New nitrogenous compounds from a Red Sea sponge from the Gulf of Aqaba

1 Introduction

The Gulf of Aqaba is one of the two northern branches of the Red Sea with a total length of 170 km, a width of 4–26 km, and a depth of up to 1830 m. It is located in a subtropical arid area between longitude 34°25′–35°00′E and latitude 28°00′–29°33′N [1]. The Jordanian coastline runs south for about 27 km from the most northern tip of the Gulf [2]. Waters of the Gulf of Aqaba are typically oligotrophic, with very low nutrient concentrations and a high biodiversity of the benthic community. The marine environment consists of a wealthy source of plants, animals, and microorganisms, which due to their adaptations to this unique habitat produce a wide variety of secondary metabolites, unlike those found in terrestrial species [3].

The majority of marine natural products currently used as drugs or in clinical trials are produced by invertebrates [4] such as sponges (Halaven^®), tunicates (Yondelis^®), mollusks (Adcetris^®), and bryozoans (bryostatin 1) [5]. Ecologically, these bioactive metabolites are thought to protect sessile or soft bodied marine invertebrates from predators [6] or to fight off neighbors competing for space [7]. Sponges, members of the phylum Porifera, are the simplest and evolutionarily oldest living group of the Metazoa. Fossil records revealed that they first appeared during the Cambrian period, over 550 million years ago [8]. Sponges are important components of all modern coral reef communities. Their biomass and range of ecological tolerance frequently exceeds that of the coral reef building species. They have a considerable impact on the marine environment by effectively filtering large quantities of water, modifying the reef framework, and providing shelter for numerous species of fish and invertebrates [9–11]. Despite their simplicity, sponges are highly diverse and well-adapted organisms. They manage to adapt and survive longer than any other Metazoa [12]. Their successful evolution and wide distribution in modern reef habitats make them interesting for ecological and chemical studies alike.

Sponge-derived natural products, especially from the phylum Porifera, have been shown to be important sources of new lead structures for pharmaceutical products [5]. These natural products have a wide range of therapeutic properties, including antimicrobial, antioxidant, antihypertensive, anticoagulant, anticancer, antiinflammatory, wound healing and immune modulator, and other medicinal effects [13].

In this study, one new natural product, 3-amino-1-(2-amino-4-bromophenyl)propan-1-one (1), as well as 7-bromoquinolin-4(1H)-one (2), which was previously known only as a synthetic product, and caulerpin (6), which is described from sponge material here for the first time, together with three known compounds (3–5), are reported from an unknown sponge from the Gulf of Aqaba (Figure 1). The structural elucidation of the new natural products 1 and 2 is described.

Figure 1:

Chemical structures of the isolated compounds 3-amino-1-(2-amino-4-bromophenyl)propan-1-one (1), 7-bromoquinolin-4(1H)-one (2), (Z)-5-(4-hydroxybenzylidene)-hydantoin (3), (Z)-6-bromo-3′-deimino-2′,4′-bis(demethyl)-3′-oxoaplysinopsin (4), 6-bromoindole-3-carbaldehyde (5), and caulerpin (6).

2 Results and discussion

The sponge was chosen for analysis as it is taxonomically unknown and its extract inhibited growth (85% inhibition at a dose of 10 μg/mL) of the murine cancer cell line L5178Y. The methanol extract of the unknown sponge from the Jordanian coast of the Gulf of Aqaba was partitioned between ethyl acetate and water. The resulting ethyl acetate phase was fractionated by semipreparative reversed-phase HPLC to yield six compounds, 1–6.

Compound 1 was isolated as a yellowish amorphous solid. It showed UV absorbances at λ_max (MeOH) 231.8, 268.4, and 361.2 nm. The HRESIMS showed an isotopic cluster of [M+H]⁺ ions in the ratio of 1:1 at m/z 243.0125 and 245.0105, indicating the molecular formula C₉H₁₂BrN₂O with m/z 244.0153 [M+H]⁺.

The ¹H NMR spectrum of 1 showed a doublet of doublets at δ_H 6.73 ppm (dd, J=1.9, 8.6 Hz) and two doublets at δ_H 7.63 ppm (d, J=8.6 Hz) and 6.99 ppm (d, J=1.9 Hz) assigned for H-5′, H-6′, and H-3′, respectively (ABX system). The signals of H-2 and H-3 were overlapping with the water signal. The ¹³C NMR and DEPT spectra revealed signals for two sp³ carbons (δ_C 36.7, 36.3 ppm) assigned for C-2 and C-3, respectively, and three sp² carbon signals (δ_C 120.7, 119.3, 132.7 ppm) which were attributed to the aromatic carbons C-3′, C-5′, and C-6′ (Table 1).

The deshielded signal of C-1 was observed at δ_C 199.7 ppm. The ¹H¹H COSY spectrum indicated a continuous spin system from H-3′ to H-6′. The structure of compound 1 was corroborated by analysis of the HMBC spectrum. H-2 and H-3 showed correlations to C-1, whereas H-6′ showed strong correlations with C-1 and C-4′, and a weaker correlation with C-5′ and C-1′. Furthermore, H-5′ showed correlations with C-3′ and C-1′. In addition, H-3′ showed strong correlations to C-5′ and C-1′ and a weaker correlation to C-4′, thus confirming the structure of 1. The assignment of the NH₂ and Br substituents to C-2′and C-4′ was corroborated by the chemical shifts of the respective carbons at δ_C 154.0 ppm and δ_C 130.8 ppm (Table 1).

The remaining compounds (2–6) (Figure 1) were identified on the basis of their ¹H NMR, ¹³C NMR, and mass spectrometric data and by comparison with published data as 7-bromoquinolin-4(1H)-one (2) (Table 2) [14], (Z)-5-(4-hydroxybenzylidene)-hydantoin (3) [15], (Z)-6-bromo-3′-deimino-2′,4′-bis(demethyl)-3′-oxoaplysinopsin (4) [16], 6-bromoindole-3-carbaldehyde (5) [17], and caulerpin (6) [18]. Caulerpin (6) was isolated from a sponge for the first time in this study.

Comparison of the structures of compounds 1 and 2 indicates that these compounds are biogenetically closely related. Their skeleton may be derived from tryptophan via the kynurenine pathway, as 1 is a 4-bromo-derivative of the decarboxylated kynurenine and 2 a 7-bromo-derivative of kynurenic acid. Compound 1 furthermore shows resemblance with the 4-chloro-kynurenine residue of the recently isolated taromycin A from marine origin [19]. Interestingly, compounds 4–6 also appear to be derived from tryptophan, indicating the metabolism of this amino acid to be a prominent biogenetical pathway within this particular sponge.

All isolated compounds were tested for cytotoxic activity toward the lymphoma cell line LY1578 but proved to be inactive when tested at a concentration of 10 μg/mL each. Thus, the growth inhibition that was observed when testing the crude extract is probably due to other unkown compounds that were not isolated in this study.

3 Materials and methods