Six Polymorphic Crystal Forms of α‐Cobratoxin

Six different crystal forms of the long neurotoxin α‐cobratoxin isolated from the venom of Naja naja Siamensis have been obtained and the conditions for their crystallization are described. Heavy atom derivatives for two of these forms have been prepared and have proven suitable for high‐resolution X‐ray studies and, with one of them, a 0.28‐nm resolution structure has been determined. All but one of the crystal forms were grown between pH 2 and pH 3. Two novel suggestions are made which may be applicable to other protein crystallization studies. One involves cocrystallisation with mércurous iodide and the other describes a two‐step microdialysis procedure, first against salt solution, followed by dialysis against poly(ethylene glycol).