Photoreceptor matrix

For directional responses, the photoreceptor molecules (or an array of those) are necessarily — at least for the duration of the photoreaction — located in a matrix with a fixed orientation to the bending organelle, or to the whole body of freely swimming organisms. 

The current organic photoreceptors are triarylamines, triarylmethanes, hy-drazones, oxadiazoles, pyrazolines, oxazoles, and more recently, stilbene derivatives. The polymer matrix, on the other hand, is constituted by polyesters and polycarbonates (Fig. 5). The common presence of aromatic amines as substituents in all these materials contributes to efficient hole transport [44]. The nonbonding electron pair on the nitrogen atom, in fact, confers on these molecules a low oxidation potential, and consequently, the production of a chemically stable radical cation with the possibility of an effective overlap of nonbonding molecular orbitals between neighboring molecules. 

In contrast, proteins vary markedly in their lateral mobility. Some proteins are nearly as mobile as lipids, whereas others are virtually immobile. For example, the photoreceptor protein rhodopsin (Section 32.3.1). a very mobile protein, has a diffusion coefficient of 0.4 pm s f The rapid movement of rhodopsin is essential for fast signaling. At the other extreme is fibronectin, a peripheral glycoprotein that interacts with the extracellular matrix. For fibronectin, D is less than 10-4 pm2 s f Fibronectin has a very low mobility because it is anchored to actin filaments on the inside of the plasma membrane through integrin, a transmembrane protein that links the extracellular matrix to the cytoskeleton. 

In azo-dye-based laminar photoreceptor systems, neither the dissociation efficiency nor the primary yield of bound pairs is dependent on excitation wavelength [11 13]. There is a strong positive linear correlation between the dissociation efficiency and the peak value of the dipole moment associated with the transition between the ground and first excited states in the azo photosensitizer (obtained from electroabsorption data) [14]. It is likely that in these systems the efficiency of dissociation is constrained by the electron transport process away from the interface into the bulk of the sensitizer material, because transport of holes through the donor matrix is much more rapid. 

The interstitial space between RPE and photoreceptors contains a sticky inteiphotoreceptor matrix consisting of glycoproteins, proteoglycans, and hyaluronic acid that helps the retina adhere to the back of the eye. Retinal adhesion is also promoted by extrusion of water from the RPE to choroid. Basolateral Cl channels and apical Na/K/2C1 transporters are particularly important for water transport out of the RPE. Because photoreceptors are not physically bound to the RPE, the retina can detach from the RPE with a strong blow to the eye, fluid build-up behind the retina (rhegmatogenous... 

Figure 7. Fluorescence micrograph of a 1-pm-thick section of the lamina of a male fly that shows stained photoreceptor axon terminals cut parallel to their long axis. Gap junctions between the six peripheral cells occur in the outer 10 pirn of the axon terminal, just below the somas of the monopolar neurons. Because recorded cells are located in reference to the corneal facet matrix, the location and orientation of that cell in the underlying neuropil is predetermined, so it can be sectioned in any given plane and recovered for histological and histochemical examination.

Baranov, P., Michaelson, A., Kundu, J., Carrier, R. L., Young, M. (2014). Interphotoreceptor matrix-poly(s-caprolactone) composite scaffolds for human photoreceptor differentiation. Journal of Tissue Engineering, 5, 1—8. 

The polymeric photoconductors used in practice are based on two types of system. The first one is based on polymers in which the photoconductive moiety is part of the polymer, for example a pendant or in-chain group. The second system involves low molecular weight chromophores imbedded in a polymer matrix. These so-called molecularly doped polymers are widely used today. They are described in Chapter 10 in this handbook by Law. Almost 100% of all xerognq>hic photoreceptors are made from organic photoconductors [5]. The main areas of application of polymeric photoconductors are as follows ... 

The basic principle in the molecular mechanism of biological photoreceptors appears to be the interaction between a photochromic moiety and a macromolecule. The protein-ligand complex as a whole is known to be embedded in a membrane matrix. In a very general way the results of our studies can be summarized by stating that in photochemical energy transductions both features are very important. That is, the... 

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