Many trees, of a wide variety of species, respond to injury
by exuding yellowish viscous fluids, which harden on exposure to
the atmosphere, producing glassy nodules. These are the plant
gums (1), which are among the most complex polysaccharides
known. In molecular structure, they resemble the mucilages and
the bacterial polysaccharides; in fact, there are no general
structural differences between the gums and the mucilages(2 -8).
The only distinction lies in their mode of origin, since the
mucilages are isolated only by the extraction of seeds or other
plant material, in which they apparently serve as food stores
or as moisture reservoirs. It is therefore necessary, for the
purpose of this thesis, to give a restricting definition of
plant gums as uronic acid containing polysaccharide exudates.
This definition also excludes resinous exudates of terpenoid
structure, and non- exuded neutral polysaccharides which are
known colloquially as gums, e.g. carob seed gum, which is a
galactomannan (9).The origin of the gums is still uncertain. They are
commonly, although not exclusively, Produced in hot, dry
climates, and healthy trees tend to exude less gum than those
in poor condition. For this reason, it has been suggested
that they are the result of infection, and a few gums, including chagual (10) and honey locust gums (11), are in fact known to
be pathological products. On the other hand, some gums, such
as gum tragacanth, are exuded copiously immediately after
incision of the bark, and are obviously natural products of
the plant's metabolism. It also seems unlikely that gums
produced on a commercial basis are the products of infection.
In general, it is probable that a tree exudes gum in order to
seal off the injured part, and to prevent the spread of infection.
The similarity of the gums and the bacterial polysaccharides,
and the cross -reactions which can take place
between gums and some Pneumococcus sera (12), may be significant
in this context, and the complexity of structure of the
gum polysaccharides may be connected with the necessity for
dealing with a variety of attacking bacteria.The commercial use of gums is almost as old as civilization.
The Egyptians used them in embalming, and for the
last few hundred years they have been common ingredients of
medicines and of 'aids to beauty'. Today, they are still
used in the fields of pharmaceuticals and cosmetics, but a
wide range of manufacturing processes also employs them as
emulsifiers, adhesives, thickeners, binding materials, etc.
Being harmless and tasteless, they find many uses in the food
industry.In view of their commercial importance, the scientific study of gums is worthwhile, and of course is also important
for its intrinsic biochemical interest. But perhaps the most
important reason for carrying out investigations lies in the
resemblance to bacterial polysaccharides. It may be possible,
by drawing analogies, to gain. insight into the structure and
formation of the latter, and thus to proceed to a fuller understanding
of the bacteria themselves.A typical gum possesses a highly branched structure, containing
anything from two to four different neutral sugar
residues, and a uronic acid residue; each may exist in more
than one type of linkage. Several gums recently investigated
are further complicated by the fact that they contain two
different uronic acid residues. The most common neutral sugars
are D-galactose, D-mannose, L-arabinose, D-xylose and L-rhamnose,
but L-fucose and D-tagatose have also been observed. The
uronic acids of gums are D- glucuronic acid, 4 -0- methyl-D-glucuronic
acid and D-galacturonic acid.In the natural state, many guns exist as neutral salts of
such cations as calcium and magnesium. Some, e.g. the
Sterculia gums and Cochlosoermum gossypium, are acetylated, and
give off a distinct odour of acetic acid. Varying molecular
weights have been quoted, ranging from 2 - 300,000 for gum
arabic, to 9,500,000 for Karaya gum (13). The usual means of
measurement is by sedimentation techniques. Because of the structural complexity of the molecules concerned, chemical
methods of molecular weight determination are in general
unsatisfactory
