Diffusion Coefficient and the Secondary Structure of Poly-l-glutamic Acid in Aqueous Solution

Abstract

The diffusion coefficients (D) of poly-L-glutamic acid (PLG) at various pHs are investigated by the laser-induced transient-grating method with a new photoreactive probe molecule. The pH dependence of D is compared with that of the helical content of PLG measured by circular dichroism. It is found that the pH dependences of both quantities are very similar. Since the frictions of the translational diffusion of charged and protonated carboxyl groups are found to be similar each other, it is concluded that the conformation of the main polymer chain is the main factor in determining the diffusion process; in other words, the alpha-helix conformation makes the molecular diffusion faster. This result indicates that the conformational change of a protein can be detected by monitoring the diffusion coefficient.

Equilibrium unfolding of the poly(glutamic acid)20 helix

“…[41][42][43][44][45]47 Nonetheless, it was important to verify that the ABZ-E 20 -Y and ABZ-E 20 -YNO 2 peptides were not significantly aggregated in any of the samples measured in the present study. This was especially important in samples studied at or below pH 4, where prolonged exposure to heat and time has been shown to promote amyloid-like aggregates.…”

“…[39][40][41][42][43][44][45]60 These structural models are largely based on circular dichroism data but are also supported by FTIR and SAXS measurements. 39,40,[42][43][44][45]60 CD spectra of the E 20 peptide (Figure 2a) are consistent with an a-helical structure at pH 4 and a random coil structure at pH 6. The E 20 helix-coil transition, measured by CD, is also clearly captured by steady state FRET data (Figure 2b).…”

“…In most helix-coil studies, it is often assumed/concluded that the equilibrium a-helix state has a single contiguous run of helical turns. 41,43,45,60 In the case of E 20 , this does not appear to be the case. 42,72 The polyE helix-coil transition has been studied for over 30 years.…”

“…[39][40][41] Recently, the apparent simplicity of this homopolymer has stimulated the use of polyE as a helix-coil model system for novel experimental and theoretical techniques investigating protein structure and folding kinetics. [42][43][44][45]60 Interestingly, these studies reveal some different new hypotheses regarding the folding mechanism and structure of polyE.…”

“…[39][40][41] Recently, polyglutamic acid has been used as a model system to study the helix-coil transition owing to the fact that a-helix folding can be initiated from a completely unfolded state using a rapid decrease in pH from 8 to below 5. [42][43][44][45] This has been particularly important in kinetic studies of a-helix folding by CD and FTIR since shifts in pH result in minimal optical perturbation. [42][43][44] Through appropriate solvent conditions, polyglutamic acid also can be completely folded and unfolded between its a-helix and random coil states.…”