Vision and photosynthesis

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 example of vision demonstrates the profound influence of a protein matrix on the photochemistry of its constituent cofactor (guest molecule). This occurs by stabilization of unstable conformers and strained geometries and by fixation of the relative arrangements of systems of co-factors and generation of contacts between co-factors. Although the complexity of the structure of the protein precludes their use in everyday laboratory control of photoreactions, the lessons learned from the example of vision (and photosynthesis) are useful in designing media that provide better control of photoreactions than that obtained in isotropic solution. Let us compare the site (termed the reaction cavity) at which the reaction occurs in a protein and an isotropic solution medium. 

To obtain as complete a picture as possible of the kinetics, it is important to obtain a maximum time resolution. Many biological reactions occur in a very short time (in fact, down to picoseconds in vision and photosynthesis) so that slower, more traditional techniques such as stopped-flow often lack adequate time resolution. Under fortunate circumstances where the system studied is photosensitive, or where the free-energy change between reactants and products is sufficiently small, then the concentrated energy impulse from a laser is a marvelous tool that can provide a wealth of information concerning the system. The intent of the review is not to offer an exhaustive review of the literature, which would be an overwhelming task. Rather we hope to delineate areas where lasers have played an important role, to discuss the importance of these investigations, and to point to areas where their potential is still untapped. 

Chemical kinetics is the study of how fast reactions take place. Many familiar reactions, such as the initial steps in vision and photosynthesis happen almost instantaneously, whereas others, such as the rusting of iron or the conversion of diamond to graphite, take place on a timescale of days or even millions of years. 

To treat photobiology quantitatively we often invoke concepts of chemical kinetics, so we need to see how the mathematical techniques discussed in Chapters 6-8 can be used to describe the fates of excited electronic states as they participate in such processes as vision and photosynthesis. 

Attention is focused on carotenoids and polyenes, which are known to be chemically very unstable as isolated entities but to acquire great stability when they are suitably surrounded by a protein cage and become the active elements in the mechanism of vision and photosynthesis. The CL dependence of the in situ Raman spectra of the carotenoids as naturally occurring pigments in bird feathers was studied by Veronelli et al. [65]. Later attention was focused on the bacterial membrane protein bacteriorhodopsin (bR). a small protein (—26,000 daltons) whose potential application in optical and electro-optical devices has been explored by many authors. The justification of such interest lies in the fact that bR contains all-/rfl/ .v retinal, which acts as a lightabsorbing center and makes bR a naturally reversible photochromic system. All-optical switching can be achieved by proper illumination of bR with yellow or blue light. 

Action spectroscopy is a general approach toward identifying the receptor pigment for a particular photobiological response or effect. The early identification of major chromophores, such as rhodopsin and chlorophyll, depended on comparison of the action spectra for vision and photosynthesis, respectively, with the absorption spectra of candidate pigments. 

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