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  • br Results and discussion The soil

    2023-11-17


    Results and discussion The soil fungus PSU-RSPG37 (BCC56870) was isolated from soil samples collected from Rajjaprabha Dam, Suratthani Province, Thailand. The fungus was deposited at BIOTEC Culture Collection, National Center for Genetic Engineering and Biotechnology (BIOTEC), Thailand Science Park. It was identified based on morphological and molecular data. The fungal colony is wooly to cottony with white aerial mycelium and orange to brownish pigment in agar medium. Hyphae are septate and hyaline. Macroconidia are multiple-celled with slightly curved at the pointed ends which are the characteristics of the genus Fusarium (Barnett and Hunter, 1998). Microconidia are one-celled, hyaline and ovoid shape. Its ITS rDNA sequence was closely related to the sequence of F. solaniAM412603 with nucleotide identity of 96.1%. Therefore, a fungus PSU-RSPG37 can be identified as F. solani. All isolated metabolites were obtained using various chromatographic techniques. Their structures were elucidated by analysis of spectroscopic data, including UV, IR, 1D and 2D NMR, and MS. The structures of the known compounds were determined and confirmed by comparison of their spectroscopic data with those in the literatures and the relative configurations were assigned using NOEDIFF data. Fusarpyrone A (1) was obtained as a pale yellow gum with the molecular formula C10H12O3 as deduced from HRESI-TOFMS. The IR spectrum showed NG25 bands at 3346 (hydroxyl group) and 1716 (conjugated ester group) cm−1. The UV spectrum displayed maximum absorption bands at λmax 249 and 306nm, indicating the presence of a conjugated pyrone chromophore (Lee et al., 1995). The 1H NMR spectrum consisted of signals for two olefinic protons [δ 7.32 (1H, d, J=7.2Hz) and 6.16 (1H, d, J=7.2Hz)], one 2-substituted-2-butenyl unit [δ 6.67 (1H, q, J=7.2Hz), 1.88 (3H, s) and 1.85 (3H, d, J=7.2Hz)] and a hydroxymethyl group (δ 4.50, 2H, s). The olefinic proton, H-4 (δ 7.32), was correlated with the other olefinic proton, H-3 (δ 6.16), in the 1H–1H COSY spectrum. In the HMBC spectrum, H-4 showed cross peaks with C-2 (δ 158.3), C-5 (δ 123.1) and C-6 (δ 164.0) while H-3 was correlated with C-2 and C-5. These results together with the HMBC correlations between the hydroxymethyl protons, H2-5′ (δ 4.50), with C-4 (δ 139.1), C-5 and C-6 indicated that 1 had a pyrone ring with the hydroxymethyl group at C-5. The 2-substituted-2-butenyl unit was located at C-2 of the pyrone ring on the basis of a HMBC correlation of the methyl protons, H3-1′ (δ 1.88), with C-2. The configuration of the double bond in the 2-substituted-2-butenyl unit was E on the basis of the NOEDIFF enhancement of H3-1′ signal upon irradiation of H-3′. These data indicated that 1 differed from 8 in the substituent at C-5. Therefore, fusarpyrone A (1) was assigned as 5′-hydroxyl derivative of 8. Fusarpyrone B (2) with the molecular formular C8H8O4 from HRESI-TOFMS was obtained as a yellow gum. Its UV and IR spectra were similar to those of 1. Its 1H NMR spectrum was similar to that of 1 except that signals of the C-2 substituent in compound 1 were replaced by an acetyl group (δ 2.54, 3H, s) in 2. The presence of the acetyl substituent was supported by signals of ketone carbonyl and methyl carbons at δ 193.0 and 25.2, respectively, in the 13C NMR spectrum. The location of this unit at C-2 (δ 155.1) was confirmed by a HMBC correlation of H3-2′ (δ 2.54) with C-2 of the pyrone ring. Consequently, fusarpyrone B had the structure 2. The antimalarial (against Plasmodium falciparum K1), antimycobacterial (against Mycobacterium tuberculosis H37Ra), and cytotoxic [against oral human carcinoma cells (KB), human breast cancer cells (MCF-7), lung cancer cells (NCI-H187) and noncancerous Vero cells] activities of compounds 1, 2, 5 and 8 were evaluated. Compounds 3–4 and 6–7 were tested only against NCI-H187 cell lines because they have been previously reported to exhibit antimalarial, antimycobacterial, and cytotoxic (against KB, MCF-7 and Vero cells) activities (Trisuwan et al., 2010). All of the tested compounds (1, 2, 5 and 8) as well as compounds 4 and 6 were inactive against P. falciparum and M. tuberculosis. The pyrone derivatives 1, 2 and 8 showed no cytotoxic activity against all tested cell lines while the naphthoquinone 5 displayed very mild activity against these cell lines. In addition, the naphthoquinones 3, 4, and 7 exhibited weak activity against NCI-H187 lung cancer cell lines (Table 1). These results indicated that a 5,8-dihydroxy-2-methoxy-1,4-naphthoquinone subunit would play an important role in cytotoxic activity. Compound 3 with a 3,4-disubstituted-2H-pyran ring displayed the best activity to all cancer cell lines. Consequently, the 3,4-disubstituted-2H-pyran ring would enhance the cytotoxic activity.