COMPRESSION EXPANSION CURVES OF CYCLOSPORINE-A MONOLAYERS ON SUBSTRATES OF VARIOUS IONIC STRENGTHS

At pH 6, monolayers of cyclosporin A (CyA) had a limiting area of 314. 5 angstrom2 per molecule; when the monolayer collapsed (at a surface p ressure of 30 mN m-1), its area was 165 angstrom2 per molecule, i.e. 1 5 angstrom2 per amino acid, a value typical of the area occupied by ea ch amino acid in densely packed protein monolayers. During compression to collapse, the surface pressure increased steadily, the surface pre ssure-area (pi-A) curves exhibiting no flat plateau. These findings su ggest that collapse involved the expulsion of segments of CyA molecule s from the interface, although strong intermolecular attraction must h ave prevented total immersion in the subphase. The hysteresis exhibite d when successive compression-expansion cycles were carried out sugges ts that the return of the expelled CyA segments to the interface is a slow equilibrium process. When spread on 3 M NaCl, CyA monolayers coll apsed only when a surface pressure of 64 mN m-1 and a molecular area o f 40 angstrom2 per molecule were reached, and the pi-A curves exhibite d a flat plateau 100 angstrom2 per molecule long at a surface pressure of 36 mN m-1. The plateau is interpreted as reflecting a change in th e orientation of the CyA peptide ring from parallel to perpendicular t o the surface.