Unfolding of Helical Poly(<i>L</i>-Glutamic Acid) in <i>N</i>,<i>N</i>-Dimethylformamide Probed by Pyrene Excimer Fluorescence (PEF)

• Main Authors: Weize Yuan, Remi Casier, Jean Duhamel
• Format: Article
• Language: English
• Published: MDPI AG 2021-05-01
• Series: Polymers
• Subjects: poly(glutamic acid); pyrene excimer fluorescence; fluorescence blob model; denaturation
• Online Access: https://www.mdpi.com/2073-4360/13/11/1690

The denaturation undergone by α–helical poly(<i>L</i>-glutamic acid) (P<i>L</i>GA) in <i>N</i>,<i>N</i>-dimethylformamide upon addition of guanidine hydrochloride (GdHCl) was characterized by comparing the fluorescence of a series of P<i>L</i>GA constructs randomly labeled with the dye pyrene (Py-P<i>L</i>GA) to that of a series of Py-P<i>DL</i>GA samples prepared from a racemic mixture of <i>D</i>,<i>L</i>-glutamic acid. The process of pyrene excimer formation (PEF) was taken advantage of to probe changes in the conformation of α–helical Py-P<i>L</i>GA. Fluorescence Blob Model (FBM) analysis of the fluorescence decays of the Py-P<i>L</i>GA and Py-P<i>DL</i>GA constructs yielded the average number (<<i>N</i>_blob>) of glutamic acids located inside a <i>blob</i>, which represented the volume probed by an excited pyrenyl label. <<i>N</i>_blob> remained constant for randomly coiled Py-P<i>DL</i>GA but decreased from ~20 to ~10 glutamic acids for the Py-P<i>L</i>GA samples as GdHCl was added to the solution. The decrease in <<i>N</i>_blob> reflected the decrease in the local density of P<i>L</i>GA as the α–helix unraveled in solution. The changes in <<i>N</i>_blob> with GdHCl concentration was used to determine the change in Gibbs energy required to denature the P<i>L</i>GA α–helix in DMF. The relationship between <<i>N</i>_blob> and the local density of macromolecules can now be applied to characterize the conformation of macromolecules in solution.