Starch is one of the most important natural products.
It finds many industrial uses and applications, and, in
different forms, is used throughout the world as a staple
item of the human diet.
Consequently, the structure and properties of starch
have deservedly formed the subject of extensive
investigations both by chemists and bio-chemists.
Because of the inherent difficulties in the many problems
which arose, and the continual need for development of
new techniques, progress prior to 1925 was slow and
uncertain. However, the subsequent elucidation of the
precise structural configurations of the simpler sugars
e.g. glucose, maltose and cellobiose, gave a firm
foundation for future work, and steady progress ensued.
One of the most important advances was the realisation of
the apparent inhomogeneity of starch; this was followed
in 1941 by the first quantitative fractionation into
simpler components.
As a result, the highly polymeric nature and main
structural features of the molecule are now well estab¬
lished. In general, starches can be separated into two
distinct entities, both of which are high polymers of
α-D-glucopyranose units joined by α-1:4-glycosidic
linkages: (a) Amylose - a mixture of very long, essentially
inbranched, chains of these units. (b) Arnylopectin - a
mixture of much shorter, highly branched chains of these
units, the inter-chain, or branching, linkages being
essentially α-l;6.
Yet many problems still remain. Some are concerned
with the clarification of controversial issues on which
the present experimental evidence, as reported by various
workers, is either at variance or considered to be
inconclusive; the majority, however, are concerned with
the fundamentally important details of fine structure and
the molecular size and shape of the starch components.
It is now accepted that there is little prospect of
progress on these particular problems unless full use is
made by the carbohydrate chemist of physical and enzymic
methods to supplement purely chemical approaches.
When the researches reported in this thesis were
begun, it was apparent that, before the established
physical techniques used in polymer chemistry for the
determination of molecular size and shape could be
profitably applied, work on the following topics was an
essential preliminary in order that the materials to be
studied could be adequately and accurately characterized:-
(1) The development of a differential potentiometric
iodine titration technique for the determination
of the % amylose in starches and their fractionated
components, and for the possible characterization
of other a-1:4-glucosans. It was desirable that
the apparatus should be considerably more sensitive
and accurate than any of those previously reported,
yet also be capable of simple operation as a
routine analytical method.
(2) A study of the oxidation of starches by the
periodate ion in view of the low quoted percentage
accuracy of the technique, and the then existing
lack of agreement in the literature on the following
points; (a) conditions for the quantitative
estimation of the formic acid released, (b) the
length of time required for complete oxidation, and
the possibility of over-oxidation, (c) the presence
of intact glucose residues after oxidation,
suggesting that inter-chain linkages other than
1:6 existed in amylopectin.
(3) A study of the effect of the presence of contaminating protein on results obtained by use of
the potentiometric iodine titration and periodate
oxidation techniques.
(4) The preparation, and purification, of starches
from several botanical sources which had not previously
been studied, so that the use at any stage of industrial samples of unknown origin, treatment, and history
could be avoided. By preparation of the starches under
the mildest possible conditions, so as to minimise
degradation, these same samples could later be used for
physico-chemical studies of molecular size and shape.
The opportunity was taken, after the extraction of
each starch, to apply a graded series of extractions
to the residual plant material; the fractions isolated
were analysed for protein, ash, and uronic acid content,
and the polysaccharide content was investigated by
chromatographic analyses of the sugars liberated
on hydrolysis.
