Studies on Long Chain Alkyl Amides of Water-soluble Poly-α-amino Acids. I. Amphipathic Properties of Poly-DL-alanine Dodecylamide

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Poly-DL-alanine alkyl amide has been synthesized by the polymerization of N-carboxy-DL-alanine anhydride initiated with alkyl amine at different anhydride-initiator ratios ([A]/[I]). The amphipathic properties of poly-DL-alanine dodecylamide (PADA) with [A]/[I] = 5 to 20 have been investigated in comparison with poly-DL-alanine. The degree of polymerization of poly-DL-alanine part can be easily controlled by the initial [A]/[I] ratio. The number average degree of polymerization (x) is found to be approximately equal to [A]/[I]. The solubility of PADA in water is higher as x increases, and PADA with x>17 is transparently soluble. PADA with any x value is soluble in aqueous acid solutions, the pH of which is lower than 5.5. The titration curve with an aqueous acid solution shows a break at this pH. The infrared spectra of PADA in the solid state indicate that PADA with higher x is almost in the random conformation and the portion of the β-conformation increases as x becomes lower. Surface tension of PADA in aqueous acid solution exhibits typical characteristics of amphipathic substances. The degree of surface tension lowering at low concentration region is the largest for PADA of x=10. The surface film of PADA is of condensed type and the limiting area is larger as x increases. PADA with higher x becomes more spread as surface film, more DL-alanyl residues being on the aqueous surface.
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