Arginine-deriving polyamidoamine stereoisomers: synthetic chiral macromolecules showing self-organization in solution

Linear polyamidoamines (PAA) are synthetic polymers obtained by polyaddition of prim- amines or bis-sec-amines with bisacrylamides. The PAAs obtained by polyaddition of D-, L-, and (D,L)-arginine with N,N’-methylenebisacrylamide gave the corresponding PAA stereoisomers named as D-, L- and (D,L)-ARGO7. In aqueous solution, the CD spectra of D- and L-ARGO7, but not those of (D,L)-ARGO7, suggested pH-dependent self-organization. A theoretical study based on Molecular Mechanics and Molecular Dynamics methods was performed to study the conformational properties of ARGO7 isomers. Here we show through atomistic molecular dynamics simulations that, in aqueous solution, these PAAs with chiral-substituted ter-amine groups in the main chain adopt a relatively compact conformation with elongated parallel strands in a sort of “hairpin arrangement” of the backbone. Such self-organization is attributed to the conformational properties of the main chain as well as to the dipolar and/or electrostatic interactions, including also a few H-bonds, involving both the side chains among themselves and the main chain.