Two New Coumarins from Talaromyces flavus

Two new coumarins, talacoumarins A (1) and B (2), were isolated from the ethyl acetate extract of the wetland soil-derived fungus Talaromyces flavus BYD07-13. Their structures were elucidated by spectroscopic data (NMR, MS) analyses. The absolute configuration of C-12 in 1 was assigned using the modified Mosher’s method, whereas that of C-12 in 2 was deduced via the circular dichroism data of its corresponding [Rh2(OCOCF3)4] complex. Compounds 1 and 2 were evaluated for their anti-Aβ42 aggregation, cytotoxic, and antimicrobial activities. The results showed that the two compounds had moderate anti-Aβ42 aggregation activity, and this is the first report on the Aβ42 inhibitory aggregation activity of coumarins.


Introduction
Fungi of the genus Talaromyces have been reported to produce a series of bioactive compounds [1][2][3][4][5]. Metabolites produced by T. flavus have also been reviewed [5]. In our searching for bioactive secondary metabolites from fungi, we have previously isolated a series of polyesters [6], as well as one sequiterpene [7] from the wetland derived fungus Talaromyces flavus BYD07-13, which was collected from a soil sample in Baiyangdian (Hebei Province, China). Ongoing chemical study on this fungus has now resulted in the isolation and identification of two new coumarins, named talacoumarins A (1) and B (2) (Figure 1). In this paper, we describe the isolation, structure elucidation, as well as the anti-Aβ42 aggregation, cytotoxic, and antimicrobial activities of 1 and 2.
Compound 2 possessed the same molecular formula and UV absorption characteristic as that of 1, suggesting 2 may be a coumarin isomer of 1. The NMR spectroscopic data (Table 1) suggested 2 was very similar to 1, except for the position of the methoxy group, which was located at C-8 for 2 instead of at C-6 based on the HMBC correlations from 8-OCH3 to C-8, from H-5 to C-7/C-9, and from H-7 to C-8/C-9 ( Figure 2). With the aid of the 1 H-1 H COSY, HSQC, and HMBC correlations, the planar structure of 2 was established and all the 1 H-and 13 C-NMR signals were assigned. The optical rotation of 2 ([α] 20 D +55.6 (c 0.5, CH3OH)) was consistent with that of 1, which suggested that 2 had the same configuration. The absolute configuration at C-12 was determined on the basis of the circular dichroism of the complex formed in situ with [Rh2(OCOCF3)4] [11,12], with the inherent contribution of the ligand subtracted. Upon addition of [Rh2(OCOCF3)4] to a solution of 2 in CH2Cl2, a metal complex was formed, acting as an auxiliary chromophore. It has been demonstrated that the sign of the E band (at ca. 350 nm) can be used to correlate the absolute configuration of a secondary and tertiary alcohol by applying the bulkiness rule. In this case, the Rh complex of 2 displayed a positive E band (Figure 4), correlating with a 12S absolute configuration. Hence, the structure of 2 was established as shown in Figure 1 and named to be talacoumarin B.  So far, natural products from fungi with the 3-alkyl-6,8-dioxycoumarin scaffold are relatively rare, and only eight such compounds have been reported [8,[13][14][15]. The inhibitory activities against Aβ42 aggregation of compounds 1 and 2, along with that of the crude extract, were tested by a thioflavin T (ThT) assay [16] with epigallocatechin gallate (EGCG) as the positive control. Compounds 1 and 2 showed moderate anti-Aβ42 aggregation activities, with relative inhibitory rates of (49.33 ± 3.16)% and (44.99 ± 3.64)% [the positive control EGCG had a relative inhibitory rate of (67.23 ± 2.51)%] at the concentration of 100 μM, while the crude extract has no activity. This represents the first report of the anti-Aβ42 aggregation activity of coumarins. Compounds 1, 2, and the crude extract were also evaluated for the cytotoxicity against five human tumor cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW480) and the antimicrobial activity against Escherichia coil, Staphylococcus aureus, Candida albicans, and Aspergillus niger. However, none of them showed any cytotoxic (IC50 > 40 μM) or antimicrobial activities (MIC > 1.0 mg/mL).

Fungal Material and Culture
The strain of Talaromyces flavus (No.BYD07-13) was identified on the basis of the morphological characters and gene sequence analyses. The ITS, beta-tubulin, and calmodulin sequences of the strain have been deposited at GenBank as KF917583, KF917584, and KF917585, respectively. It was deposited in the culture collection at the Institute of Traditional Chinese Medicine and Natural Products, College of Pharmacy, Jinan University, Guangzhou, China. The fermentation of No.BYD07-13 was described in our previous paper [6].

Preparation of (S)-and (R)-MTPA Esters of 1 (1a and 1b)
A solution of 1 (0.5 mg) in pyridine-d5 (0.5 mL) was treated with (S)-MTPA chloride (8 µL) under an atmosphere of nitrogen in an NMR tube. The mixture was stirred at room temperature for 20 h to obtain the (R)-MTPA ester (1b). The same procedure was used to prepare the (S)-MTPA ester (1a) with (R)-MTPA chloride.

Absolute Configuration of the Secondary Alcohol in 2
According to a published procedure [11,12], 2 (0.5 mg) was dissolved in a dry solution of the stock [Rh2(OCOCF3)4] complex (1.0 mg) in CH2Cl2 (200 μL). The first CD spectrum was recorded immediately after mixing, and its time evolution was monitored until stationary (about 10 min after mixing). The inherent CD was subtracted. The observed sign of the E band at ca. 350 nm in the induced CD spectrum was correlated to the absolute configuration of the C-12 secondary alcohol moiety.

ThT Fluorescence Assay
The ThT fluorescene assay was performed as described in our previous paper [16], with EGCG as the positive control. Biological activity was determined as relative inhibitory activity (Vi) for each sample according to the formula: Vi = 100 − [(Fi − Fb)/F0] × 100, where Fi is the fluorescence value of the sample, Fb is its bank value, and F0 is the fluorescence value for free aggregation of a sample of Aβ42 incubated in the same buffer/HFIP/DMSO system and in absence of inhibitors.

Cytotoxicity Assay
Cytotoxic activity was tested against five human cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW480) using the MTT method as described in our previous paper [6]. Cisplatin and paclitaxel were used as the positive controls.

Antimicrobial Assay
The antimicrobial activity against E. coil, S. aureus, C. albicans, and A. niger were evaluated by an agar dilution method, which described in our previous paper [7]. Tobramycin was used as the positive control.

Conclusions
Our investigation on the metabolites of the wetland soil-derived fungus T. flavus BYD07-13 resulted in the isolation of two new coumarins, named talacoumarins A (1) and B (2). Compounds 1 and 2 both have the 3-alkyl-6,8-dioxycoumarin moiety, which is relatively rare in fungal metabolites. The absolute configurations of 1 and 2 were determined by the modified Mosher's method and the CD analysis of the in situ formed [Rh2(OCOCF3)4] complex. It is noteworthy that the absolute configuration of branched chain alcohol of 3-alkyl-6,8-dioxycoumarin was determined for the first time. Compounds 1 and 2 showed moderate anti-Aβ42 aggregation activity, making this the first report on the Aβ42 inhibitory aggregation activity of coumarins.