Abstract
The existence of insulin submolecules having a very low molecular weight has been confirmed by studying surface monolayers of this protein in a range of very low surface pressures (0.05 to 0.5 dyne/cm). By the method of Guastalla and Bull, a molecular weight of the order of 5,000 to 6,000 can be calculated for insulin particles spread on 0.01 N hydrochloric acid. When insulin is spread on phosphate buffers at a constant ionic strength of 0.1 and pH increasing from 2.5 up to 7, a strong variation of the molecular weight is observed, values of the latter passing through a maximum of 120,000 at pH 5.6 and going down to 30,000 at pH 7. This behavior is similar to what happens in the bulk phase, in the restricted pH range (2 to 4), where the solubility of insulin allows measurements to be made. Chloride ions do not influence these processes, while thiocyanate ions considerably lower the molecular weight between pH 4.5 and 7, a range where they actually promote solubilization.
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