Stability and Folding Dynamics of Polyglutamic Acid

Abstract

The thermal stability and folding dynamics of polyglutamic acid were studied by equilibrium circular dichroism (CD), Fourier-transform infrared (FTIR), and time-resolved temperature-jump infrared (IR) spectroscopy. Polyglutamic acid (PGA) forms α-helical peptides in aqueous solution and is an ideal model system to study the helix–coil transition. Melting curves were monitored with CD and FTIR as a function of pD. At low pD, PGA aggregates at temperatures above 323 K, whereas at pD >5, unfolding and refolding are reversible. At pD 5.4, a helix–coil transition occurs with a transition temperature T m of 307 K. At slightly higher pD of 6.2, the peptide conformation is already in a coil structure and only small conformational changes occur upon heating. We determined the equilibrium constant for the reversible helix–coil transition at pD 5.4. The dynamics of this transition was measured at single IR wavelengths after a nanosecond laser-excited temperature jump of ∆T ~ 10 K. Relaxation constants decreased with increasing peptide temperature. Folding and unfolding rates as well as activation energies were extracted based on a two-state model. Our study shows how equilibrium and time-resolved infrared spectroscopic data can be combined to characterize a structural transition and to analyze folding mechanisms.