SPECTROSCOPY OF CHLOROPHYLL IN BIOLOGICAL AND SYNTHETIC SYSTEMS*

Abstract. –This review discusses recent spectroscopic studies aimed at discovering the structure, orientation, and function of chlorophyll in vivo. In plant membranes there appear to be at least two distinct types of chlorophyll a. The greater part, over 99%, is antenna chlorophyll which absorbs and transfers radiant energy to a few specialized chlorophyll molecules in a reaction center where the actual charge separation occurs. A dimer-oligomer model for antenna chlorophyll has been proposed on the basis of comparative studies of the absorption spectra of chlorophyll in various dry solvents and in vivo. Unfortunately a similarity between essentially structureless broad spectra is very weak evidence for their original identity. Also the requirement of an anhydrous environment for most of the chlorophyll in biological material is an unlikely postulate. A cross-linked, linear polymer model of chlorophyll in vivo has also been proposed. Recent Resonance Raman spectroscopic results appear to rule out, in large part, either polymer model and once again suggest that it is the various attachments of chlorophyll to proteins which determine its function as antenna pigment in vivo. Circular dichroism measurements of chlorophyll in various plant materials have also led to the conclusion that antenna chlorophyll has strong interaction with protein. However, some doubt still exists as to the interpretation of these CD results. New studies of fluorescence, polarized fluorescence and Resonance Raman spectroscopy of various plant species corroborate the original proposition, based upon deconvolution of absorption spectra, that antenna chlorophyll occurs in vivo in at least five discrete pools, and that each pool is likely to be located in the same environment in different plants. A new model-systems approach to simulating chlorophyll in vivo has come through the use of lipid bilayers and liposomes. Charge transfer has been observed between chlorophyll in a lipid phase and phycobiliproteins or cytochrome c. The most promising, newly synthesized model for the reaction center, P700, is a covalently bound dimeric derivative of pyrochlorophyllide a. Its properties are similar to P700 in several respects except for reversible photooxidation which has not yet been observed. By detergent treatments chlorophyll-protein complexes having about 20–40 chlorophyll a molecules for every P700 have been isolated from different plants, and their spectroscopic properties are under investigation in several laboratories. The several hypotheses to explain the shape of the oxidized minus reduced absorption difference spectrum of P700 have not yet been reconciled. The nature of the photosystem II reaction center chlorophyll, P680, is also a subject of active investigation. Its absorption difference spectrum appears to have two kinetic components.