A great many polysaccharides are known to occur
in nature in the form of sulphuric acid esters. To
give but two classes of such compounds we have (a) some of the gel-forming carbohydrates from various
algae, particularly marine algae.
(b) many of the gluco-proteins are protein derivatives
of complex substances which are essentially sulphuric acid esters.
In the former class the sulphuric acid ester is
usually soluble and can be extracted from the algae
by treatment with cold or hot water. In some cases
extraction is carried out by cold dilute acids or
by cold dilute alkali.
Only a few of these gel-forming carbohydrates
have been subjected to any detailed investigations.
In the few cases that have been the subject of re search
the results are of an extremely vague nature and
rather inconclusive due to the experimental difficulties