Self-Structuring in Water of Polyamidoamino Acids with Hydrophobic Side Chains Deriving from Natural α-Amino Acids

This paper reports on synthesis, acid-base properties and self-structuring in water of chiral polyamidoamino acids (PAACs) obtained by polyaddition of N,N'-methylenebisacrylamide with L-alanine, L-valine and L-leucine (M-L-Ala, M-L-Val, M-L-Leu) with potential for selective interactions with biomolecules. The polymers maintained the acid-base properties of amino acids. In water, the circular dichroism spectra of PAACs revealed pH-dependent structuring in the range 3-11 and in the wavelength interval 200-280 nm. Taking as reference the values at pH 3, the differential molar ellipticities were plotted in the pH interval 3-11. Sigmoidal curves were obtained presenting inflection points at pH 8.1, 6.8 and 7.3 for M-L-Ala, M-L-Val and M-L-Leu, respectively, corresponding to the amine half-ionization. Theoretical modeling showed that PAACs assumed stable folded conformations. Intramolecular interactions led to transoid arrangements of the main chain reminiscent of protein hairpin motif. Oligomers with ten repeat units had simulated gyration radii consistent with the hydrodynamic radii obtained by dynamic light scattering.