Synthesis of proposed structures and structural revision of marine Roseobacter sulfur amino lipids (SALs)

The Roseobacter Clade Bacteria (RCB) play a crucial role in marine ecosystems, particularly Ruegeria pomeroyi and Phaeobacter inhibens, which utilize organosulfur compounds such as 2,3-dihydroxypropanesulfonate (DHPS) and dimethylsulfoniopropionoate (DMSP). Recently, a new class of sulfonolipids, sulfur amino lipids (SALs), was identified in these bacteria, with possible structures proposed by Smith et al. (ISME Journal, 2021, 15, 2440-2453). This study aims to confirm or revise the proposed structures for SAL-656 and SAL-672. Two candidates for SAL-656 and SAL-672 were synthesized, namely 3-acyloxyacylamides of homotaurine and cysteinolic acid, respectively. Tandem mass spectrometry (MS/MS) analysis of synthetic and natural SALs revealed significant discrepancies, leading to the exclusion of proposed structures. Further exploration of lipid extracts from R. pomeroyi and P. inhibens identified related lipoforms of SAL-656 and SAL-672, which form two distinct families based on LC-MS/MS and molecular network analysis. While the mass spectrometric data allow exclusion of previously proposed structures and provide insights into acyl groups and headgroups, the complete structures of SAL-656 and SAL-672 remain elusive. Nonetheless, the data are consistent with revised structures based on cysteinolic acid or 3-amino-2-hydroxypropanesulfonic acid wherein both hydroxyl and amino groups are acylated. Our findings reveal the SALs as a group of sulfonolipids that are distinct from more studied classes of sulfonolipids, with implications for understanding their biosynthesis and ecological roles in marine environments.