Natural antifungal compounds from the peels of Ipomoea batatas Lam

Abstract Three antifungal compounds have been isolated for the first time from the peels of Ipomoea batatas Lam. Their structures were established on the basis of 1D and 2D NMR spectra data as well as ESI-MS and IR analysis. Urs-13(18)-ene-3β-yl acetate was found to possess a weak activity against Sporothrix schenckii and Trichophyton metagrophytes fungi with an MIC value of 50 μg/mL each. Stigmasterol and 3-friedelanol were equally active against T. metagrophytes.


Introduction
Sweet potato (Ipomoea batatas Lam) is a member of the Convolvulaceae family and it is the seventh most important crop worldwide. It is widely grown around the world due to its hardy nature and its important folkloric applications, especially in China and amongst other nations (Li 1999;Andrade et al. 2009). Several compounds such as batatins (Edgar & Rogelio 2007), aminoacyl sugars (Irene et al. 2006), coumarins (Yuan et al. 2005), triterpenoids (Luo & Kong 2005) and esters of caffeic and coumaric acids (Muyinza et al. 2010) have been earlier ABSTRACT Three antifungal compounds have been isolated for the first time from the peels of Ipomoea batatas Lam. Their structures were established on the basis of 1D and 2D NMR spectra data as well as ESI-MS and IR analysis. Urs-13(18)-ene-3β-yl acetate was found to possess a weak activity against Sporothrix schenckii and Trichophyton metagrophytes fungi with an MIC value of 50 μg/mL each. Stigmasterol and 3-friedelanol were equally active against T. metagrophytes. isolated from the aerial parts and tubers of the plant. Other phytochemicals which have been sourced from the plant include anthocyanins ( Van-den et al. 2010;Xinying et al. 2013) and from Ipomoea obscura which is also from the Convolvulaceae family, sesquiterpene-rich essential oil (Rajesh 2015).
Conducting a study on a Nigerian variety of sweet potato is important considering the fact that ecological factors play a major role in the kind of phytochemicals that are biosynthesised in plants. Furthermore, it is also suspected that the peel might contain significant and medicinally important phytochemicals that are not obtainable in the flesh (Ogundana et al. 1983).
In our previous study (Oluyori et al. 2013), we reported the isolation of an alkyd resin from the peels of the plant. Further investigation into the peels of this plant has led to the isolation of three known compounds which are being reported for the first time from the peels of I. Batatas Lam and which exhibited undocumented biological activity.

Results and discussion
Compound I (0.082% of the n-hexane fraction) was obtained as a crystalline compound with melting point 239-241 °C. Positive result from Liebermann-Burchard test indicated that I was a triterpenoid. The compound showed a characteristic tlc pink spot peculiar to triperpenoids (R f value 0.49, EtOAc/n-hexane 1:50) after the developed plate was dipped in a saturated solution of Ce(SO 4 ) 2 in 65% H 2 SO 4 and heated to 110 °C. The molecular formula of I, C 32 H 52 O 2 was determined from the ESI-MS (M + + Na) at 492.3 (calcd for 469.3378 ESI-HRMS. The 1 HNMR spectrum of I exhibited a hump between 0.5 and 2.2 ppm which is peculiar to steroids and triterpenoids. The signals at δ 4.4813-4.5224 (dd, J = 5.4 Hz, 11.5 Hz) is the characteristic of H-3 of a pentacyclic triterpenoid skeleton. In addition, the proton NMR revealed the presence of six CH 3 singlets at δ 0.7845, 0.8546, 0.8644, 0.9475, 0.9973 and 1.0500 (each 3H, s) along with two secondary methyls; δ 0.8866-0.9032 (3H, d, J = 6.6 Hz) and δ 0.9300-0.9475 (3H, d, J = 6.6 Hz). The 13 C NMR (Table S1) and DEPT spectra displayed 32 carbon atoms including 8 quartenary, 8 methyl, 10 methylene and 5 methine protons. Two of the quartenary carbons were olefinic carbons (131.49 and 142.23). Based on the data above, HSQC, COSY, HMBC spectra, as well as comparison with literature (Niaz 2013), the structure of I is identified as urs-13(18)-ene-3β-yl acetate (Figure 1). This compound is an isomer of the well-known compound alpha amyrin acetate and its occurrence is reported from this plant family for the first time. It has been reported that alpha amyrin acetate has antispasmodic activity amongst other properties (Niaz 2013 (Table S1), DEPT spectra of II were compared well with reported data (Anjoo & Ajay 2011), and thus II was identified as Stigmasterol, a well-known compound (Figure 1). The economic importance of stigmasterol stems from its use as a precursor in the production of semi-synthetic progesterone and cortisone. The compound has also been reported to possess antiosteoathritic, antioxidant, antihypercholesterolemic, antitumour, hypoglycaemic and thyroid inhibitory potential (Navpreet et al. 2011). This further explains the reason why this stigmasterol, has been shown as a constituent of several crude plant extracts of biological importance by various researchers.
Compound  (Table S1) and DEPT spectra of III with literature (Trinh et al. 2007), the structure of III was identified as 3-friedelanol ( Figure 1). 3-Friedelanol is an antitumour compound (Kundu et al. 2000) and is useful in the preparation of dietary supplements or cosmetics that modulate ageing-associated diseases (Yang et al. 2011).
As shown in Table S2, urs-13(18)-ene-3β-yl acetate showed a weak activity against Sporothrix schenckii and Trichophyton metagrophytes with an MIC value of 50 μg/mL, while stigmasterol and 3-friedelanol showed similar activity against only T. metagrophytes (MIC 50 μg/mL). These compounds are probably biosynthesised by the plant to help provide resistance against some microbial infections. In the antifungal study of α and β amyrin derivatives carried out by Johann et al. (2007) against Candida species, the most active of the derivatives were the formiate and the acetate derivatives with MIC values between 30 and 250 μg/mL. The MIC value of urs-13(18)-ene-3β-yl acetate falls within this range thus confirming its antifungal potential.
Although triterpenoids/sterols are economically important, their synthesis remains an uncompetitive option because of their structural complexity. Plant tissue culture therefore becomes a wise means of enhancing the yield of these biologically active compounds. The peel of sweet potato could be used as an explant for the development of a plant with better yields of compounds I-III.
A potential method of enhancing triterpene synthesis is to increase the flux of IPP and DMAPP by overexpressing their genes (Jacinda & Ian 2009). In a recent study, overexpression of SQS gene in Panax ginseng resulted in hyperproduction of triterpenes and phytosterols (Lee et al. 2004). This method could be adopted in order to conveniently increase the bioavailability of the antifungal triterpenes/steroids which naturally occur in the peels of I. batatas Lam. For example, specifically increasing α-amyrin synthase, a triterpene synthase, should ultimately lead to an increased yield of urs-13(18)-ene-3β-yl acetate (Compound I).

Conclusion
Peels which are generated as waste from sweet potato during domestic and commercial processing contains phytochemicals which can be used to ameliorate skin infections. The yield of these compounds can be enhanced by the cloning of genes in their biosynthetic pathways.

Supplementary material
Experimental details relating to this paper are available online along with relevant spectra, tables S1 and S2.