ASSEMBLY OF AMPHIPHILIC COMPOUNDS AND RIGID POLYMERS .1. INTERACTION OF SODIUM DODECYL-SULFATE WITH COLLAGEN

To elucidate the role of fixed charge and chain rigidity on the assemb ly of amphiphilic compounds over a polymer substrate, we studied the i nteraction between sodium dodecyl sulfate (SDS) and crystalline or sol uble collagen. Shrinkage temperatures and the equilibrium degree of sw elling were measured for crosslinked tendons under isoelectric (pH = 6 .0) conditions or in acid (pH = 2.5) solutions. Viscosities and solubi lities were measured for soluble collagen to construct pseudo phase di agrams delimiting the field of stability of the helical, random coiled , and crystalline forms. Under isoelectric conditions, increasing conc entrations of SDS (up to similar or equal to 0.1 M) cause a depression of transformation temperatures which is much larger than that observe d with salts, with aliphatic alcohols, or with sodium methyl sulfate. This is attributed to the prevailing hydrophobic interaction of the su rfactant with the random coiled form of collagen, as evidenced by adso rption of the surfactant onto the higly swollen, denatured tendon. The cooperativity of the binding process at pH = 6.0 confirms that peculi ar micelle-like assemblies are formed by SDS within the disordered ran dom coiled network. Under nonisoelectric conditions the transition tem perature is not much affected by conformation. SDS brings about a dram atic collapse of the lateral dimension of tendons at pH = 2.5, as well as precipitation of soluble collagen, even at very low concentrations , revealing a cooperative phenomenon leading to an increased interheli x interaction. The extent of adsorption of the surfactant over the pro tein, considerably larger at this pH, emphasizes the strength of elect rostatic interaction enhanced by the hydrophobic tail of the surfactan t. However, poor cooperativity is shown by the binding data at pH 2.5, suggesting that ordered assembly of cylindrical micelle-like aggregat es is prevented by the low charge density of collagen. Tendons at low pH might be described as highly swollen oriented liquid crystalline ne tworks and may undergo a phase transition to the crystalline state as a result of the strong binding between fixed charges and dodecyl sulfa te ions.