Absorption spectra of Müller cell intermediate filaments: Experimental results and theoretical models

Experimental spectra of Müller cell (MC) intermediate filaments (IFs) isolated from porcine retina are reported in this work. The absorption spectra recorded at different MC IF concentrations were used to estimate their absorption cross-sections at different wavelengths. The average absorption cross-section of a single MC IF was ca. (0.97 … 2.01) × 10^-10 cm² in the 650-445 nm spectral range. To interpret these experimental absorption spectra, we made ab initio calculations of the optical spectra of α-helix polypeptides, and also used a simplified theoretical approach that modeled an IF by a conductive wire. The energy spectra of the refractive index, extinction coefficient (absorption cross-section), energy loss and reflectivity functions for different photon polarizations, with strong anisotropy with respect to the system axis, were calculated ab initio for polyglycine α-helix molecule containing 1000 glycine residues. Strong anisotropy of these parameters was explained by photons interacting with different electronic transitions. Note that similarly strong anisotropy was also obtained for the optical absorption cross-sections in the simplified model. Both modeling approaches were used for calculating the absorption cross-sections of interest. As a result, the absorption cross-section for photons propagating axially along MC IFs was much larger than their geometrical cross-section. The latter result was explained taking into account the density of electronic states, with numerous electrons contributing to the transition intensity at a given energy. We found that the simple conductive wire model describes the MC IF absorption spectrum better than the ab initio spectra. The latter conclusion was explained by the limitations of ab initio analysis, which only took into account one α-helix with 1000 aminoacids, whereas each porcine Müller cell IF is assembled of thousands of protein molecules, reaching the total length of ca. 100 μm. The presently reported results contribute to the understanding of the quantum mechanism of high-contrast vision of vertebrate eyes.