Chemical Analogs to Vision Chemistry

Research in vision has followed a relatively isolated path that is so common in science. Very few references are found in the vision literature to concepts brought over from other fields except in the popular field where the eye is frequently compared to a camera. When inter-disciplinarians have tried to contribute to the vision field, they have encountered massive resistance. (Rose 1976). 

Based on some early work in the 1930 s, the assumption has been that the chromophores of vision are based on retinol or retinal coupled to a protein, opsin, in such a way as to form a chromophore. The variation in the location and possibly the style of couphng has been assumed to account for the actual spectral absorption of the material. After massive efforts, this premise has still not been demonstrated. However, it has caused a great deal of concentration on retinol and retinal as the fundamental structure of the chromophores. To this day, no laboratory experiments have shown either of these two retinoids exhibit a spectral absorption characteristic matching those of vision, human or otherwise. The retinols are neither chromophores nor analogs of the chromophores of vision. 

This work takes a completely different pathbased on the application of scientific tools unknown before the 1960 s, and still essentially unknown in the biological literature. It also incorporates a set of retinoid molecules into the theory of vision that can be shown to have the required chromophoric properties that Vitamin A aldehyde clearly does not possess. Neither retinol modified by a Schiff base nor further protonation of that Schiff base offers the required absorption spectrum18. 

The photoreceptors of the eye utilize dyes to convert the incoming light into at least the precursor to a nervous system signal. The field of dye chemistry is extremely well developed and broad. It is also extremely complex. For many years, dye chemistry delineated itself between natural dyes and man-made dyes. This was done for a number of reasons the natural dyes were generally inferior in absorption, stability and applicability to all but the first few manmade counterparts and the natural dyes involved grossly different fabrication techniques. However, the chemical rules derived during the development of man-made dyes are also applicable to the natural dyes. In recent years, the separation between natural dye chemistry and man-made dye chemistry has begun to disappear. Many natural dyes can now be classified in the same tables as the man-made dyes. This will be demonstrated below. 

The term dye is frequently used to describe two major chemical classes colorants that affect the perceived color of other materials, and transducers that accept photons of a specific wavelength and generate a technical response that can take on a number of forms. Dyes as colorants are not total analogs to visual photo-reception, but the chemistry of photography is. This field has nearly all of the same constraints and requirements as does vision chemistry. It can be very instructive to review dye chemistry as it apphes to photography and transition that analog to the natural vision chemistry arena. 

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