EXTREMELY SWEET STEROIDAL GLYCOSIDES
Both Osladin and Polypodoside A are extremely sweet saponins or sapogenin steroidal glycosides that occur in the roots and rhizomes of both Common Polypody (and in
Liquorice Fern which is also called
Sweet Root (Polypodium glycyrrhiza), but which is not found in the UK). Compared with a 6% w/v aqueous solution of
Sucrose, Polypodoside A is 600 times sweeter than sugar. But as with most other intensely sweet substances, it is so sweet that the sensation of sweetness lingers for over an hour. A direct relative of Polypodoside A, Osladin, is one of the sweetest known natural steroidal glycosides, variously reported as being 500 or 3000 times sweeter than sucrose.
Whether any particular steroidal glycoside is intensely bitter or intensely sweet depends upon both the identity and arrangement of the glycosides, and their stereochemical juxtapositions.
There are about a dozen known intensely sweet steroidal glycosides known, one of which being
Glycyrrhizin which occurs in Glycyrrhiza glabra, and is the well known 'spanish' liquorice root. Other sweet steroidal glycosides (some of which may be synthetic) are
Arbruside A (from Arbus precatorius); Stevioside and
Rebaudioside (from Stevia rebaudiana, and
Baiyunoside (which occurs in
Phlomis betonicoides), but none of these plants are to be found within the UK.
Polypodoside A is chemically: Polypodogenin-3-O-α-L-rhamnopyransyl-(1→2)-β-D-glucopyranosyl-26-O-α-L-rhamnopyranoside.
Osladin is 22R,26R-Epoxy-6-oxo-25S-5α-cholestan-3β,26-diol-3-O-[α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranoside]-26-O-[α-L-rhamnopyranoside]
The stereochemistry is important for taste. Excepting the stereochemical differences, the main difference between Osladin and Polypodoside A is the double bond on the steroid moiety which is absent on Osladin (bottom, centre).