Iodination and purification of oxytocin

Oxytocin was iodinated using the thallium chloride method. Purification of iodination reaction mixture on Sephadex G-10 and G-15 was compared. Sephadex G-10 effected the separation of iodo-oxytocin from excess free radioiodide. Sephadex G-15 succeeded in separating iodinated from non-iodinated oxytocin as well as from readioiodide. It was found that the storage of unfractionated iodination mixture resulted in an apparent increase in the molecular size of iodinated hormone and that this process was accelerated at temperatures of -20 degrees in comparison with 4 degrees.