Primary processes of vision

W. Rettig, Charge separation in excited states of decoupled systems-TICT compounds and implications regarding the development of new laser dyes and the primary processes of vision and photosynthesis, Angew. Chem. Int. Ed. Engl. 25, 971-988(1986). 

The polyene chromophore retinal or vitamin A aldehyde is the active molecule in the primary process of vision in almost all animals. Its ll-c form (Figure 12.13) easily inserts itself into a well-defined cavity in between seven parallel a helices in the protein opsin. With retinal present in this cavity, the protein is called rhodopsin. 

Retinal (Figures 12.13 and 12.14) is roughly half of the P-carotene molecule (Figure 12.12). Most animals get vitamin A from leaves, while carnivores (meat eaters) get vitamin A from other animals. AU vertebrates (animals with a backbone) use retinal, while invertebrates use the hydroxylated forms of retinal. We are interested in the primary process of vision, which takes less than a millionth of the time for the whole process of seeing, that is, noticing and becoming aware of light. 

Liu, R. S. H., Asato, A. E. (1985). Photochemistry of polyenes. 22. The primary process of vision and the structure of bathorhodopsin - A mechanism for photoisomerization of polyenes. Proceedings of the National Academy of Sciences of the United States of America, 82(2), 259-263. 

Photochemical cis-trans isomerization in a conjugated polyene system is thought to be the crucial primary process in vision. The visual pigment (rhodopsin) is derived from 11 -crs-retinal by reaction of the aldehyde group with an amino substituent in a protein (opsin). There is considerable distortion in the geometry of this chromophoric group anyway, because of the spatial requirements of the protein... 

The primary photochemical process of vision is therefore the cis-trans isomerization of retinal in rhodopsin. The free protein opsin then leads to the production of the nerve impulse through a secondary biochemical process... 

AIMD simulations have also been carried out on the chromophore present in the rhodopsin photoreceptor (retinal). In the primary event of vision, retinal passes from the ground state (GS) to an excited state (ES) and isomerizes from 11-cis to all-trans within 200 fs. A series of papers [46-50] have analyzed the GS isomerization process. More recently, calculations were extended to the first singlet ES [51] within a recently developed scheme for singlet state dynamics [52]. This work characterizes structural and energetic changes during the photoisomerization process and points to the crucial role of environment effects. 

The laser supercontinuum source has found numerous applications in the physical, chemical, and biological sciences. Many studies have concerned elementary photochemical and photobiological reactions such as those involved in the primary processes of photosynthesis and vision (von der... 

In the case of 5-membered rhodopsin, only a long-lived excited state (r = 85 ps) was formed without any ground-state photoproduct (Fig. 4.5D), giving direct evidence that the CTI is the primary event in vision [39]. Excitation of 7-membered rhodopsin, on the other hand, yielded a ground-state photoproduct with a spectrum similar to photorhodopsin (Fig. 4.5C). These different results were interpreted in terms of the rotational flexibility along the C11-C12 double bond [39]. This hypothesis was further supported by the results with an 8-membered rhodopsin that possesses a more flexible ring. Upon excitation of 8-membered rhodopsin with a 21 ps pulse, two photoproducts - photorhodopsin-like and bathorhodopsin-like products - were observed (Fig. 4.5B) [40], Photorhodopsin is a precursor of bathorhodopsin found by picosecond transient absorption spectroscopy [41]. Thus, the picosecond absorption studies directly elucidated the correlation between the primary processes of rhodopsin and the flexibility of the Cl 1-02 double bond of the chromophore, and we eventually concluded that the respective potential surfaces were as shown in Fig. 4.5 [10,40]. 

The first study in which a full CASSCE treatment was used for the non-adiabatic dynamics of a polyatomic system was a study on a model of the retinal chromophore [86]. The cis-trans photoisomerization of retinal is the primary event in vision, but despite much study the mechanism for this process is still unclear. The minimal model for retinal is l-cis-CjH NHj, which had been studied in an earlier quantum chemisti7 study [230]. There, it had been established that a conical intersection exists between the Si and So states with the cis-trans defining torsion angle at approximately a = 80° (cis is at 0°). Two... 

Stereo-isomerizations are quite common photochemical processes with unsaturated organic molecules (the primary photochemical reaction of vision is of this type). 

Fig. 20. Model for the primary event in vision. Isomerization of the 11.12 bond leads to charge separation at Ihe Schiff base site. This process, as shown, can possibly be followed by proton transfer, the latter resulting from the charge separation. In rhodopsin, the second negative charge responsible for wavelength regulation is shown close to the 11,12 bond of the polyene chain. This model assumes that hypsorhodopsin is the unprotonated form of the Schiff base, and that it is formed possibly by proton transfer from the Schiff base nitrogen in some pigments. From Honig ct al. [207].

On examination of the data it now appears that the primary event in vision is a cis-trans isomerization of the retinal moiety proton(s) could be translocated on the protein, concomitant with or subsequent to the isomerization process, but not before. The isomerization of the 1 1-cw-retinal would lead to a batho product having a distorted transoid chromophore and a ground-state energy of ca. 35 kcal above rhodopsin. 

The human retina contains two types of photoreceptors, rods and cones, that are the primary recipients of visual stimulation. Cones are Involved In color vision, while rods are stimulated by weak light like moonlight over a range of wavelengths. The photoreceptors synapse on layer upon layer of Interneurons that are Innervated by different combinations of photoreceptor cells. All these signals are processed and Interpreted by the part of the brain called the visual cortex. 

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