Lepidosperma sp.

King, Douglas I., Hamid, Kaiser, Tran, Van H., Duke, Rujee K. & Duke, Colin C., 2021, Kangaroo Island propolis types originating from two Lepidosperma species and Dodonaea humilis, Phytochemistry (112800) 188, pp. 1-11 : 4-5

2.2. Lepidosperma sp. Flinders Chase resinous exudates foraged by honey bees to produce propolis

In the north west coastal De Mole River area of Kangaroo Island honey bees were observed collecting resin from Lepidosperma sp. Flinders Chase. Resin samples were collected from plants ( Fig. 2S ) and bee legs and, also, freshly deposited propolis was collected from nearby beehives. 1 H NMR spectral analyses of ethanol extracts of the resin samples and S 2 type propolis showed that they closely matched in constituent profile indicating that the plant was the botanical source of the resin collected by the bees to produce the propolis sample from the hive ( Fig. 3 ).

TLC analytical ( Fig. 1S ) and 1 H NMR spectral profiles of 73 Lepidosperma sp. Flinders Chase resin samples collected over KI showed two distinct patterns for the distribution of quantities of the compounds 1 to 3, and 6 and 7. For 60 samples one pattern was observed that showed variations for 1, 3, and 6 from rich to depleted and that 7 was always a minor constituent over all samples. These plant resin samples came from plants widely distributed over KI and isolated yields for compounds 1, 3, 6 and 7 were 4%, 12%, 10% and 0.5% respectively.

3

For 13 plant samples collected from Cape Borda and Stunsail Boom exposed coastal locations another pattern was observed that showed only compounds 2 and 3 as major constituents with isolated yields of 18% and 16% respectively.

Compound 1 is the methyl ester of 3-phenyl-2-(E -cinnamoyloxy) propanoic acid. The free acid form, commonly known as O -(trans -cinnamoyl)-L- β -phenyllactic acid, has been much studied is the past as a substrate for the zinc-containing metalloenzyme carboxypeptidase A ( Awazu et al., 1967). Recent interest is focused on β -phenyllactic acid which exhibits broad spectrum antimicrobial activity and is also of current interest as a food additive as an alternative to antibiotics for food preservation ( Chaudhari and Gokhale, 2016). β -Phenyllactic acid has been found in honey at levels of 100–800 mg /kg.

3-(8- E -Methoxy-8-oxo-3,7-dimethyloct-2-enyl)-4-hydroxy- E -cinnamic acid (2) and 3-(E -6,7-dihydroxy-3,7-dimethyloct-2-enyl)-4-hydroxy- E -cinnamic acid (3) appear to be metabolites of 3-geranyl-4- hydroxy- E -cinnamic acid where, from position 6 of the geranyl group, the group is oxidized. Both of these metabolic modifications are uncommon and no similar structures were found. By contrast, metabolites of 3-geranyl-4-hydroxycinnamic acid from Chrysothamnus pulchellus (Asteraceae) were hydroxylated at the allylic 4-positions on the geranyl group ( Jakupovic et al., 1990).

Known stilbene, 2-prenyl-3,5-dihydroxy- E -stilbene (6) was previously isolated from Glycyrrhiza flavescens (Fabaceae) ( Kusano et al., 2002) and, also, the known stilbene 2-prenyl-3-methoxy-5-hydroxy- E -stilbene (7), was previously isolated from Lonchocarpus violaceus (Fabaceae) ( Delle Monache et al., 1977) and Cajanus cajan (Fabaceae) ( He et al., 2019) ( Fig. 4 ).

Figures

Fig. 2. Key heteronuclear multiple bond correlation (gHMBC) of compounds 1 to 5.

Fig. 3. 1H NMR spectra of Lepidosperma sp Flinder Chase resin (A), plant resin from bee legs (B) and freshly deposited propolis from bee hives (C).

Fig. 1. Cinnamic acid derivatives and prenylated dihydrochalcones identified in this study.

Fig. 4. Known prenylated stilbenes found in propolis and plant source resin in this study.