Behaviour of insulin Langmuir monolayers at the air–water interface under various conditions

Bovine insulin (INS) monolayers were spread from dilute HCl solution on aqueous subphases under various conditions of temperature (15–25 °C), pH (1–10) and salt content (NaCl, Na3PO4, ZnCl2), and plots of surface pressure (π) against mean area per amino acid residue (A) were recorded during their compression at fast and slow barrier speeds. Although the characteristics and behaviour of the monolayers were significantly influenced by these variables, under none of the conditions studied did molecules of the film appear to migrate into the subphase. In most of conditions mentioned above, compression of these films seems likely to cause the partial submersion of the insulin molecules in the subphase, but these structural changes are relatively slow and essentially reversible. The formation of a relatively rigid, compact film structure, by molecular association, seems to be favoured by high subphase pH and by the presence of Zn2+ ions, while low pH and the presence of PO43− ions appear to favour the dissociation of oligomers, the extension of the monomers and the total submersion of their A chains in the subphase during compression. The presence of salts in the subphase also reduces the reversibility of the changes occurred under film compression.