D-, L- and D,L-Tryptophan-Based Polyamidoamino Acids: pH-Dependent Structuring and Fluorescent Properties

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.