Electric birefringence study of an amyloid fibril system: The short end of the length distribution

Abstract

Abstract. In this article, a system of amyloid fibrils, based on the protein β-lactoglobulin, is studied by transient electric birefringence. Single pulses of an electric field were applied to the solution, and the initial rise and subsequent decay of birefringence analysed. The decay takes place on a range of relaxation times, and therefore contains information about the length distribution of fibrils in the system. The information can be extracted using theories of the electric polarisability of polyelectrolyte rods, since the fibrils are an example of these. Despite the long-standing complications of such theories, useful quantitative information about the system can still be obtained. Using the Fixman model of polyelectrolyte polarisability, we obtain a measurement of the short end of the length distribution which shows the fibril concentration as a function of length rising linearly from 0.02-2µm. The short end of the length distribution was unobtainable in our previous study using rheo-optics (Rogers S. S. et al., Macromolecules 38, 2948Macromolecules 38, (2005, Iss. 8), but reasonable agreement between the two techniques shows they are complementary.

Investigating the permanent electric dipole moment of β‐lactoglobulin fibrils, using transient electric birefringence

“…37 In our previous study, we found that our -lactoglobulin fibril solutions obeyed the predicted relationship between the initial rise gradient of the birefringence, and the field strength, assuming instantaneous induced polarization; D _ n init ! E 2 , 21 where the gradients were measured on a time scale $1 s. Using Fixman's prediction for the magnitude of the induced polarization, and treating this polarization as instantaneous, we found that the birefringence response from 240 s pulses led to a prediction of the length distribution of the fibrils, in reasonable agreement with the results of our earlier flow birefringence technique. 20 However, these results do not preclude the existence of additional slower components of the polarizability, provided that if slower components exist, their relaxation times are much greater than the pulse time of 240 s, or that they have a much smaller contribution than D according to Fixman. Because of the previous results, it is reasonable to model the induced polarization of the fibrils as instantaneous, with a magnitude given by Fixman's theory.…”

“…As calculated in Ref. 21, a prediction of D _ n init ! E 2 is obtained, reminiscent of the Kerr law, though it applies to the initial rise of birefringence rather than the steady state at long times.…”

“…In our previous study, 21 we found that Fixman's theory of the polarizability of polyelectrolyte rods 26 gave a reasonable prediction of D for -lactoglobulin fibrils. Fixman predicted…”

“…15,17 In previous articles, we studied this system's length distribution using flow birefringence 20 and electric birefringence relaxation. 21 As a model amyloid fibril system, -lactoglobulin fibrils have some interesting characteristics. The simplicity of their cross-sectional structure is in contrast with many other fibril systems where complex arrangements of filaments are observed, and they are composed of a complete protein rather than a short peptide sequence.…”

“…In our previous study, 21 TEB was performed on a solution of -lactoglobulin fibrils, and the electric field pulses was purposefully made as short as possible to minimize the effect due to the permanent electric dipole moment of the fibrils. The measurements were consistently described using Benoît's model, 22 where the polarizability has a magnitude given by Fixman's theory.…”