One new furanone analogue from the deep-sea fungus Purpureocillium sp. SCSIO 06693

Abstract A new furanone analog, (E)-2-(8,9-dihydroxy-6,8-dimethyldec-4-en-2-yl)-met-hylfuran-3(2H)-one (1), together with six known compounds, including two diterpenoids (2 and 3), one butyrolactone (4) and three isocoumarins (5–7), were isolated from a deep-sea fungus, Purpureocillium sp. SCSIO 06693. Among them, compound 1 existed as two tautomeric forms (1a and 1b) differing in configuration of the furan ring. The chemical structures were elucidated by the basis of spectroscopic evidences, including HRESIMS, NMR and optical rotation. Isolated compounds were evaluated for their cytotoxic, antiviral, antibacterial, antioxidant, acetyl cholinesterase (AChE) and pancreatic lipase (PL) enzyme inhibitory activities. Biological evaluation results revealed that compound 4 showed modest antioxidant activity against DPPH with IC50 value of 72.03 μM. In addition, compounds 1–4 exhibited PL enzyme inhibitory activities.


Introduction
Deep-sea fungi have become an important source of marine natural products (Carroll et al. 2020). According to the literature, the total amount of new compounds isolated from fungi from deep-sea sediments has exceeded one-third of the number of natural products derived from marine fungi (Blunt et al. 2012). About 76% of deep-sea natural products have biological activity, and more than 50% of those compounds show significant cytotoxic activity against human tumor cell lines (Danielle 2008). Among marine fungi sources, Purpureocillium sp. is a saprobic filamentous fungus commonly isolated from soils, which is one of the most promising and commercialized agents against various pests and pathogenic nematodes (Liu et al. 2020). However, in recent years this fungus has been increasingly found is a cause of infection in man and other vertebrates (Luangsaard et al. 2011).
In order to search for new bioactive compounds from marine fungi (Chen et al. 2021;Pang et al. 2018), one strain identified as Purpureocillium sp. SCSIO 06693 was isolated from a deep-sea sediment at Western Pacific. Our chemical investigation of ethyl acetate extracts from Purpureocillium sp. SCSIO 06693, led to the isolation of seven metabolites (Figure 1). Furthermore, the secondary metabolites of this fungus were studied for their antitumor, antibacterial, antioxidant and enzyme inhibitory activities, respectively. Herein, we present the fermentation, isolation and biological activities of these compounds.
Each pair of 1 H NMR signals showed similar multiplicity pattern suggesting that compound 1 is an epimeric mixture and named 1a and 1b. The significant differences between 1a and 1b was the chemical shift of C-2 0 (d C 87.8 and d C 88.5) and C-4 0 (d C 104.7 and d C 109.2), indicating that 1 likely existed as a pair of interconverting geometric isomers in the solution. The isomeric behavior of 1 can be explained by a tautomerism of the absolute configuration at C-2 0 , which could transform between 1a and 1b, respectively, via a speculated enol intermediate (1c). (Kwon 1999;Mikhailov and Logatcheva 2007;Pertejo et al. 2021). What's more, the NMR data showed the most favorable tautomer for compound 1 is the keto one.
All isolated compounds were evaluated for their cytotoxic (LNCa, 22Rv1, PC3, and DU145), antiviral (ZIKV, DFV, and HSV), and antibacterial activities (Staphyloccocusaureus, Methicillin-resistant Staphylococcus aureus, Acinetobacter baumannii, and Klebsiella pneumonia). However, no compounds showed cytotoxicity at the concentration of 10 lg/mL and significant antibacterial activities. In radical scavenging activities of all isolated compounds were tested on against DPPH, and compound 4 showed modest radical scavenging activity against DPPH with IC 50 value of 72.03 lM (Rathnayake et al. 2018). Besides, all compounds were also assessed for their inhibitory activities against AChE and PL enzyme. Compounds 1-4 showed PL enzyme inhibitory activities with 77.13%, 60.64%, 44.73% and 50.20% inhibitions at the concentration of 50 lg/mL, respectively.

Fungal strain
The fungal strain, Purpureocillium sp. SCSIO 06693, was isolated from deep-sea sediment of Western Pacific (126.9753333 E;27.546 N;Depth 1240 m). The isolated fungal strain was stored on Muller Hinton broth (MB) agar (malt extract 16 g, agar 18 g, seasalt 30 g, water 1 L, and pH 7.4-7.8), slants at 4 C and then deposited in CAS Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China. The ITS1-5.8S-ITS2 sequence region (550 base pairs (bp), GenBank accession no. OM980710) of strain SCSIO 06693 was amplified by PCR, and DNA sequencing showed it shared significant homology to several species of Purpureocillium. The 550 bp ITS sequence has 99% sequence identity to that of Purpureocillium lilacinum (GenBank accession no. NR111488), so it was designated as a Purpureocillium sp. and was named Purpureocillium sp. SCSIO 06693.