New Secondary Metabolites from the Marine-Derived Fungus Talaromyces mangshanicus BTBU20211089

Seven new compounds, namely talaromanloid A (1), talaromydene (2), 10-hydroxy-8-demethyltalaromydine and 11-hydroxy-8-demethyltalaromydine (3 and 4), talaromylectone (5), and ditalaromylectones A and B (6 and 7), together with seven known compounds were identified from a marine-derived fungus, Talaromyces mangshanicus BTBU20211089, which was isolated from a sediment sample collected from the South China Sea. Their chemical structures were determined using spectroscopic data, including HRESIMS, 1D, and 2D NMR techniques. The absolute configurations of 1 and 2 were elucidated by comparing experimental and calculated ECD spectra. Compounds 1, 2, 6, and 7 are new compounds possessing a novel carbon skeleton. Compound 6 is a dimeric molecule of 3 and 9. Compound 7 shared a unique structure of the cyclized dimer of 3 and 4. All the compounds were tested for their bioactivities against Staphylococcus aureus, Escherichia coli, and Candida albicans.


Introduction
Marine-derived fungi represent the most prolific source of new chemical entries with diverse bioactivities [1,2]. Talaromyces species had been included in the Penicillium subgenus Biverticillium and were classified as a valid genus by McNeill [3]. The Talaromyces fungus, widely distributed in marine and terrestrial environments, is an important natural resource producing enzymes and pigments of industrial importance, and sometimes can cause invasive mycosis. Different classes of secondary metabolites, such as polyene and isocoumarin [4], oxaones and oxaphenalenones [5], meroterpenoids [6], oxaphenalenone [7], diphenyl ether derivatives, and sesquiterpene-conjugated amino acids [8,9], have been characterized from marine-derived Talaromyces strains.
Compound 4 was isolated as a colorless powder. The molecular formula of 4 was determined to be C 10 H 11 NO 5 based on the HRESIMS spectrum (m/z [M + H] + 226.0716, calcd for C 10 H 12 NO 5 , 226.0710), accounting for six degrees of unsaturation ( Figure S21). The 1 H, 13 C NMR, and HSQC data ( Figures S22-S24, Table 2) shared high similarity with those of 3. The hydroxyl of C-11 was confirmed by the HMBC correlations (Figure 2 and Figure S25) between H 3 -10 and C-3, C-9, and C-11, and ROESY correlation (Figure 4 and Figure S26) between H 2 -11 and H 2 -4. Thus, the structure of 4 was determined and named 11-hydroxy-8-demethyltalaromydine.
The biosynthesis of 2-9 most likely proceeds via the same precursors, and plausible biosynthetic relationships of 3-5 and 7-9 are presented in Figure 5. Precursors 15 and 16 may be derived from the tricarboxylic acid cycle [21] and then form 9 by an amidation reaction. Compound 8 is produced by the oxidization of 9. Compound 5 is proposed to be generated after the oxidization, cyclization, and amidation of 15 or 16. Compound 7 is proposed to be derived from the cyclization of 9 and dehydration of 4.

Biological Activity
These compounds were evaluated for their antibacterial activities against Candida albicans ATCC 10231, Staphylococcus aureus ATCC 25923, and Escherichia coli ATCC 25923. Compounds 6 and 13 showed an inhibitory effect against C. albicans with an MIC value of 200 μg/mL. Compounds 13 and 14 exhibited antibacterial activity against S. aureus with MIC values of 50 μg/mL.

ECD Computation Method
Conformation searching was performed using OpenBabel by a genetic algorithm (GA) with the default settings [23]. The conformers were subsequently optimized using the DFT method at the B3LYP/TZVP level with GAUSSIAN 09 [24]. The TDDFT calculations of their low-energy conformations within 0-2.5 kcal/mol were performed at the same level with 40 single excited states. The solvent effect was taken into account by using the polarizable continuum model (PCM).

Biological Activity
Compounds 1-14 were evaluated for their antimicrobial activities in 96 well plates according to the Antimicrobial Susceptibility Testing Standards outlined by the Clinical and Laboratory Standards Institute document M07-A7 (CLSI) and our previous report [13]. The MIC was defined as the minimum concentration of the compound that prevented visible growth of the microbes.
Compound 7 possessed a unique novel carbon skeleton structure of a cyclized dimer of 3 and 4. Compounds 6 and 13 showed an inhibitory effect against C. albicans with an MIC value of 200 µg/mL. Compounds 13 and 14 exhibited antibacterial activity against S. aureus with MIC values of 50 µg/mL.