Brevetoxin B: Chemical Modifications, Synaptosome Binding, Toxicity, and an Unexpected Conformational Effect

Five naturally occurring brevetoxins and seven synthetically modified brevetoxins were examined for their affinity for site 5 of the voltage-gated sodium channel and their toxicity to mosquito fish, Gambusia affinis. All but three of the toxins studied still retained some affinity for their receptor site (IC₅₀'s in the range of 1-100 nM). Compound 7, having all three carbon-carbon double bonds reduced, is almost 3 orders of magnitude less strongly bound than 4, which has only two carboncarbon bonds reduced. This large effect resulting from H-ring reduction was unexpected, due to the similarity of this region of the molecule to the corresponding region of brevetoxin-A, which has a fully saturated eight-membered G-ring and is the most strongly bound toxin of those studied. Conformational analysis revealed that the unsaturated H-ring of brevetoxin B favors the boat-chair conformation as does the saturated G-ring of brevetoxin A. Upon reduction, the H-ring of brevetoxin B shifts to a crown conformation. This subtle change in conformational preference induces a significant change in the gross shape of the molecule, which we believe is responsible for the loss of binding affinity and toxicity.