  Ultrastructural fluctuations at nanoscale are fundamental to assess
properties and functionalities of advanced out-of-equilibrium materials. We
have taken myelin as a model of supramolecular assembly in out-of-equilibrium
living matter. Myelin sheath is a simple stable multi-lamellar structure of
high relevance and impact in biomedicine. Although it is known that myelin has
a quasi-crystalline ultrastructure there is no information on its fluctuations
at nanoscale in different states due to limitations of the available standard
techniques. To overcome these limitations, we have used Scanning micro X-ray
Diffraction, which is a non-invasive probe of both reciprocal and real space to
visualize statistical fluctuations of myelin order of the sciatic nerve of
Xenopus Laevis. The results show that the ultrastructure period of the myelin
is stabilized by large anti-correlated fluctuations at nanoscale, between
hydrophobic and hydrophilic layers. The ratio between the total thickness of
hydrophilic and hydrophobic layers defines the conformational parameter, which
describes the different states of myelin. Our key result is that myelin in its
out-of-equilibrium functional state fluctuates point-to-point between different
conformations showing a correlated disorder described by a Levy distribution.
As the system approaches the thermodynamic equilibrium in an aged state the
disorder loses its correlation degree and the structural fluctuation
distribution changes to Gaussian. In a denatured state at low pH, it changes to
a completely disordered stage. Our results clarify also the degradation
mechanism in biological systems by associating these states with variation of
the ultrastructural dynamic fluctuations at nanoscale.
