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Processes in Biological Vision PROCESSES IN BIOLOGICAL VISION: including, ELECTROCHEMISTRY OF THE NEURON This material is excerpted from the full β-version of the text. The final printed version will be more concise due to further editing and economical constraints. A Table of Contents and an index are located at the end of this paper. James T. Fulton Vision Concepts [email protected] April 30, 2017 Copyright 2001 James T. Fulton Photochemistry 5- 1 5 The Photochemistry of Animal Vision 1 “The concept in psychophysics that the visual spectral sensitivities are unknowable and irrelevant is unthinkable.” (This Author) “Solid state events involving conduction are evident in animate aggregations and may well be an essential characteristic of life, which may be an electromagnetic phenomenon.” Gutmann, Keyzer & Lyons (1983)2 5.1 Introduction Saari said in 1994; “Nature has exploited the relatively simple retinoid structure to full advantage. The molecule mediates a bewilderingly complex set of biologic functions with only a single functional group and a set of conjugated double bonds.”3 This chapter will show that when Nature added a second functional group, resulting in two distinctly different sets of conjugated double bonds, She expanded this exploitation considerably. The vision community within the field of biology has sought to determine the detailed nature of the chromophores of animal, and particularly human, vision for a very long time. Even after the industrial and technological revolutions, these chromophores are not known, except in conceptual form, in the vision literature. This is unfortunate since the necessary scientific knowledge has been available since the 1930-50's in other non biological disciplines. In the absence of the transfer of this knowledge into the vision community, the community has suffered from a lack of knowledge about exactly what they are seeking and the conditions required in the laboratory to isolate it. On the other hand, the specific conditions under which the chromophores of vision can be isolated and identified are extremely demanding compared to conventional chemistry. Many conventional chemical tests lack the specificity required to isolate the chromophores completely. This Chapter will provide the background and detailed conditions required to isolate the chromophores of vision, and the exact formula and state of matter of the chromophores of vision. Example: The four chromophores of animal vision are resonant conjugate retinoids that only exhibit their unique spectral performance when in the liquid crystalline state. They must not be attacked by strong detergents or either oxidizing or reducing agents. Their spectra can only be measured in the transient mode unless they are in quantum contact with another material that can continually de-excite the excitons created by the incident photons. CHAPTER SYNOPSIS [xxx some duplication with above insert ] This Chapter will present the actual chromophores of vision, labeled the Rhodonines and derivable from a number of feedstocks, including the retinol family, consist of relatively small molecules with a molecular weight of either 285 (R5 & R9) or 299 (R7 & R11). They are retinoids of the resonant conjugate type. They are also carboxylic-ion systems and exhibit a negative charge in their fundamental form. The molecules are relatively easily generated in the laboratory in pure form. However, they exhibit a number of unique properties that have made their isolation difficult. They only exhibit the properties of a visual chromophore when in the liquid crystalline state. Their absorption characteristic is a transient one unless a means of de-exciting the molecules of the liquid crystal is present. Finally, they are extremely sensitive to destruction by oxidants and alkali metal ions. The Rhodonine chromophores consist of a family of twelve molecules; four related to Retinol (Vitamin A1 and found in saline-based animals), four related to 3,4 dehydroretinol (Vitamin A2 and associated with the freshwater-based animals) and four related to 3-hydroxyretinol (Vitamin A3 and found primarily in the two-winged flies, the Diptera Order of Arthropoda). The functional absorption spectra of these three sets, when in the liquid crystalline state, are virtually identical. This is due to the primary quantum-mechanical mechanism involved and the identical form of their chromophoric structure. The members of each set are spaced at 95 ± 2 nm. Each Rhodonine molecule exhibits a large number of functional groups. It requires careful laboratory technique to differentiate them from simpler structures which may not have all of their characteristics. They satisfy all of the 1 Released: April 30, 2017 2Gutmann, F. Keyzer, H. & Lyons, L. (1981) Organic semiconductors: Part B Malabar, FL: Robert E. Krieger Publishing Co. Pg. 319 3Saari, J. in Sporn, M. Roberts, A. & Goodman, D. (1984) The retinoids. NY: Academic Press pg. 351 2 Processes in Biological Vision historical tests for a retinene but are not retinenes. Their carboxylic-ion structure exhibits the characteristics of both an alcohol and an aldehyde simultaneously. This fact has caused confusion in the literature for years. When utilized in the vision process, the Rhodonine chromophores are formed into a liquid crystalline state on the surface of a substrate, known generically as the protein opsin. It appears that the chromophores are held to the opsin substrate by very weak bonds of the hydrogen bond type. This linkage does not disturb the unique electronic configuration of the chromophoric material. The unique spectral absorption of the Rhodonines contains two visual band components, an isotropic absorption associated with the conjugated dipole molecular structure of the molecule, and a anisotropic absorption associated with an additional resonant slow-wave structure intimately associated with the triplet electrons of the oxygen atoms of the molecule. The unusual relaxation properties of these molecules are also associated with these triplet state electrons. The Rhodonines do not fluoresce or phosphoresce significantly while in a dilute liquid solution. The chromophores are aggregated into a liquid crystalline structure wherein they are able to conduct the excited electrons resulting from photoexcitation at a given location to a second location where they are de-excited in the process (developed in Chapter 11) of generating a signal within the dendrites of the photoreceptor cells. The chromophores of vision are produced in the RPE cells of the retina and not in the photoreceptor cells as conventionally assumed. This complex procedure is developed in Chapter 7. This Chapter begins with a comprehensive review of the quantum-mechanical properties of organic molecules and how this affects their photon excitation. A series of detailed definitions and concepts are presented that are not normally found in biological treatises. These concepts are vital to an understanding of the mechanisms involved in the photochemistry of vision. 5.1.1 The conventional wisdom of the vision community The vision community has had great difficulty in describing the chromophores of vision. The nomenclature has varied from being based on the color of light absorbed, the color of light not absorbed (their appearance by reflection or transmission), the chemical chromogen they include, whether they are sensitive to a given short, medium or long wavelength region, the numerical value of the absorption peak, the animal or cell from which they were obtained, or some combination of the above4. The problem has been compounded by the different perspectives adopted by the biological, electrophysiological and psychophysical communities. Since the time of Wald’s demonstration that rhodopsin, the conceptual chromophore of vision relied upon retinol as a chromogen and Hubbard’s contemporary proposal that photodetection involved an isomerism of the chromophore, the conventional wisdom has adopted that position. To support these proposals, Collins proposed that a Shiff-base was the mechanism connecting the protein opsin to the retinoid retinol in forming rhodopsin. This structure is defined as N- retinylidene-opsin. Since this proposal was found to have serious problems on energy grounds, an additional conceptual proposal was made by Bownds suggesting protonation of the Schiff-Base, to form N-retinyl-opsin. Hubbard proceeded to promulgate a complex series of chemical reactions leading to the transition of the initial 11-cis-retinol ligand to all- trans-retinol. However, the community has not been able to demonstrate the accuracy of those proposals or to confirm any of these reactions under biological conditions. An extremely large volume of literature has used their unconfirmed proposals as their foundation. As reviewed more fully in Section 5.5.2.1, no independent laboratory confirmation of the Wald, Hubbard, Collins, or Bownds proposals have appeared from outside of their institution in over fifty years. Goldsmith concurred in this position5. 5.1.1.1 Putative isomerism in the chromophore The experimental work based on the assumption that rhodopsin consists of a conjugated protein consisting of opsin and retinol joined via a Schiff base has been studied for over 60 years. The work has failed to discover how the chromophores of vision achieve their high spectral absorption, broad spectral line widths and their specific central 4Goldsmith, T. (1994) Ultraviolet receptors and color vision: Evolutionary implications and a dissonance of paradigms. Vision Res. vol. 34, no. 11, pp
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