ones span two or
even three clusters, thereby providing connections between these.
When viewed with a polarizing microscope, with crossed polarizer and
analyzer, native starch granules show markedbirefringence, which means
that they can be seen.
Note A birefringent material has two or even three optical axes,
which causes the refractive index to vary with the direction of the
wave vector of the polarized light.Consequently, the state of polarization of the light passing through the
material is generally changed, and part of the emerging light can pass the
analyzer; a nonbirefringent material remains dark. Birefringence is caused
by a regular ordering of molecules over distances of about half a wavelength
or more. This does not necessarily imply that the material is crystalline, since
stacks of polymer molecules with a roughly parallel orientation also show
strong birefringence. Birefringence can be positive or negative with respect
to the direction of the wave vector, according to the direction at which the
FIGURE 6.21 Schematic illustration of the structure of a small part of an
amylopectin molecule. The lines denote stretches of linear 1?4 linked anhydroglucose
units, the branching points are 1?6 linkages. The molecule grows from left to right.a
denotes the approximate thickness of a microcrystalline region,bthe repeat distance
of such regions. (Redrawn from S. Hizukuri.Carbohydr. Res.147 (1986) 342.)