Covalently linked chlorophyll a dimer: A biomimetic model of special pair chlorophyll

Abstract

The synthesis of a covalent dimer of chlorophyll a which possesses properties strikingly similar to those exhibited by P700 special pair chlorophyll in vivo is described. The covalent dimer is characterized by several spectroscopic techniques. Hydrogen bonding nucleophiles, such as water, primary alcohols, and primary thiols, are effective in generating a species from solutions of 10 ,sM covalent dimer in hydrophobic solvents which absorbs light near 700 nm. Formation of this in vitro special pair is a rapid, spontaneous process at room temperature. The range of nucleophiles which promote this process suggests that amino acid residues may function in a similar fashion to form P700 in chlorophyll-protein complexes. The photochemical properties of this in vitro special pair mimic those of in vivo P700 species. The 697 nm a sorption of the in vitro special pair undergoes photo-bleaching rapidly in the presence of iodine that results in the production of a cation radical which exhibits an electron spin resonance signal similar to that of oxidized P700 observed in Chlorella vulgaris. Investigations into the molecular organization of chlorophyll in green plants strongly suggest that the chlorophyll molecules act cooperatively in the primary light absorption process of photosynthesis (1-3). The light energy is captured by an extended array of antenna chlorophyll molecules that absorb strongly at 678 nm (4). The electronic excitation produced by the light absorption is transmitted via the antenna chlorophyll to a special pair of chlorophyll molecules absorbing at about 700 nm, that functions as a low energy trap (5). The trapping of the excitation energy by the P700 special pair results in one electron oxidation of the pair to yield the special pair cation radical, and an electron which ultimately furnishes the reducing potential of photosystem 1 (6). There is good evidence from studies in vivo that the species designated P700 is actually a pair of chlorophyll a (Chl a) molecules.In the visible spectra of systems in vivo, P700 undergoes bleaching upon illumination with red light to a greater extent than does P680 (7-10). The subsequent appearance of a photo-induced electron spin resonance signal of about 7 gauss linewidth with g = 2.0025 suggests the formation of P700t (11) Chl a. Two such interactions result in a structure possessing C2 symmetry with an approximate 8.9 A Mg-Mg distance and 3.6 A interplanar separation between the macrocycles. In this model, the carbomethoxy groups are pointed away from the center of the structure and the phytyl tails extend in similar directions from the macrocycles.Spectroscopic evidence indicates that the keto carbonyl group of Chl a is the primary nucleophilic donor moiety and is of principal importance in Chl a aggregation (14-16). This evidence also supports a minor role for the carbomethoxy group as a donor. Nevertheless, Fong has proposed a model (17) for the structure of the special pair based on the coordination of a water molecule to the magnesium of one Chl a molecu...

Mechanistic Reappraisal of Early Stage Photochemistry in the Light-Driven Enzyme Protochlorophyllide Oxidoreductase

“…We have shown that an excited state species, I675*, which was thought to be essential for the subsequent hydride and proton transfer chemistry [21], is not an obligatory catalytic intermediate in Pchlide photoreduction. Instead, we conclude that I675* represents an excited state energy transfer species formed between neighboring pigment molecules in Pchlide-Chlide dimers (possibly excimers) [34]. These findings illustrate the challenges in interpreting complex excited state spectral changes.…”

Spectral Properties of Chlorophyll a Monolayers in the Presence of an Exogenous Electron Donor and Acceptor

“…In polar and non-polar solutions absorption bands associated with aggregated forms of chlorophyll have been reported between 688 and 697 nm; the monomer band is around 665 nm [9,10,19]. The spectral properties ofaggregated chlorophyll depends on the mutual orientation of the absorption dipoles of the monomer units, size of the aggregate, bound water and perhaps bound redox components, and orientation on the aqueous surface [5,10,20,21].…”

Approaches for biological and biomimetic energy conversion

“…Dimers composed of porphyrin or chlorophyll were first shown to mimic the reaction center special pair (BChl b) (54,55). The dimers were followed by molecular dyads consisting of a carotenoid polyene and chlorophyll to mimic the properties of the light-harvesting antenna (38,51,56), because the carotenoid increases the solar absorption cross section of chlorophyll and transfers excitation energy to the chlorophyll.…”