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Synthesis of an acetylcholine receptor-specific toxin derivative regioselectively labeled with an undecagold cluster.

The regioselective modification of a snake curaremimetic toxin is described. Toxin-alpha from Naja nigricollis was derivatized with an electron-dense maleimido undecagold cluster 5. The cluster-bound obtained toxin 8 has a very high affinity for the cholinergic binding site (Ki = 70 pM) of Torpedo marmorata nicotinic receptor. It is harboring a compact heavy atom core of about 8 A which makes it very useful for electron microscopy experiments.