A new δ-valerolactone produced by marine Streptomyces sp. YIM 13591

Abstract A new δ-valerolactone compound, named 8-hydroxy invictolide 2 (1), a new natural product 4-hydroxy-3,5-dimethyl-6-(pentan-2-yl) tetrahydro-2H-pyran-2-one (2) together with four known compounds cyclo (L-Pro-L-Tyr) (3), cyclo (D-Pro-L-Tyr) (4), acetyltryptophan methyl ester (5), 2-aminobenzoic acid (6) were isolated from marine Streptomyces sp. YIM 13591. Their structures were elucidated by NMR spectroscopic data, HRESIMS and specific rotation, and the absolute configurations of the stereo centre in lactone ring 1 and 2 were further confirmed by electronic circular dichroism (ECD) calculation. The antimicrobial effects of these compounds were evaluated by the paper diffusion method. Graphical Abstract


Introduction
Lactone is a kind of intermediate which widely used in medicine, fiber and pesticide (Xu 2020). Lactone has various bioactivities, such as antibacterial activity (Wi nska et al. 2017;Samedova et al. 2018), anti-tumor activity and cytotoxic activity (Xin et al. 2018;Huang et al. 2019), anti-inflammatory effects (Jiang et al. 2021). d-valerolactone, colorless transparent to light yellow liquid at room temperature, is widely used as a drug intermediate in the field of pharmaceutical synthesis (Tamura 1957;Fang et al. 2012;Feng et al. 2015). At present, the most noted lactones are macrolide (Zhang WJ et al. 2013) and butyrolactone derivatives (Peng et al. 2013), and less research has been done on valerolactone.
The molecular formula of 2 was determined to be C 12 H 22 O 3 according to the HRESIMS data (m/z 237.1464, The NMR data of 2 was similar to that of 1, except that the hydroxy group at C-8 was absent and the resonance of C-3 was downfield shifted from d C 36.6 to d C 74.2, revealing that the CH 2 group at C-3 in 1 was replaced by a hydroxy in 2, which was further confirmed by the HMBC correlations from H-3 (d 3.77) to C-2 (d 40.0), and C-4 (d 35.5). The NOESY experiment of 2 showed three key correlations between 2-H/4-Me, 4-H/6-Me and 4-H/5-H. Therefore, the relative configuration for 2 was considered to be 2 R Ã ,3S Ã ,4R Ã ,5R Ã . We performed ECD calculation on MacroModel/B3LYP/6-311G of theory, which confidently corroborated the absolute stereochemistry the stereo centre in lactone ring of 2 to be 2 R, 3S, 4 R, 5 R. Thus, compound 2 was determined to be 4hydroxy-3,5-dimethyl-6-(pentan-2-yl) tetrahydro-2H-pyran-2-one. Moreover, compound 2 is currently only present in the intermediate product of chemical synthesis invictolide (Yadav et al. 2012), which is the reported here in to first time as natural product.

Fermentation and extraction of strain YIM 13591
Marine Streptomyces YIM13591 was cultured in modified ISP4 medium A (Yeast Extract: 4.0 g, glucose monohydrate: 1.0 g, malt extract 10 g, agar: 15.0 g) with a total volume of 10 L at 28 C for 11 d. The fermented cultures were diced and then extracted three times overnight with ethyl acetate: methanol: acetic acid (80:15:5, v/v/v) at room temperature to obtain a crude extract (28.5 g). The crude extract was partitioned between equal ethyl acetate and H 2 O to afford the ethyl acetate extract (15.2 g). The ethyl acetate extract was subsequently extracted by petroleum ether and methanol to afford the petroleum extract (4.6 g) and methanol extract (2 g).

ECD calculation
Conformational analyses were carried out with conformation searching of MacroModel module in Maestro 10.2 program using the OPLS3 force field method to calculate the conformational energy, and those conformations whose energy were less than 3.0 kcal/mol were selected for quantum chemical optimization. The conformers were reoptimised using DFT at the B3LYP/6-311G (2d,p) level with the Gaussian 09 program. The 60 excitations states of the dominant conformation (boltzmann specific gravity greater than 1) were calculated with the TD-DFT methodology at the B3LYP/6-311G (2d,p) level. The ECD profiles of compounds were fitted by SpecDis software and compared with the experimental profiles.

Antimicrobial assay
Pathogenic bacteria were inoculated into liquid LB medium and cultured at 37 C at 200 r/min for 12 h, candida albicans was inoculated into SAB medium and cultured at 28 C at 200 r/min for 24 h, then diluted to 1 Â 10 6-7 cfu/mL in liquid medium for later use.
The antimicrobial activities of the six compounds were carried out with paper diffusion method (Digdem et al. 2015;Selemani et al. 2021). Kanamycin (30 lg), ampicillin (10 lg) and amphotericin B (30 lg) were used as positive control. The diameters of the inhibition zones including the 5 mm disc diameter were measured after 24 h of incubation at 37 C. With 50 lg as the measured dose, 16 strains of indicator microbes were selected for the antimicrobial activity test.

Conclusion
In summary, chemical investigation of marine Streptomyces YIM13591 afforded a new compound 8-hydroxy invictolide 2 (1), one new natural product 4-hydroxy-3,5dimethyl-6-(pentan-2-yl) tetrahydro-2H-pyran-2-one (2) and four known compounds (3-6). Six compounds showed certain antimicrobial activities. In particular, compound 1 showed obvious antimicrobial effect on the 3 Candida albicans and compound 6 has obvious antimicrobial activities to Salmonella typhimurium v 8956, Pseudomonas aeruginosa PA01 and Bacillus subtilis ATCC 6633. It showed implying its potential application as an antimicrobial agent and more in-depth investigation should be carried out to evaluate this compound for drug development.