Simply stated, organosulfur compounds are organic molecules that contain the element sulfur. Natural products are compounds synthesized in nature from plant, animal or microbial sources. Depending on structure, the presence of sulfur in an organic molecule is often indicated by a distinctive and oftentimes unpleasant and ‘loud’ odor. However, organosulfur compounds can also confer pleasant odor characteristics, as is observed in garlic and onions. The aroma and flavor molecules in garlic and onions are derived from precursor compounds that are derivatives of the amino acid cysteine.

Garlic and onions both contain 1-5% dry weight of cysteine derivatives in which the proton at sulfur in cysteine is replaced with an alkyl or alkenyl substituent, and the sulfur atom is itself oxidized to the sulfoxide. The cysteine sulfoxide derivatives found in onions and garlic are indicated in Figure 1.

Onions contain propiin, isoalliin and methiin (compounds 1-3), whereas garlic contains isoalliin, methiin and alliin (compounds 2-4, Figure 1). Alliin exhibits considerable biological activity.

The distinct flavors of garlic and onion reflect varying amounts of cysteine sulfoxides in each plant, most particularly isoalliin (higher amount in onion) and alliin (higher amount in garlic). Isoalliin is the precursor of thiopropanal S-oxide, the volatile sulfine in onion that causes tearing. The cysteine sulfoxide derivatives are contained in the cytoplasm of the plant cells. In the vacuoles of these cells is contained a class of enzymes known as C-S lyases. If the plant tissue is disrupted by cutting/slicing, chopping, chewing etc, the C-S lyase is released, and it subsequently acts upon the cysteine sulfoxide derivatives, cleaving the C-S bond between the b-carbon and sulfur (Figure 2). This cleavage results in two fragments; a putative sulfenic acid intermediate, and a-aminoacrylic acid. The latter compound spontaneously decomposes to ammonia and pyruvic acid while the former condenses with a second sulfenic acid molecule to form a class of compounds known as thiosulfinates. The importance of the thiosulfinates derives from the fact that they have been shown to exhibit a variety of biological activities, including antibacterial, antifungal, antiviral and anticancer properties, among others. Thiosulfinates also serve as the primary flavor and odor producing molecules in freshly prepared garlic and onion macerates. The thiosulfinates participate in a variety of subsequent reactions which afford a considerable number of organosulfur volatiles, such as sulfides, di- and trisulfides and dithiins (Figure 3). These compounds impart additional flavor, odor and biological activity characteristics to longer standing and/or heat-treated macerates.

Because much early work established that the aforementioned chemistry occurred in garlic and onions, and since both crops are members of the allium family, this chemistry is often referred to as ‘allium chemistry’. However, there are numerous other plants unrelated to the allium genus whose organo-leptic properties imply the presence of organosulfur compounds. Indeed, we have shown that similar chemistry occurs in other plants, such as Petiveria alliacea, Tulbaghia violacea, and Nectaorscordum siculum.

P. alliacea
T. violacea
N. siculum
 

Petiveria alliacea:
P. alliacea L. (family Phytolaccaceae) is a perennial shrub indigenous to the Amazon Rainforest and widely distributed in other areas including tropical America, the Caribbean, Africa, Sri Lanka, and the southeastern Unites States. It is known by many names among which “anamu”, “apacin”, “guiné”, “pipi”, “tipi”, and “garlic guinea henweed” are noteworthy. It has commonly been used in folk medicine, and various preparations made from this plant are considered to have antiinflammatory, antimicrobial, antispasmodic, diuretic, and stimulant effects, among others.

We have isolated and characterized four new cysteine sulfoxide derivatives from P. alliacea, christened ‘petiveriins A and B’, and ‘6-hydroxyethiins A and B’ (Figure 4). 2-Hydroxyethyl cysteine was also observed. C-S lyase mediated cleavage of these cysteine sulfoxides, followed by condensation of the resulting sulfenic acids, yields a total of four thiosulfinates (Figure 5), all of which we have observed to have antimicrobial activity against various bacteria and fungi.

An interesting feature of P. alliacea is its ability to cause cows that ingest it in the field to “cry”. In working with the plant in the laboratory, we also observed it to have potent lachrymatory properties. We have since determined that the lachrymatory principle of this plant is the sulfine thiobenzaldehyde S-oxide, as a mixture of Z and E isomers in the ratio 99.95:0.05 respectively. This interesting compound is only the third naturally occurring sulfine to be reported, with the other two being (Z)-thiopropanal S-oxide (the onion lachrymator) and (Z,Z)-(±)-2,3-dimethylbutane dithial S,S’-dioxide (also from onion).

Additional studies on this plant are ongoing, and among other things, we are working to purify and fully characterize the P. alliacea C-S lyase, as well as a putative L-F synthase which may be responsible for the formation of the P. alliacea lachrymator.

Tulbaghia violacea:
T. violacea Harv. (Alliaceae) is a small bulbous herb indigenous to Natal, Transvaal and the eastern Cape region in South Africa where it grows in rocky grasslands. The evergreen leaves of T. violacea exhibit a garlic-like smell when bruised and have been used in some cultures as a substitute for garlic and chive. The plant is known by several common names including “society garlic”, “sweet garlic”, and “wild garlic”. These names originated from the belief that, in spite of its garlic-like flavor, the consumption of T. violacea is not accompanied by the development of bad breath as is the case with the consumption of the real garlic (Allium sativum L.). T. violacea has traditionally been used for the treatment of fever and colds, asthma, tuberculosis, and gastrointestinal ailments. However, extensive consumption of this plant has been associated with a variety of undesirable symptoms, such as abdominal pain, inflammation, and gastroenteritis. It has also been reported that society garlic deters moles and that the Zulus of South Africa grow this plant around their homes to repel snakes.

We have isolated S-(methylthiomethyl) cysteine-4-oxide from the rhizomes of this plant
(Figure 6). We have also observed the thiosulfinate marasmicin (Figure 6) which is presumably formed from C-S lyase mediated cleavage of the cysteine sulfoxide precursor, followed by sulfenic acid condensation. We are currently working on this plant to determine if it has any biological activity.

Nectaorscordum (Allium) siculum:
Nectaroscordum (Lindl.) Gren. & Godr. (Alliaceae) is a small subgenus of the genus Allium consisting of only two species, Allium siculum (Ucria) Lindl. and Allium tripedale (Trautv.) Grossh. Both are rare ornamental bulbous plants used in gardening. The former is native to Asia Minor, southern France and Sicily (hence the trivial name Sicilian honey garlic) where it grows in damp shady woods. It is still sometimes referred to by the synonymous names Allium nectaroscordum, Nectaroscordum siculum Ucria, A. dioscorides auct., or A. meliophilum Juz. The second member of the subgenus, A. tripedale (syn. N. tripedale Trautv. or N. persicum (Bornm.) Bornm.), is indigenous to Armenia, Iran, and Iraq.

Both members of the Nectaroscordum subgenus, A. siculum and A. tripedale, are very closely related to other plants of the genus Allium L. Due to their close morphological similarities, the relationship between these two groups has long been embroiled in taxonomic controversy. At present, classification of Nectaroscordum as a subgenus in the Allium genus is generally accepted. The chromosome basic number of x = 9, special and unique characteristics of most flower parts, and other morphological peculiarities of Nectaroscordum species were the main arguments in support of separating this oligotypic group at a generic level.1-3

A. siculum attracted our attention because of its odor which is notably different from that of common alliaceous plants. We have isolated, and for the first time, definitively shown the presence of the S-butyl cysteine sulfoxide in any plant (compound 5, Figure 7). We also isolated the S-methyl and S-1-propenyl cysteine sulfoxides (Figure 7). C-S lyase mediated cleavage of the cysteine sulfoxides yields a variety of symmetrical and mixed thiosulfinates, as shown in Figure 7. Several of these were observed to exhibit modest antimicrobial activity. We are continuing to study this plant to isolate and characterize the C-S lyase responsible for the formation of the S-butyl containing thiosulfinates.

 


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