Octasulfur in Alcyonarians and Ascidians

Abstract

The finding of S₈ in the extract of the alcyonarian Sinularia humilis in 2021 was not the first record for marine invertebrates, because a similar finding was published in 1995 for the ascidian Polycitor sp.

To the editor

The finding of a biologically active metabolite having no NMR signals in the alcyonarian Sinularia humilis (family Alcyoniidae, suborder Alcyoniina, order Alcyonacea, subclass Octocorallia, class Anthozoa, phylum Cnidaria), namely octasulfur S₈, which was characterized by X-ray analysis, seems to be very uncommon and interesting. ¹ The authors noted: “To the best of our knowledge, this is the first occasion on which cyclooctasulfur was obtained from a marine invertebrate.” However, investigations by the scientists of G. B. Elyakov Pacific Institute of Bioorganic Chemistry of the Russian Academy of Sciences, and published in 1995, ² reported the isolation of this compound from an ascidian. This work used a slightly different methodology, but with the same result.

The authors of this work, Tatiana N. Makarieva, Valentin A. Stonik, and their colleagues, isolated the known polysulfide phenolic compound varacin (1)^2,3 having 5 sulfur atoms in a seven-member ring from the ascidian Polycitor sp. (family Polycitoridae, suborder Aplousobranchia, class Ascidiaceae, subphylum Urochordata, phylum Chordata). They also isolated several new minor related compounds, including varacin A (2) ² having 3 sulfur atoms in a five-member ring (Figure 1) and obtained their acetates 1a and 2a.

Figure 1.

The equilibrium between varacin, varacin A and octasulfur in an ascidian Polycitor sp. extract. 1—varacin; 1a—varacin acetate; 2—varacin A; 2a—varacin A acetate.

The authors reported the slow conversion of 1 at room temperature in CD₃OD and other solvents into varacin A (2) and octasulfur. The formation of varacin A was confirmed by NMR and mass spectra. The presence of S₈ was determined from the mass spectrum. Moreover, the addition of S₈ to the reaction mixture induced the shift of the equilibrium to 1. Similar experiments have been made for the acetates 1a and 2a (Figure 1). Hence the simultaneous presence of 1 and 2 in the methanolic extract of the ascidian is a clear direction of the presence of S₈ in the extract. Among marine invertebrates, this was first found in ascidians, but not in an alcyonarian. This information will be a good addition to the interesting finding of S8 in an alcyonarian. ¹