Probing chiral interactions between L- and D-arginine-based polymers and sodium deoxycholate solutions

Nowadays the spontaneous self-organization of a polymer into an ordered structure is a soughtafter property of many smart materials, whose applications might range from catalysis [1] to drugdelivery [2]. However, literature regarding the role played by these specific conformations in chiral recognition remains scarce. In this context, polyamidoamino acids (PAACs) are an emerging class of stimuli-responsive bioinspired synthetic polymers able to self-assemble into pH depend conformations [3,4]. PAACs are an off-spring of polyamidoamines (PAAs), a family of polymers obtained by the Michael-type polyaddition of prim-monoamines or sec-diamines with bisacrylamides. The reaction occurs in aqueous solution at pH = 8–9 and at room temperature. By using α-amino acids as monomers, PAACs are obtained. The first example of PAAC was named ARGO7, obtained by the stepwise polyaddition in water of L- or D-arginine to N,N’-methylenebisacrylamide. Results indicated Mn 8500, PDI 1.4 and Rh of 1.2 nm [3]. Molecular dynamics (MD) and circular dichroism (CD) showed ARGO7 folded into a rigid structure, reminiscent of the hairpin conformation, solely driven by the polymer main chain. Due to its ability to self-assemble in solution forming chiral structures, L- and D-ARGO7 may selectively interact with biological components. To assess chiral recognition, sodium deoxycholate (NaDC), one of the components of bile salts, was chosen as a chiral model surface. In aqueous solution, NaDC showed three different pH dependent behaviour: homogeneous solution (pH>8), gel phase (pH 7-8) and aggregation/flocculation (pH<6.5). Notwithstanding the ability of NaDC to self-assemble into different conformations at each pH interval, signs of chiral recognition were found in NaDC gel phase only. Conformational modifications were probed by circular dichroism spectroscopy: both D- and L-ARGO7 changed shape and magnitude of the CD pattern, whereas D,L-ARGO7 did not modify the CD spectra of NaDC. After 8 days, NaDC compact structure loosened, ended up being fluid and the CD pattern were completely modified due to NaDC and D- or L-ARGO7 interactions. Incoming diffusion NMR and SANS studies will probably highlight the mechanisms and dynamics of the chiral interactions in these polyelectrolyte-micelle systems. [1] Luo, R.; Zhu, M.; Shen, X.; Li, S. J. Catal. 2015, 331, 49. [2] Quiñones, J. P.; Peniche, H.; Peniche, C. Polymers. 2018, 10, 3, 235. [3] Manfredi, A.; Mauro, N.; Terenzi, A.; Alongi, J.; Lazzari, F.; Ganazzoli, F.; Raffaini, G.; Ranucci, E.; Ferruti, P. ACS Macro Lett. 2017, 6, 987. [4] Lazzari, F.; Manfredi, A.; Alongi, J.; Mendichi, R.; Ganazzoli, F.; Raffaini, G.; Ferruti, P.; Ranucci, E. Polymers 2018, 10, 1261.