d-, l- and d,l-Tryptophan-Based Polyamidoamino Acids: pH-Dependent Structuring and Fluorescent Properties

Abstract

Chiral polyamidoamino acids were obtained by polyaddition of N,N’-methylenebisacrylamide with d-, d,l- and l-tryptophan (M-d-Trp, M-d,l-Trp and M-l-Trp). l-tryptophan/glycine copolymers, M-G-l-Trp5, M-G-l-Trp10, M-G-l-Trp20 and M-G-l-Trp40, were prepared from l-tryptophan/glycine mixtures. These polymers were amphoteric, with acid-base properties similar to those of the parent amino acids. The l-tryptophan/glycine copolymers with high glycine content were water soluble in the pH range 2-12. M-G-l-Trp40 showed a solubility gap centred at pH 4.5 and all tryptophan homopolymers were soluble only at pH > 7. Dynamic light scattering measurements performed in their solubility ranges, namely 2-11 M-G-l-Trp5, M-G-l-Trp10 and M-G-l-Trp20 and 7-11 for M-G-l-Trp40, M-d-Trp, M-l-Trp and M-d,l-Trp, showed that the size of all samples did not significantly vary with pH. Both M-l-Trp and M-G-l-Trp copolymers showed pH-dependent circular dichroism spectra in the wavelength interval 200–280 nm, revealing structuring. All samples were fluorescent. Their emission spectra were unstructured and, if normalized for their tryptophan content, almost superimposable at the same pH, providing evidence that only tryptophan governed the photoluminescence properties. Changing pH induced in all cases a slight shift of the emission wavelength maximum ascribed to the modification of the microenvironment surrounding the indole ring induced by different protonation degrees.

pH-Dependent Chiral Recognition of D- and L-Arginine Derived Polyamidoamino Acids by Self-Assembled Sodium Deoxycholate

“…This characteristic was shared by other PAACs prepared from L-alanine, L-valine, L-leucine [26]. Subsequently the PAAC derived from L-tryptophan [27] was also prepared. As a further development, the controlled synthesis of L-ARGO7 and its copolymers with L-alanine was performed [28].…”

“…A different class of stimuli responsive polymers based on α-amino acids, named polyamidoamino acids (PAACs), has been obtained by the Michael-type polyaddition of α-amino acids with bisacrylamides, as for instance N,N'-methylenebisacrylamide [24][25][26][27][28][29][30], 2,2'-bis(acrylamidoacetic) acid [29][30][31], and 1,4-bis(acryloyl)piperazine [32]. In particular, the PAAC named L-ARGO7 was obtained from N,N'-methylenebisacrylamide and L-arginine [24].…”

Hydrogen Bonding in a l-Glutamine-Based Polyamidoamino Acid and its pH-Dependent Self-Ordered Coil Conformation

“…The circular dichroism (CD) spectra of M-L-Gln in 0.1 M NaCl and in the pH range 3-11 demonstrated the presence of self-structuring ( Figure 3). Like all PAACs [31][32][33][34], M-L-Gln showed pH-dependent CD patterns due to the conformational changes induced by the changes in the ionization state. However, unlike other PAACs, the M-L-Gln spectra obtained at acidic pH exhibited positive molar ellipticity with maxima centered at 217 nm and only modest negative peaks with minima centered at 210-215 nm.…”

“…Chiral synthetic and hybrid polymers may also exhibit structuring ability [12][13][14][15] and open interesting perspectives for applications such as catalysis [16][17][18][19], drug-delivery [20,21], chiral recognition [22,23], chiral resolution [24][25][26], biosensing [27][28][29], and bioimaging [30]. Recently, the chiral representatives of a new polymer family deriving from natural α-amino acids, polyamidoamino acids (PAACs), have been shown to assume stable conformations in water [31][32][33][34]. Polyamidoamino acids are obtained by the step-wise Michael The peculiarity of PAACs, compared to other natural and synthetic polymers prepared from or inspired by α-amino acids, for instance, polypeptides, polypeptoids, and poly-N-acrylamido acids, consists in the retention, in addition to the chirality, of the amphoteric properties of the starting α-amino acids.…”

“…The PAACs studied so far have been classified based on the structural features of the α-amino acid side substituents, namely, basic PAACs, obtained from l-, d-or d,l-arginine (called ARGO7 stereoisomers) [31,32] and alkyl-substituted PAACs, obtained from l-alanine (M-l-Ala), l-valine (M-l-Val), and l-leucine (M-l-Leu) [33]. In addition, homo-and copolymeric l-tryptophan-based PAACs were studied [34].…”