Dysidinoid A, an Unusual Meroterpenoid with Anti-MRSA Activity from the South China Sea Sponge Dysidea sp.

An unusual meroterpenoid, dysidinoid A (1), was isolated from the South China Sea sponge Dysidea sp. Its structure was elucidated by extensive spectroscopic methods including HRESIMS and 2D NMR, and its absolute configuration was determined by single-crystal X-ray diffraction analysis. Dysidinoid A (1) is the first meroterpenoid from Nature bearing a 9,4-friedodrime skeleton and a 2,5-dionepyrrole unit. Dysidinoid A (1) showed potent antibacterial activity against two strains of pathogenic bacteria methicillin-resistant Staphylococcus aureus (MRSA) with MIC90 values of 8.0 μg/mL against both.


Introduction
Infectious diseases are the leading cause of death worldwide. Emerging infections due to methicillin resistant Staphylococcus aureus (MRSA) pose a significant threat to patients [1,2]. It has been estimated that in the United States more people die from MRSA related infections than from HIV [3]. Infections involving drug resistant bacteria are more difficult to treat due to increased costs and decreased efficacies [4,5]. One important approach to drug discovery for the treatment of MRSA is through natural products research.
In our efforts to search for new anti-MRSA agents from marine sponges collected from the South China Sea, chemical investigation of an active fraction from the sponge Dysidea sp. resulted in the isolation of a novel meroterpenoid, dysidinoid A (1) (Figure 1). It is the first meroterpenoid from Nature bearing a 9,4-friedodrime skeleton and a 2,5-dionepyrrole. Antibacterial evaluation showed that dysidinoid A showed potent antibacterial activity against two strains of pathogenic bacteria MRSA with MIC90 values of 8.0 μg/mL against both. Details of structural elucidation and antibacterial activity of dysidinoid A (1) were reported herein.  showed resonances attributable to two olefinic protons at δH 5.16 (H-3) and 6.26 (H-18), three tertiary methyl groups at δH 1.55 (H3-11), 1.00 (H3-12), and 0.88 (H3-14), a secondary methyl group at δH 0.95 (H3-13). In addition, the spectrum showed resonances due to an exchangable amine proton at δH 7.33 (20-NH), as well as partially overlapping signals with complex coupling patterns between δH 1.08 and 2.61 that could be attributed to several aliphatic methylene and methine units. The 13   Unambiguous assignment of NMR data of 1 was achieved by a combination of COSY, HSQC, and HMBC experiments, as depicted in Figure 2. In the 1 H-1 H COSY spectrum, the correlations of H2-1/H2-2/H-3, H2-6/H2-7/H-8/H3-13, and allylic coupling correlations of H-3/H3-11 revealed the presence of two fragments (thick lines in Figure 2). The two spin systems and their connectivity with the remaining atoms enabled assembly into the final planar structure based upon the HMBC spectrum of 1. The HMBC correlations from H3-11 to C-3, C-4, and C-5, from H3-12 to C-4, C-5, C-6 and C-10, from H3-13 to C-7, C-8, and C-9, and H3-14 to C-8, C-9, C-10, and C-15 indicated the presence of 9,4-friedodrime sesquiterpene skeleton with four methyl groups at C-4, C-5, C-8, and C-9, respectively. This assignment was confirmed by the HMBC correlations from H-10 to C-2, C-4, C-5, C-9, C-12, C-14, and C-15. Furthermore, the olefinic proton H-18 showed HMBC correlations with C-15, C-16, and C-19, in combination with the chemical shifts of the proton and carbon resonances, suggested the presence of a 2,5-dionepyrrole substructure. In addition, HMBC correlations from the methylene protons H2-15 to C-8, C-9, C-10, C-14, C-16, C-17, and C-18 supported the linkage of C-9 and C-16 via the methylene CH2-15 between the 9,4-friedodrime sesquiterpene moiety and 2,5-dionepyrrole substructure. Therefore, the gross structure of 1 was determined as shown in Figure 2. The relative configuration of 1 was deduced from NOESY correlations in combination with coupling constant values. The large coupling constant between H-1β and H-10 (J = 12.4 Hz) and the NOESY correlations of H-1β/H3-12 and H3-14 indicated the axial orientations of these protons and methyls and also revealed the trans fusion of the two six-numbered rings [15,18]. The NOESY correlation of H3-13/H3-14 and H3-12/H3-14 revealed the three methyl groups are all β-orientation, while NOESY correlations from H-8 to H-6α, and H-10 suggested the three protons were α-orientation.

Results and Discussion
Fortunately, crystals of 1 suitable for single crystal X-ray diffraction analysis were obtained from a methanol solution. The relative configuration of 1 was unambiguously established by its X-ray crystal structure ( Figure 3). Besides, a final refinement of the CuKa diffraction data resulted in the assignment of the absolute configuration of 1 as 5S, 8S, 9R, and 10S.

Animal Material
Samples of Dysidea sp. were collected along the coast of Yongxing Island in Xiasha on 12 April 2010. The voucher number for this collection is XD10401, and a voucher sample is maintained at the Key Laboratory for Marine Drugs, Department of Pharmacy, Renji Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai, China. The sponge was identified by Professor Jin-He Li (Institute of Oceanology, Chinese Academy of Science).

Extraction, Isolation and Characterization
The animals (200 g, dry weight) were soaked in EtOH (250 mL, 25 °C, 72 h) repeatedly to give 24.6 g of a crude EtOH extract after solvent removal. The extract was dissolved in 250 mL H2O, and partitioned five times with the same volume of CH2Cl2 to yield after concentration 12.0 g of a CH2Cl2 solvent extract, The CH2Cl2-soluble fraction was subjected silica gel column chromatography eluting with a gradient of CH2Cl2 and MeOH, yielding four subfractions (D1-D4). Fraction D3 (1.4 g) was passed through an ODS chromatography column eluted with a gradient of aqueous MeOH, size-exclusion chromatography Sephadex LH-20 eluted with CH2Cl2/MeOH (1:1), and then purified by reversed-phase HPLC (YMC-Park Pro C18, 10

Antimicrobial Assays
Minimum inhibitory concentration (MIC) was determined according to Clinical and Laboratory Standards Institute (CLSI) guidelines. The MIC90 values were recorded using a spectrophotometer. For antibiotic sensitivity assays, bacteria in 96-well plates (Corning) were incubated with dysidinoid A (1) or antibiotic standards at final concentrations of 0 to 256 mg/mL. The plates were incubated at 37 °C and read at 24 h.

Conclusions
Marine sponges provide a rich source on drug discovery for the treatment of MRSA infectious diseases. In this paper, dysidinoid A (1), an unusual meroterpenoid, was isolated from the South China Sea sponge Dysidea sp. Its structure was determined based on extensive spectroscopic data, and the absolute configuration of 1 was established by single-crystal X-ray diffraction analysis. Dysidinoid A (1) showed potent antibacterial activity against two strains of hospital-acquired pathogenic MRSA with MIC90 values of 8.0 μg/mL against both.