Anesthetic-protein interaction. Random versus helix polylysine monolayers and interaction with 1-alkanols

Penetration of 1-alkanols into monolayers of hydrophobic polypeptides, poly(epsilon-benzyloxycarbonyl-L-lysine) and poly(epsilon-benzyloxycarbonyl-DL-lysine), was compared with their adsorption on the air/water interface in the absence of monolayers. The polypeptide prepared from L-lysine is generally considered to be in the alpha-helical form whereas DL-copolymer polypeptide contains random-coiled portions due to the structural incompatibility between the two isomers. The free energy of adsorption of 1-alkanols on the air/water interface at dilute concentrations was -0.68 kcal X mol-1 per methylene group and 0.15 kcal X mol-1 for the hydroxyl group at 25 degrees C. In the close-packed state, the surface area occupied by each molecule of 1-alkanols of varying carbon chain-lengths showed nearly a constant value of about 27.2 A2, indicating perpendicular orientation of the alkanol molecules at the interface. About 75% of the water surface was covered by 1-butanol in this close-packed state. The mode of adsorption of 1-alkanols on the vacant air/water interface followed the Gibbs surface excess while the mode on the polypeptide membranes followed the Langmuir adsorption isotherm, indicating that the latter is characterized by the presence of a finite number of binding sites. The free energies of adsorption of 1-alkanols on the L-polymer monolayers were more negative than those on the vacant air/water interface and less negative than those on the DL-copolymer monolayers. Thus, the affinity of 1-alkanols to the interface was in the order of vacant air/water interface less than L-polymer less than DL-copolymer. The difference between the air/water interface and L-polymer was about 0.54 kcal X mol-1 and that between L-polymer and DL-copolymer was 0.17 kcal X mol-1 at 25 degrees C: the adsorption of 1-alkanols to the DL-copolymer was favored compared to the L-polymer. The polar moieties of the backbone of the DL-copolymer may be exposed to the aqueous phase at the disordered portion. Dipole interaction between this portion and 1-alkanol molecules may account for the enhanced adsorption of the alkanols to the DL-copolymer.