Occurrence of (RS,RC)- and (SS,RC)-marasmin in Tulbaghia violacea Harv.

The cysteine sulphoxide marasmin is a natural sulphur-containing aroma precursor, occurring in several plant species, as well as in some members of the genus Marasmius. After disruption of a cell containing it, it is cleaved by a C-S-lyase. The generated sulfenic acids lead to the formation of the thiosulfinate marasmicin which is the main odourous compound of Marasmius spec. and some of these plants. Marasmicin has been drawn attention to for further investigation due to bioactivity against fungi and Mycobacterium tuberculosis. [1]

Marasmin was firstly identified as precursor of the garlic odour in Marasmius spec. [2]. In the fungi it appears as glutamyl-dipeptide with (S)-configuration at the sulphur atom. 11 years later it was found in the fruits of the tree Scorodocarpus borneensis Becc., where it exists in form of the free sulphoxide in (R)-configuration [3]. In the following marasmin was detected in several plants of the Amaryllidaceae including Allium, Ipheion, Leucocoryne and Tulbaghia [4]. As in S. borneensis only the (R_S,R_c)-marasmin could be detected in all plant material investigated so far.

Recent research on the South African plant Tulbaghia violacea Harv., known as 'society garlic', revealed the presence of both configurations of marasmin (Fig.), with (R_S,R_c)-marasmin being the major compound. This is an example of the rare appearance of both isomers of one cysteine sulphoxide in the same plant as well as the first report of (S_S,R_c)-marasmin within the plant kingdom. This leads to the conclusion that either the oxidizing enzyme is not very specific or there is more than one enzyme involved in the oxidation of cysteine derivatives in this plant.

References:

[1] Kusterer J, Fritsch RM, & Keusgen M, J Agric Food Chem. (2011) 59(15):8289 – 97

[2] Gmelin R, Luxa H-H, Roth K, & Höfle G, Phytochem. (1976) 15(11):1717 – 1721

[3] Kubota K, Hirayama H, & Sato Y, Phytochem. (1998) 49(1):99 – 102

[4] Kubec R, Krejčová P, Mansur L, & García N, J Agric Food Chem. (2013) 61(6):1335 – 42