New antimicrobial bromotyrosine analogues from the sponge Pseudoceratina purpurea and its predator Tylodina corticalis

Abstract

Bioassay-guided fractionation of extracts from temperate Australian collections of the marine sponge Pseudoceratina purpurea resulted in the isolation and characterisation of two new and six known bromotyrosine-derived alkaloids with antibiotic activity. Surprisingly, a single specimen of the mollusc Tylodina corticalis, which was collected while feeding on P. purpurea, contained only a few of the compounds found in the sponge suggesting selective accumulation and chemical modification of sponge metabolites.

Figure 1

Low resolution-LCMS chromatograms (λ_max 210 nm) of ethyl acetate partitions of P. purpurea (A) and T. corticalis (B); Panels (C) and (D) are expansions of the 19–21 min regions (λ_max 280 nm).

Figure 2

Bromotyrosine-alkaloids isolated from the marine sponge P. purpurea.

Figure 3

Structure and selected COSY (black) and HMBC (red) correlations of new bromotyrosine compound 1.

Figure 4

Selected HMBC correlations of 1: (a) from H-19 (δ 2.51) to C-20 (δ 122.4) and C-24 (δ 117.8) of the imidazole moiety. H-19 lies under the large DMSO peak (2.49 ppm). (b) from H-8′ (δ 6.83) ³J couplings to C-24 (δ 117.8) of the imidazole moiety, and to C-6′ (δ 149.6), C-10′ (δ 128.9) and a ²J coupling to C-9′ (δ 137.4) of the uranidine moiety.

Figure 5

Structure and selected 2D-NMR correlations of new bromotyrosine compound (2). (Colours: red (HMBC), green (ROESY), black (COSY)).

Figure 6

Selected HMBC correlations found in the unknown bromotyrosine (2; DMSO-d₆, 600 MHz). A weak correlation between H-16 (δ 3.88) and C-18 (δ 151.3) indicates the link of substructures B and C.

Figure 7

Bromotyrosine derivatives tentatively identified from the ethyl acetate extracts of P. purpurea and T. corticalis based on UV and HR-MS analysis. The boxed structures are possible isomeric forms present. Stereochemistry shown as reported in the literature.