Supramolecular Assembly of Biological Molecules Purified from Bovine Nerve Cells: from Microtubule Bundles and Necklaces to Neurofilament Networks
Zach Wissner-Gross (March 16, 2009)
Supramolecular assembly of biological molecules purified from bovine nerve cells: from microtubule bundles and necklaces to neurofilament networks
Daniel J. Needleman, Janya B. Jones, Uri Raviv, Miguel A. Ojeda-Lopez, H. P. Miller, Y. Li, L. Wilson, and C. R. Safinya
Journal of Physics: Condensed Matter, 2005, 17, S3225-S3230
Soft matter keywords
Self-assembly, packing structure
Needleman et al. examine the behavior of neurofilaments (NFs) in vitro in solutions containing different polyelectrolytes (i.e., in different salt solutions). In short, they found that the NFs formed hexagonal bundles in solutions with containing cations with a greater charge and larger size, while the NFs formed what the authors term "living necklace bundles" in solutions containing bivalent cations (Figure 1). The authors go on to quantify this bundling via scattering experiments, although optical and electron micrographs seem to sufficiently reveal the bundling behaviors.
Soft matter discussion
I found it rather remarkable that NFs exhibit such different bundling behaviors in the presence of different cations. In this paper, the authors avoid any substantial discussion of why the cations induce different self-assemblies, but this is primarily because the work presented in this paper was originally published a year earlier in PNAS . However, the authors discuss their results strictly empirically in that paper as well: in their conclusion, they state that the main purpose of the work was to demonstrate how their diffraction methodology can quantifying supramolecular structures.
While I won't stake a claim as to how the different cations reacted with the NFs, I will point out some mesoscopic physics from the PNAS publication: the authors observed that NFs exposed to large, multivalent cations exhibited rigid bends in their shapes which were smaller than the persistence length of the polymers (Figure 2). Higher valencies resulted in more bending. The authors state that local defects on the nano-scale likely cause this bending behavior. In that case, the multivalent cations must somehow be stabilizing these defects.
This somewhat resembles the "pinning" of a receding water drop that we have been discussing in class, in that structural defects can be stabilized by surface (or in this case, solution) heterogeneity. Pointing out this analogy is the best I can do, since the theory of NF self-assembly is not discussed in the paper. But I would be very curious to see the authors discriminate between the effects of the size and charge of the cations -- in this study, they only used large, multivalent cations and small, bivalent cations. How would NTs assemble in the presence of a large, monovalent cation? Is it the cation's size or charge density that is relevant here?