Life with Boron: Steroid Architecture and the Chemistry of Marine Boronosteroids

Marine invertebrates produce a remarkable diversity of polyhydroxylated steroids and secosteroids whose structural features-particularly vicinal (1,2-)diols, 1,3-diols, and clustered hydroxyl arrays-make them well suited for coordination with boron species. In the marine environment, where boron is abundant, chemically stable, and predominantly present as borate under mildly alkaline conditions, such interactions are not only plausible but may be widespread. This review examines the chemistry of boron-steroid complexation in marine systems, emphasizing how rigid steroidal frameworks preorganize diol motifs to form reversible yet stable borate esters under environmentally relevant conditions. We discuss how polyhydroxy steroids may exist in dynamic equilibria between free and boron-bound forms, with speciation governed by pH, boron concentration, and local microenvironmental factors rather than enzymatic control. Boron complexation can modulate key physicochemical properties, including solubility, conformation, and membrane affinity, thereby influencing the biological activity of marine steroids without covalent modification of the carbon framework. By integrating examples from sponges, echinoderms, and corals together with well-characterized model polyols, this review highlights boron complexation as an underrecognized but potentially important factor influencing the structure, function, and bioactivity of marine steroid metabolites.