Properties of collagen monolayers formed at the water-air interface: The effect of pH and the ionic strength of a subphase

Monolayers of collagen isolated from the sclera of the eyes of a pig were s tudied at the water-air interface upon the variation of subphase pH from 3. 0 to 12.0 and the variation of concentration of KCl in the subphase up to 4 .0 M. It was shown that the two-dimensional pressure isotherms for collagen monolayers are characterized by regions of liquid-expanded and liquid-cond ensed states with a clearly pronounced plateau dividing these regions. The plateau reflects the formation of fibrils from horizontally oriented single collagen molecules. The largest collapse pressure pi(c) (60 mN/m) of a mon olayer composed of fibrils at the water surface is observed in the protein isoelectric state and decreases while passing to the acidic and alkaline re gions due to the electrostatic repulsion of macromolecules. As the ionic st rength of a subphase increases to 4.0 M KCl, pi(c) rises to 69.6 mN/m. Hyst eresis of isotherms was found for monolayers constituted both from fibrils and fibers formed by fibrils. The compression moduli of elasticity of colla gen monolayers were calculated at various pH and KCl concentrations. The la rgest value of the modulus of elasticity is typical of a collagen monolayer in the isoelectric state. It was concluded that self-assembling, two-dimen sional structures are formed from collagen molecules and fibrils at the sur face of water.