Title
Spectroscopic study of the pH dependent interaction of an achiral molecular photo-switch with poly-Glutamic acid
Author
Julie Fitz, Angela Mammana
Year
2020
Journal
Journal of Photochemistry and Photobiology A: Chemistry
Abstract
In this manuscript we present a spectroscopic study of the supramolecular interaction of photo-switchable 4,4′-Azobenzene dicarboxylic acid (ADA) with L and D poly-Glutamic acid (Poly-Glu). ADA is water-soluble and undergoes photo-isomerisation between trans and cis geometries when irradiated with UV and visible light. Non-covalent assembly of the photo-switch with the polypeptide is induced by reducing the pH of an aqueous solution containing both the ADA and Poly-Glu and characterized with circular dichroism (CD) and UV–vis spectroscopy. The properties of the aggregates depend on multiple factors such as the molecular conformation of the photo-switch, which is selected with UV and visible light, the pH of the solution and the manner in which the pH is reduced (rapid vs. slow reduction). The polypeptide acts as a chiral template for trans-ADA chromophores as demonstrated by the appearance of an induced CD signal of the achiral photo-switch in the presence of Poly-L-Glu at reduced pH. In neutral aqueous solutions trans-ADA can be photo-isomerised to cis-ADA, however, its aggregated form loses this capability. In contrast to the trans geometry, CD and UV–vis spectroscopy show that Poly-Glu is unable to function as a chiral template during the aggregation of cis-ADA. Nevertheless, the polymer is able to induce chirality when cis-ADA aggregates are photo-isomerised to the trans form by irradiation with visible light, as shown by the appearance of an induced CD signal. Distinct differences observed in the CD and UV–vis spectra depending on the mode of pH reduction, indicates that assembly occurs under hierarchical control.
Instrument
V-630, J-815
Keywords
Circular dichroism, Secondary structure, Polymers, Chemical stability, Stereochemistry, Induced circular dichroism, Absorption, Aggregation, Kinetics, Biochemistry