The Roles of Prolyl Residue in Polypeptide Monolayers. I. On the Chain Configurations Deduced from Surface Pressure and Potential Measurements

To elucidate the roles of prolyl residue in polypeptide monolayers, the surface films of poly-Dl-alanine, poly-l-prolyl-l-leucylglycine, copoly-1 : 1 : 1-(l-proline, l-leucine, Dl-alanine), poly-l-proline and poly-l-hydroxyproline were studied by the surface pressure and potential measurements. The area of minimum surface compressibility and the smallest area of constant surface moment were assigned to each of the first three polypeptides which were found to form monomolecular films. In the region of large area their surface moments were equal to one another and, therefore, the orientations of carbonyl groups might be the same. From these data their chain configurations at the interface were deduced. Poly-Dl-alanine exists in the β-configuration and the other two have the similar configuration except for the bonds attached to prolyl residues. Based on the departure in surface pressure behavior of copoly-1 : 1 : 1-(l-proline, l-leucine, Dl-alanine) from that of poly-l-prolyl-l-leucylglycine, the arrangement of prolyl residues in the sequence of the former polypeptides was discussed. In the poly-l-proline film it is likely that the parts of cis peptide bond form the film and the parts of trans bond dissolve into the aqueous subphase. The polypeptide films were found to be more expanded as the prolyl content was higher, although the arrangement of prolyl residues also affected the expansion of film. From the comparison of poly-l-proline film with poly-l-hydroxyproline, the evidence was obtained that the expansion of film comes from the decrease in number of hydrogen bonds because of the presence of prolyl residues. The condensation or expansion and its related properties of the film were interpreted in terms of the chain rigidity or flexibility and the intrachain interaction.