Surface Chemistry of Synthetic Protein Analogues. I. Surface Pressure-Area Relation of Synthetic Polypeptides as the Model of Proteins

Abstract

Abstract The surface pressure against area (F-A) curves of the monolayers of poly-Dl-α-aminocaproic, poly Dl-α-aminocaprylic, poly Dl-α-aminocapric, poly Dl-α-aminolauric acids, poly γ-methyl-l-glutamate and poly ε-aminocaproic acid (amilan) was measured. The results obtained were as follows: (1) The F-A curve of poly Dl-α-aminocaproic acid having four carbon atoms in the side chain resembled in shape very closely to those given by natural proteins such as the egg albumin or pepsin. This fact is due to the resemblance of the carbon number in the side chains of both of the polypeptide and proteins. (2) The plateau which is a region of very high compressibility appeared in the F-A curves of the polypeptides having more than six carbon atoms in the side chains. The curves of these polypeptides were entirely different from those of natural proteins. (3) The area per amino acid which was found for every polypeptide to be in the closest packing of polypeptide backbone was all 14.7 Å.2 This value is in good agreement with the area calculated from x-ray data for an arrangement of β-keratin configuration. (4) Poly γ-methyl-l-glutamate occupied the limiting area of 9.8 Å.2/amino acid residue on distilled water. This value can be sufficiently accounted for, if this polymer has a structure of α-configuration on water surface and the model of α-keratin configuration presented by Shimanouchi and Mizushima and by Ambrose et al. is accepted. On substrates containing hydrochloric, formic, or acetic acid, however, this polypeptide occupied the limiting area of 14.7 Å.2/residue, this value corresponding to the area of the other polypeptides which are arranged in close packed β-configuration. The α→β transformation of this polypeptide could be demonstrated by monolayer technique. (5) The monolayer of the polymer of ε-aminocaproic acid is inadequate as a model of protein films.