Surface Chemistry of Synthetic Protein Analogues. V. On the Effect of Salt on the Monolayers of the Copolypeptide of l-Lysine, l-Phenylalanine and l-Glutamic Acid

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Abstract
By studying the surface pressure-area and surface viscosity-area curves of the copolypeptide of l-lysine, l-phenylalanine and l-glutamic acid over a range of pH values and salt contents in the substrate, the following four distinct results were obtained: (1) When the concentration of potassium chloride in the substrate was kept constant (0.07 m/litre), the (A)F-pH curve has a minimum area region between pH 5.6 and 8.0 which is parallel to the pH axis and this region corresponds to the isoelectric band of this copolypeptide at this salt concentration. The width of this band was the function of the salt concentration; when the salt content was increased this band became wider. (2) When the pH’s of the substrates was kept constant (pH 2.2), the F-A curves were more expanded at low salt concentrations. This fact could be interpreted by the increment of surface pressure due to the free energy of the electrical double layer which is formed between the ionized groups of the monolayer (NH3+ ions) and the counter ions (Cl− ions). The values of surface potentials calculated from the F-A curves and from Gouy’s equation agreed with each other. (3) The surface viscosities became higher when the salt concentrations in the substrate at constant pH’s were decreased and became maximum at its isoelectric point. The surface viscosity depends markedly on the number of inter- or intra-salt-linkages between -NH3+ and COO− ions and not on the shape of each polymer molecule. (4) The (A)F-pH and (η)A-pH curves could be interpreted in terms of effective free charges and the number of salt-linkages.